dual quads


The Grumpy Grease Monkey mechanical engineer.
Staff member
the first thing I suppose I should point out is that many, but not all, low rise dual quad and cross ram dual quad intakes are 30-60 year old designs, based on what was originally a need to increase air flow past what was available from any available at the time single quad (4 barrel carb) intake available, since the advent of EFI and the HOLLEY DOMINATOR SERIES of carbs they are mostly a NOSTALGIC throw back, they look "COOL" but they don,t offer a huge performance advantage over a single larger carburetor on a , more modern and properly designed matching intake,any longer. back when the dual quad and three deuce intakes were factory options on some cars the available carburetor air flow was limited.There are at least THREE distinctly different DUAL QUAD INTAKE LAYOUTS
Ive always felt the rhythmic sewing machine sound , of the of a well adjusted solid lifter cam engine, and a rumbling gurgle in the exhaust note , with the instant explosive rpm acceleration and three stage progressive stepped, moan as the secondaries on a set of correctly adjusted dual quads, was and is what a properly built performance street/strip engine SHOULD sound like , tromp on a high compression ratio 454- 496-540 cubic inch big block thats properly built with the correct matched components, like a couple 600 cfm holleys on a dual quad intake and watch the many yards of dark black tire marks rapidly recede in the rear view mirror clouded with tire smoke.
the low rise inline typical of early muscle cars like the earlier corvettes



heres another fe 390 ford dual quad intake picture

the cross ram like the Z28 race engines used


most of the SBC cross ram intakes share a common plenum, but there are several designs with a dual plenum , where each carburetor feeds the opposite bank of cylinders exclusively







or my old 455 pontiac

and the true tunnel ram designs


TUNNEL RAM intakes are a different deal, now thats not saying you can,t make excellent power with dual quads, but they are not the only way to get there with today's technology and they take some experience to tune correctly, experience few guys have or understand, so you will frequently see test results showing that dual quads make less power than a properly tuned single 4 barrel car intake.
If you have a typical dual quad inline low rise intake most are set up for the Edelbrock or AFB style carbs, only a few have room for twin Holley carbs to fit mounted end to end due to fuel bowl length, on the Holley carbs
you can buy two 500cfm


or two 600 cfm edelbrock carbs
Ive always preferred the two 1406 carbs
keep in mind when your using DUAL carbs with 8 venturies they effectively reduce the engine created vacuum under each carb significantly , compared to the same size single carb on the same engine, so a carb rated at 500cfm as a single carb probably flows less than 350cfm as a twin carb, if your new to dual quad set-ups your going to be thinking 2 500cfm carbs equals 1000cfm, in reality your looking at about 700cfm-750cfm , and two 600cfm carbs flow about 800-850cfm due to the reduced vacuum signal under each carb.
Ive used both sizes and got good results from both. the 500cfm might be easier to tune for most applications but either will work.
4 barrel carbs are rated at 1.5" of vacuum doubling the available number of venturies from 4 too-eight effectively reduces the vacuum by at least 40% thus each carb will flow less air thru each venturie, while a single 500 cfm rated carb flows 500 cfm at 1.5 inches of vacum its only flowing 365 cfm at a bit less than half the draw like it would see in a dual quad setup at W.O.T.
lets say your intake plenum vacuum reads 15" of vacuum at idle and 1" at wide open throttle and your running a single 650 cfm carb,lets assume you install a dual quad intake and install twin 650 cfm carbs, at idle. not much changes simply because all 4 pairs of venturies are closed , but remember the dual quads have the potential to have 4 sets of dual venturies open this now doubles the effective flow potential but reduces the vacuum reading under any set of venturies, you have doubled the number of venturies that can potentially flow fuel & air but they will NOT all be open at the same time in a progressive set-up so until there's more than the original air flow , youll see minimal changes. and as each additional set opens in progression the vacuum in the plenum is further reduced, thus the individual sets of venturies will generally see a REDUCTION in flow due to the reduced vacuum in the plenum, but you also now have four separate, steps of increased air flow entering the plenum,and with proper tuning this can provide not only more total flow but a smoother transition and more total air flow.
4 barrel carbs are factory rated at 1.5" of mercury vacuum.
youll find a properly tuned multi carb intake generally has very good fuel/air distribution, but total flow is significantly lower than the factory rated carbs individually rated flow.

BTW IF YOU THINK OVER CARBING AN ENGINES A HUGE CONCERN,HERES A VIDEO OF A GUY RUNNING 4 750 quadrajet 4 barrel carbs on a car engine, and once tunned for the application it seems to run decently


READ THESE, and sub links carefully


http://www.vetteweb.com/tech/1305_1958_ ... uel_monty/


you really should read these related threads also











http://www.superchevy.com/how-to/projec ... ld-part-5/








if your using two holley vacuum secondary carbs on the street.....this might prove useful
to try to maintain exactly even fuel distribution between runners...which will almost never happen if the carbs throttle bores open in a totally non synchronized manor
http://www.holley.com/data/Products/Tec ... 9R8291.pdf
but this may help you








http://www.centuryperformance.com/fueli ... g-140.html





http://www.thecarburetorshop.com/Afterm ... setups.htm




http://www.popularhotrodding.com/engine ... ewall.html




the question usually goes " what size carb do I need," or something similar, this is not a guessing game, having a good understanding of what your looking at and a few basic tools will provide more info,

http://www.harborfreight.com/fuel-pump- ... 93547.html
throw a vacuum gauge on the plenum under the carb and run the engine at w.o.t. under load and get a reading
if its higher than about 2 inches of vacuum you might benefit from a slightly larger carb.

4v carbs are rated at a flow rate determined with a vacuum or pressure drop of 1.5" of mercury, your best power AT WIDE OPEN THROTTLE AT MAX RPMS will generally be found with a carb that lowers the pressure drop or vacuum to between 0.5" and 1.0" of vacuum, not 1.5" at full throttle,more vacuum at full throttle indicates a slight restriction to flow, now on a street car that's not going to be much if any problem, but on a race cars engine its a sign that your potentially giving away some potential power.you might want to keep in mind that carbs are flow rated at 1.5 inches of mercury, heads at 28" of water and that anyone who has ever watched a vacuum gauge, will tell you the readings fluctuate rapidly, under most conditions and vacuum tends to vary with the cam,used the rpms the engines running, displacement and intake design,exhaust scavenging,ETC.
in most cases there factors are at least, mostly or on occasion, partly understood well enough that theres formulas and charts that can be used to define probable results, that parts selection will result in to a reasonable degree.

If you decide to go with dual quads, remember when you double the venturie area with two carbs youve effectively cut the depression, or flow thru each carb on the same engine about in half
4 barrel carbs are measured at 1.5",of mercury, so a 500cfm carb that flows 500cfm at 1.5"", paired with a second 500cfm rated carb would be flowing about 350cfm at 1/2 or .75" of depression giving the new twin carbs about 700cfm as a paired flow rate, obviously the true flow is determined by the true depression or vacuum under the venturies but the ratio will be similar

lets look at your common 600cfm carb some of you guys use, a 0.5 inches of vacuum it flows only about 350cfm, at 1.0" it flows about 500cfm, at 1.5" it flows about 600 cfm , rated like a two barrel at 3.0" of vacuum it flows close to 780cfm, and if you stuck it on a 600 cubic inch big block spinning 6000rpm you'll pull about 6" of vacuum and it would flow about 1000cfm plus!
now remember you'll try to stay in the .5" to 1.5" range at full throttle, to make good power.
now some of you might notice that the flow dropped NOTICEABLY once the vacuum dropped and dropping the vacuum at wide open throttle tends to help power, provided the a/f ratio is kept near 12.5-13.0:1, AND the engine is set up to USE the flow available to it.

in theory a cylinder fills to 100% full, but the limited time the valves are open and the ports restrictive flow will only allow that to happen at a narrow rpm range
your engines torque curve on an rpm scale closely mirrors the engines efficiency at filling the cylinders, on that same scale, once the cam timing and port flow become a restriction power falls off because theres less fuel /air mix burnt per power stroke, the power tends to keep going up for a little further in the rpm band simply because theres MORE ,thou slightly less effective power strokes per minute.
at 1000rpm theres 500 intake strokes per minute that's 8 per second times the intake valve opens and closes, at 6500rpm that's 54 times a second, not much time when you think about what needs to flow thru that port in the limited time....especially if you remember that of that 720 degrees in the cycle only about 240 degrees have any useful flow potential, so you just cut even that time by 2/3rds

some of you may have figured out that to get the lower vacuum or restriction, you'll want a larger carb or perhaps two carbs, remember were trying to get that .5"-1.0" of vacuum at full throttle, and that 600cfm carb is not going to flow 600cfm, at that vacuum reading but between about 350-500cfm, so if you have an engine that can take full advantage of the flow it may, and usually does require a larger carb to make max power,that 383 might require an 800-850cfm carb or two 600 cfm carbs (since you double the venturie cross sectional area with two carbs the vacuum reading is generally cut to about 1/2 what it was and the two 600 cfm carbs now flow about 350cfm each or 700cfm per pair) yet the carb size is just NOT all that critical, to making fairly decent (NOT MAXIMUM POWER) simply because as the vacuum signal goes up, so does the carbs flow rate, and as the vacuum signal strength goes down so does the flow and to some extent the RESPONSE!

up till now we are talking only FULL THROTTLE POWER, but on the street,you operate under a wide range of rpms and loads, put that larger carb on a small engine and it makes good power at wide open throttle, but it also tends to have a weak vacuum signal at off idle rpm ranges and it may run like crap! so a balance must be accepted. smaller carbs are generally more responsive, in a street combo,but slightly more restrictive with their smaller venturies.
this allows the DUAL QUAD SET UP TO HAVE SOME ADVANTAGES, remember your dealing with 8 small venturies, set up in distinct pairs, that can be set up to come in in series, or progressively not only 4 larger venturies on a single larger carb., and you can adjust most dual quads to come in in four distinctive stages, rather than just two larger ones, in most cases I set up STREET DUAL QUAD SET-UPS, so the rear carb primaries open about 1/4 of the way before the front carb primaries start to open and then the rear carbs secondaries start to open as the primaries on the front carb open about 1/4 of the way, then the front carbs secondaries open last, this is done with a progressive sliding linkage and changing the radias distance, on the carbs throttle linkage.

http://www.edelbrock.com/automotive_new ... 0/7094.pdf

http://www.streetsideauto.com/images/BD ... inkage.pdf

setting it up can be fairly easy if you understand what your doing


and having sliders, thread rod and small heim joints and springs helps
in most cases but not all,
your carb throttle opens as pressure is applied by the throttle pedal and (pull/tension) to the connected linkage,
a throttle linkage is closed due to throttle spring tension theres hundreds of different configurations,
brackets and springs and ways to accomplish this.
ITS been my experience that a properly set up,
throttle linkage using cables,
are generally less likely to bind or cause the geometry related issues

https://www.zoro.com/qa1-female-rod-end-rh-516-24-cfr5/i/G4714683/feature-product?utm_source=google&utm_medium=surfaces&utm_campaign=shopping feed&utm_content=free google shopping clicks&gclid=CjwKCAjw0On8BRAgEiwAincsHOmXtmI0qgKNxUaCfdQw0-OerkhZyEzEMz40ZPEUIPk8iaNvhkojdBoCK2MQAvD_BwE







http://www.sracing.com/Store/Hardware_C ... d_ends.htm


http://www.carcraft.com/techarticles/03 ... index.html

http://www.superchevy.com/technical/eng ... index.html

http://www.speedwaymotors.com/Throttle- ... ,4369.html

if you want to build something responsive for street/strip use, these smaller 4 barrel carbs below ,make tuning easier on a car used mostly on the street


the inline dual quad 360 open plenum, offenhauser inline intakes don,t work well in my experience

yes Ive tried several engines with that intake,
- it makes fairly decent power over 5500-rpm,
but compared to many other, intake manifolds,
its wholly lacking in low and mid rpm torque.
(a fact you can,t fully appreciate without back-to-back testing,
of several different intake manifold ,designs on the same engine)
and yes I.m fully aware that rarely happens unless either a couple of your buddies have similar dual quad intake set-ups,
and those friends are willing to spend a day swapping several intakes and tuning them,
to see the results, or you have several different intake manifolds on your shop shelf,
and again, your willing to go through the time and effort.....
.most people won,t, but being a mechanic and masochist,
(almost mandatory in a good mechanic)

I've done it a couple times, and yes theres a marked easily seen and felt,
between the designs that you can feel easily from the drivers seat.
and while a properly tuned dual quad intake can provide impressive power the tunnel ram design versions are almost universally the better designs, but of course hood clearance and the fact most people don,t want carbs sticking through the hood limits their wide spread appeal.

http://www.exeterautosupply.com/Docs/Of ... atalog.pdf
with every design change you get a compromise in some area, theres no free lunch, but there are choices to be made and at times those compromises benefit the application,
offy dual port intake manifolds, use a smaller separate lower set of intake runner passages,
to radically boost intake manifold runner air speeds too in theory increase the fuel to air mix and port filling efficiency


this actually works rather well in the lower rpms and as the secondary larger upper tandem ports open up that faster lower current of fast moving air tends to help the transition or slower air mix and keep up,the problem is that the divider wall between the ports is thick enough to limit the total available cross sectional area so that while low and mid rpm torque tends to benefit from the designs characteristic flow rates the peak power is slightly restricted over what you could potentially get from a more common, single larger, cross sectional area available in the intake runner port manifold like the wieand or edelbrock designs.
yeah Ive used several of the dual port offenhauser intakes in the past, they were dirt cheap at swap meets for years, so if your low on cash you could pick up an intake that would fit and function, (maybe not ideal but when your moneys tight you make do!) they were cheap, mostly because very few guys know how to effectively tune ANY carb or intake correctly, hell, Id bet 70% can't adjust valves correctly.
I,ve had guys that both loved and hated the offy intakes and a good many times it was not the intake but the guys engine combo and his ability to tune it that made the difference, the cross ram intake offy sells (pictured below ) can make very impressive power and it fits under many stock hoods making it a valid option if you want impressive looking dual quads on your cars engine

these work great if tuned correctly but only from about 3000-4000rpm and up and only if matched to the correct cam and heads and carbs

if you look closely at the cross ram design youll see it allows longer and straighter runners than almost any other design excepet some tunnel rams yet retains more hood clearance
those offy intakes are really nice on a few race cars but rules in most classes won,t allow them any more





we commonly used two 450cfm holley carbs and a crane 110921 solid lifter cam,
http://www.cranecams.com/index.php?show ... vl=2&prt=5
and a 11:1 cpr 383 with decent heads that flow about 250cfm @ .500 lift with that intake, a 3.90:1-4.11:1 rear gear and a manual transmission in a lightened camaro or vega engine swap, to produce a kick butt combo

if you want a current inline dual quad intake,ID suggest getting the newer edelbrock design even with the obviously higher cost



http://www.superchevy.com/how-to/engine ... sults.html

http://www.superchevy.com/how-to/engine ... rison.html

http://www.superchevy.com/news/dual-qua ... all-block/

If you can,t get the carbs to fully open or smoothly transition, its usually a linkage issue ,if your local to the west palm beach fla area I can CURE that issue in under 30-90 minutes and get it operating smoothly, but theres a couple routes you can go and all of them will be superior to the current set up, you have if it won,t open the throttle blades fully
some thread rod,1/4 20 thread heim joints , and a couple bell cranks would cure that easily


btw the rollers they use on glass slide doors can be used as a roller pivot and the custom arms brazed to them to make nice smooth pivot points CHEAPLY
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Staff member
theres a bunch of guys that say dual quads are old out dated tech,
ok, I don,t agree, IVE almost always found that properly tuned dual quads, produce excellent driving and performance characteristics.
in fact two well tuned 450-650 carb can and frequently do allow for a better hp curve than either a single dual plane or a single carb single plane, now you might give up a bit of peak hp to the single plane but you'll generally have more peak hp than a single carb dual plane and a bit more mid rpm tq than either of the other two set-ups.
naturally having them set up and tuned correctly is mandatory.
BTW what most guys forget is a 600 cfm rated carb is rated at set vacuum reading, IF you double the venturies area with twin carbs and you've just significantly reduce the vacuum under each venturie reducing the flow rate considerably.
the runner design and plenum design naturally effect results.
two 450cfm-600cfm carb will generally flow about 270cfm-370cfm each in a dual quad set-up
Ive always had good results from the dual quad set ups on big block chevys and I've used both the holley and edelbrock carb set ups in several different cfm sizes, it is just a mater of knowing how to read plugs, and set the ignition advance and jetting, accelerator pumps,power valves , or mix and match jets and mettering rods etc.and use a vacuum gauge , and use an infrared temp gun to measure exhaust temps , so you can equalize the fuel/air ratios, and know what your looking at, so you can get the fuel air ratio and ignition advance curve where you need it to be












http://www.allcarbs.com/detail.php?pid= ... n=71&stt=0

PROPERLY SET UP DUAL QUADS can be a killer set up, the problem is that so few people understand the tuning issues and get them correctly adjusted.
your correct the edelbrock low rise is a dual plane intake and properly set up its a great low and mid and upper mid rpm range intake design that frequently out performs almost all the common single carb intakes in those rpm ranges, its only in the upper rpms that the single planes tend to produce better peak hp,the OFFY is a common plenum, or split common plenum design with almost no decent low or mid rpm throttle response in my experience, but it get good upper rpm and decent but not impressive peak hp numbers.
but a TUNNEL RAM dual quad intake will generally produce better upper rpm and peak hp, so it depends mostly on your goal and parts selection, and tuning skill.
For looks and ease of tuning a couple 500cfm edelbrock carbs are usually a good choice on the street, for tunnel rams on the street I prefer twin 450 holleys, on the strip twin 1000cfm dominators on a radical high compression 500 plus inch big block have their strong points, but if youve doubled the number of venturies feeding a common plenum youve significantly reduced the plenum vacuum, so the carbs are not going to flow at near the rated flow numbers


bowtie468s excellent street setup

read this again


http://www.carcraft.com/techfaq/ccrp_07 ... quads.html

http://www.carcraft.com/techarticles/03 ... index.html

http://realstreetram.com/?gclid=COr0qsH ... 7QodiWS4pQ

keep in mind theres lots of ways to mount carbs on engines, and some work far better than others


To get the ABSOLUTE BEST RESULT out of dual quad set-up:


you need to understand its strengths and limitations

If you haven't picked a cam yet, get some advice on selecting a cam, from an older established cam maker like crower or isky that is better suited to that dual quad intake design which is basically a low restriction dual plane design. Most of today's modern cam designs are expected to be used with the flow from a single 4 bbl intake, or efi intakes, with dual quads you tune for a known predictable f/a ratio but over a limited power band ,you gear the car to stay in that power band and the cam,timing and compression ratio,are selected to match that power band, usually thats the 4000rpm-6500rpm band in most cases, if its a bit leaner than ideal off idle its no big deal because your not looking to make full power in that range and keeping the mix a bit leaner (lets say 13.3-13.7:1 helps keep the plugs from fouling)and youll set up the carbs to reach full flow at the upper end of that power band

If your cam is already in,tuning can help, but dual quads normally work best on high compression combos with tight lsa cams and matched to headers with good exhaust scavenging, and keeping the f/a ratio near 12.7:1


most dual quad induction systems require a small diam distributor or vertex mag
its simple enough to tell with a caliper or even a ruler

small cap distributors measure approximately 4" in diam.

LARGE cap distributors measure approximately 5.25" in diam.

a couple detailed pictures of both the intake and distributor , and distributor mounted in the intake showing the interference would help

If your having problems getting the distributor clearance required, generally youll need a small base HEI with remote coil or vertex magneto to get clearance







related info








heres the small base computer controlled distributor I selected for use with my custom ported and fabricated EFI intake

http://static.summitracing.com/global/i ... r10209.pdf


Holley's performance billet distributors are built with premium components to deliver consistent and reliable ignition performance under the most extreme racing conditions. The distributors are capable of performing to whatever rpm levels an engine is designed to achieve. They feature 8 degree vacuum advance and 28 degree mechanical advance settings, with 32 degrees of total advance, a magnetic pick-up, output wires for connecting to the magnetic input of most aftermarket CD ignitions, and caps and rotors that utilize brass inserts for improved electrical conductivity. Their stout billet aluminum housings work in conjunction with centerless-ground, case-hardened 0.500 in. shafts for increased durability. Caged needle bearings at the top and bottom of the housing create longevity and reliable operation up to 12,000 rpm.

Brand Holley
Manufacturer's Part Number 890-160
Part Type Distributors
Product Line Holley Performance Billet Distributors
Summit Racing Part Number HLY-890-160
UPC 090127481103

Computer-Controlled Compatible Yes
Trigger Style Hall effect
Advance Type Electronic
Cap Style Male/HEI
Mechanical Tach Drive No
Slip Collar No
Ignition Box Required No
Distributor Rotor Rotation Clockwise
Housing Material Billet aluminum
Housing Finish Natural
Quantity Sold individually.
Marine Use No
Last edited by a moderator:


Very well put, I never really thought about the drop in vacuum with the two carbs, makes alot of sense, I did go round and round with mine but when I finally got a handle on the two 600's the car drives well. I was surprised that a mechancal pump could handle the fuel load of my 468, but the Holley mech. pump handles the two carbs very well. Again Grumpy thanks for all the info, I will be a readin alot.


New Member
Really good write up and reference material Grumpy, Thanks!!!

bowtie468, what are the model # of your carbs? Are they vac sec's or mech? Is that the Edelbrock c-66 style intake?



Staff member
http://www.chevyhiperformance.com/howto ... index.html

http://www.superchevy.com/technical/eng ... index.html

http://www.thecarburetorshop.com/Multip ... ifolds.htm






Its unfortunate but accurately tuning dual quads seems to be a rather commonly ignored or lost art, even guys who are good at tuning a single carburetor some how don,t seem to grasp the concept that having two carbs working together in a co-dependent system,is not that same thing as tuning both carbs the same way you would tune a single carburetor.
now on a system like a common race engine with a tunnel ram intake where your generally running at wide open throttle most of the time this is far different than in a street dual quad set-up where part throttle operation needs a smooth transition thru all four progressive stages and the rate of air flow varies a great deal during operation, there's a huge difference in a dual quad intake that's expected to transition slowly and in a controlled and predictable manor from idle to wide open throttle compared to a typical tunnel ram where you operate at wide open throttle 90% or more of the time!..
there's sometimes almost as much power, lost or gained in the skill of the guy tuning a dual quad carburetor combo as in the parts used, and Ive seen plenty of very badly tuned dual quad set-ups by guys with little or no clue how to tune a multi carb set-up.
one factor that's frequently over looked is that a properly tuned dual quad set -up has FOUR distinct progressive stages, where the four pairs of carburetorventuries progressively allow increasing air flow to enter the intake plenum, think of it as air flow divided into four controlled steps or stages in the increased air flow, this can be a big advantage over the much more sudden and abrupt change that having only two stages in a common single carbs primary and secondary venturies produce, in the intake plenum air flow rates, that usually require an extended accelerator pump shot of fuel to prevent a bog due to the much less controllable fuel/air mix ratio that results
obviously the brand and type of carbs used and the flow rating makes a difference but remember your dealing with TWICE the venturie area and about one half the vacuum readings in the plenum compared to a single carb in some cases
both holley and edelbrock carburetor combos are common , but remember on a dual combo,a 600cfm rated carburetor, will flow significantly less in a matched pair

while the use of old school low rise dual quads may not be current cutting edge tech, they can be tuned to produce a significant amount of power in the hands of a skilled tuner
one of the best parts about setting up dual quads correctly is setting 4 separate, sets of dual venturies to open in a smooth progressive transfer from idling on just the primaries on both carbs to opening each of the 4 sets of venturies in succession, providing both a smooth steady increase in air flow and the killer sound as each stage of the transition kicks into play, done correctly the air flow increases almost exactly match the engines air flow needs as the rpms increase and you get a flat torque curve and a very rapid response to throttle changes



in most cases I set up STREET DUAL QUAD SET-UPS,
so the rear carb primaries open about 1/4 of the way before the front carb primaries start to open,
and then the rear carbs secondaries start to open as the primaries on the front carb have reached the point where they open about 1/4 of the way,
then the front carbs secondaries open last, this is done with a progressive sliding linkage and changing the radias distance, on the carbs throttle linkage.





at times youll need small pieces so its a good idea to have a collection or buy several spares

you generally start with both carbs fully closed and adjust the idle air screws out about 1 full turn, the rear carb is the primary so its set up to open about 1/4-1/2 way before the front secondary carbs primaries start too open, differences in leverage arm length and angles and a knowledge of linkage adjustment allows all four sets of venturies to progressively open in fairly uniform stages as the gas petals depressed past the 1/3 third depressed point , when properly adjusted you'll have a very smooth transition from idle to wide open throttle with zero bogs or hesitation, but it takes experience, having the fuel feed consistent and knowing how to set the ignition advance and jets accelerator pump cams etc to get it to respond almost instantly to throttle position changes without dead spots in the transition.

in years past there were no huge carbs like the dominator series and if your engine required a good deal more air flow than what was available at that time (450cfm-to maybe 600cfm, in a single 4 barrel carb,)- (and only at the middle to the end of the muscle car era, maybe,700cfm-780cfm) you were more or less forced into multi carb intake designs
If your new to tuning dual quads for street set-ups you normally use the rear carb as your primary carb, since its primary venturies are closer to the engine center, you adjust the linkage so the rear carbs primaries open about 1/4 -to-1/2 of the way before the carb linkage starts to open the front carbs primaries, as the front carb linkage continues to open, the rear carb secondaries start to open as the front carbs primaries reach about 1/4-to-1/2 open and the rear carbs primaries have reached about 2/3rds open and the front carb secondaries open only after the rear carbs secondaries are about 1/2 open, this is done with sliding linkage rods stops and adjusting the linkage rod attachment points on the two carbs throttle linkage arms

http://static.summitracing.com/global/i ... inkage.pdf

the object, is to progressively open about 1/4 more throttle area as each of the 4 sets of throttle bores open.
it should be obvious but its not to most guys that you need experience or (an educated foot), in that you don,t just mash the throttle to the floor, with dual quads you learn to roll into the throttle to blend the additional air flow as the engine can use it efficiently, to try to maintain the most consistent vacuum in the plenum as the rpms build(a vacuum gauge mounted next to your tach helps) while learning the skill, but with time and experience the sound and feel of the engine becomes second nature to an experienced tuner

think of it this way, the dual quads allow you 4 distinct transitional stages and finer control over the fuel air mix entering the intake plenum compared to a huge single quad like a dominator style carb that supply's a similar total air flow capacity

http://www.edelbrock.com/automotive_new ... 0/7094.pdf






look closely at these pictures






even back in the early 1960s they were well aware that long intake runners on race engines provided a significant inertial ram tuning benefit that increased mid range torque

http://static.summitracing.com/global/i ... thrott.pdf

below you'll find a good base line starting point,
and Id suggest you monitor coolant and oil temps,
try hard to keep coolant temp under 200F and oil temp under 220F
verify your fuel delivery pressure is consistent over the whole rpm range
take the time to graph out and verify your ignition advance curve!
a good IR temp gun to use on the header primary tubes helps locate fuel/air ratio inconsistency's

GENERALLY your fuel/air ratio should be in these ranges
Idle- up too about 2500 rpm try for 14.7:1-15:1 f/a ratio,
too get max mileage and prevent spark plug fouling

from about 2500 rpm- too about 4500 rpm,
try to smoothly and predictably transition the fuel/air ratio mix richer to about 13.5:1
for good power and less chance of detonation

from about 4500 rpm- too about 6500 rpm and higher
,try to smoothly and predictably transition the fuel/air ratio mix richer to about 12.5:1,
for good peak power and less chance of detonation.

this is only a well proven starting point on the tune,
but it generally gets you in the ball park ,
and tends to reduce the chances of the engine reaching detonation conditions.

the ignition advance curve needs to be checked, the chart below is a very good starting point to work from, and USE OF A RICHER FUEL/AIR MIX, WILL AT TIMES BE REQUIRED IN THE OFF IDLE TO 3500 rpm range, but the idea here is to keep the lower rpm and lower stress operations running at efficient fuel/air ratios to reduce plug fouling and improve mileage as youll generally spend 90% of the cars operational life on the street at under 4500 rpm.


set the plug gaps at about .045, make sure the valves are adjusted correctly

finding what works best on each unique engine in any ignition advance timing curves are a bit of trial & error
theres zero doubt that you can with experience make a darn good estimate on what will work,
but a dyno and data logger will be required to absolutely fine tune,
the result for near perfect results.
yes the old mechanics experience ,ear and seat of the pants,
reading spark plugs skills of the better and knowledgeable,
old school mechanic can get you surprisingly close...
but theres no substitute for sensors and a data log system.
you can,t always hear detonation that can eventually damage your engine,
that requires sensors and a good knowledge of spark plug reading,
and a fuel/air sensor and exhaust heat measurements ,
with an infrared temp gun and oxygen sensors,
in each exhaust header primary sure helps
this is the most consistently accurate I.R temp gun I've used for testing

http://www.testequipmentdepot.com/e...1100200223789&utm_content=All Extech Products





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Staff member
http://www.diamondbackengines.com/techn ... ual-quads/

this may help
Ill point out right out front that tuning dual quads will take a bit more experience to get it tuned correctly but Ive yet to see a properly tuned dual quad set-up beaten by a single carb
if the guy tuning selected the correct carbs,and intake and knew how to tune



btw this shows a complete lack of understanding of the basic air flow characteristics of a low rise dual quad dual plane intake design


mounted like this both primaries on both carbs only feed the same 4 cylinders, and all four secondaries feed the other 4 cylinders
mounted like this on a dual plane intake its 100% guaranteed to run like crap



what seems to have been over-looked in the upper picture is that the left side of the plenum only feeds 4 cylinder and the right side of the plenum only feeds 4 cylinders, if the carbs are mounted like this (below & correctly) each SIDE of the carburetors primaries venturies feeds four cylinders

obviously the direct intake runner port feed a tunnel ram design intake provides to the cylinder heads is one of the least restrictive designs


with some thought and fabrication skill a dual quad EFI tunnel ram is easily set up with twin 4 barrel throttle bodys

keep in mind that the newer self learning computer software available,
potentially makes the twin throttle body EFI on the cross ram, in line or tunnelram intake a far more,
realistic and much easier to tune induction system, now than it was in the 1960s-90s

jet sizes on the jets can be determined in most cases,
by simply measuring the internal hole diameter
( be well aware that, some jets are not properly marked,
others have been drilled larger than the marked size
years ago I purchased two of these assortment packs when they cost about 1/2 the current price , Ive tried hard to keep 4 jets in each size in inventory as I use those, 72 jets in assorted sizes for $52 is still a good deal , and it gets better if you buy the pack or two of them, when summit posts a discount code


if you have a complete set of drill bits they can be used to get a good idea on jet size
















the MegaSquirt Forums http://www.msefi.com/






http://www.summitracing.com/parts/dem-1901 (625cfm) (CHEVY TRANS LINKAGE)



I can,t help thinking that twin thermal street demon 625cfm carbs on a big block might be fun to experiment with and tune as theres obviously some potential there for a lower (non- tunnel ram) style intake.
theres little doubt that the combo of the newer carbs and the intake flow well, as Ive seen a few guys using edelbrock 500 cfm dual quads on that BBC intake reach 570 plus hp on what were basically fairly mild 496 cubic inch engines built with mostly reworked O.E.M engine components with reasonably mild cams and compression compatible with crappy pump octane gas, using an out of the box intake manifold, and I've seen the intakes mildly ported so they flow noticeably more air, now even the single 625 cfm carb can support 500 plus hp, so at least in theory dual quads could easily support 700-800hp or more without tapping near the ported intake manifold's flow potential , with the twin carbs after proper tuning.
which at least in my eyes means it might provide both a visually impressive intake set up and a functional induction system that might serve as a basis for an old school looking hot rod.

yes there's zero doubt that a true tunnel ram has some advantages in flow rates and potential power curve,and there's no question the dual quad dual plane design gives up some flow to the more direct and straighter runner design of the taller intake, but many guys are less than over joyed with the idea of cutting a hood and installing a sight obstructing higher hood scoop or allowing the taller tunnel ram intake to stick out of the hood.
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tunnel ram, twin Holley 950 hp's on a 700 lift solid roller 11.0:1 comp. 468 spinning just over 7000 rpm. i think it will be fine but i know there are lots of people on here with much more experience. I've done the calculation for the cfm but i know that's only so accurate when it comes to really getting down to business. thanks in advance

ok, first of all, a carb rated at 950cfm only flows that cfm at the rated vacuum, installing twin carbs effectively doubles the number of venturies feeding the plenum, so you've effectively reduced the vacuum a great deal.
in most cases you want the carb size USED to maintain a MINIMUM 1"-.50" vacuum IN THE PLENUM at WIDE OPEN THROTTLE, on a race car engine, for the street Id certainly try to look for a 1" minimum reading on the vacuum at W.O.T.
a few basic tools will provide more info,

http://www.harborfreight.com/fuel-pump- ... 93547.html
throw a vacuum gauge on the plenum under the carb and run the engine at w.o.t. under load and get a reading
if its higher than about 2 inches of vacuum you might benefit from a slightly larger carb, but if it drops under 1"-.5" of vacuum your OVER CARBED

heads are rated at 28" of water on a flow bench
Sometime during the 1950’s, engineers found that a passenger engine with a four-barrel carburetor would not maintain a vacuum of 3 inches of mercury at wide-open throttle; and by some convention 1 1/2 inches of mercury was chosen for rating 4-barrel carburetors

CFM ratings

CFM ratings are more accurate than physical carburetor sizes, as the CFM rating takes into account the venturi size of the carburetor. It is not uncommon for a given physical size (see Carburetor sizes paragraphs) to have many different internal venturi sizes. Early Stromberg and Zenith carbs could have as many as 9 different venturi sizes for a given physical size. Carburetor CFM ratings have been around since at least the 1920’s; however many O.E. (original equipment) carburetors never had published CFM ratings. I have not seen any actual agreement that stated that it had to be this way, but the early published ratings that I have seen for 1-barrel and 2-barrel carburetors were measured at 3 inches of mercury. This rating was about the amount of vacuum available on engines of the period under wide-open throttle conditions.

Sometime during the 1950’s, engineers found that a passenger engine with a four-barrel carburetor would not maintain a vacuum of 3 inches of mercury at wide-open throttle; and by some convention 1 1/2 inches of mercury was chosen for rating 4-barrel carburetors. The ratings for 1-barrel and 2-barrel carburetors were left unchanged.

To convert from one system to another (with a very small percentage of error) is relatively simple. Simply use the square root of 2 (1.414). Thus to convert a two-barrel rating into a four-barrel rating, divide the two-barrel rating by 1.414. To convert the four-barrel rating to a two-barrel rating, multiply the four-barrel rating by 1.414.

This worked very well up through the mid-1960’s, when carburetor comparison tests became popular in car magazines. One carburetor company determined that the results could be skewed by rating their carburetors “dry” (air only), instead of the conventional “wet” (a non-flamable liquid with the density property of gasoline and air mixed). Rating the carburetor dry would add approximately 8 percent to the rating (example – a carburetor rated on the four-barrel rating scale at 500 CFM would now amazingly flow 540 CFM).

As the general public was unaware of the “wet” versus “dry”, this system worked fairly well until the mid-1980’s when it seems that other scales were “needed”. No attempt will be made to explain ratings of carburetors produced after 1980. The best way to compare these units would be throttle area (which was a measurement used back in the 1950’s and 1960’s); or more accurately, the comparitive area of the main venturi. On the later carburetors, best to write to the company, and ask at what vacuum the carburetor was tested, and whether it was tested wet or dry. If this information cannot be obtained, then the rating is suspect.

So, to give an example of the 4 ratings scales prior to the mid-1980’s:

Using a Carter 4-barrel AFB carburetor that is rated wet and rated 500 CFM for an example.

To convert this rating to the 4-barrel dry scale, add 8 percent (multiply by 1.08). 500 CFM multiplied by 1.08 is 540 CFM.

To convert this rating to the 2-barrel wet scale, multiply the rating by the square root of 2. 500 CFM multiplied by 1.414 is 707 CFM.

To convert this rating to the 2-barrel dry scale, first convert to the 2-barrel wet rating as above, then add 8 percent. 707 CFM multiplied by 1.08 is 764 CFM.

Going the other direction, if we take a Rochester 2-GV carburetor that is rated wet and rated 435 CFM.

To convert this rating to the 2-barrel dry scale, add 8 percent. 435 CFM multiplied by 1.08 is 470 CFM.

To convert the rating to the 4-barrel wet scale, divide the rating by the square root of 2. 435 CFM divided by 1.414 is 308 CFM

To convert the rating to the 4-barrel dry scale, first convert to the 4-barrel wet rating as above, then add 8 percent. 308 CFM multiplied by 1.08 is 333 CFM.



http://www.thecarburetorshop.com/Carbsh ... andCFM.htm



Staff member
"I had an idea to run a tunnel ram intake on the street and reduce the plenum volume under the carbs a bit to get a quicker throttle response, I have done something similar with several single plane manifolds on the street with good results."

the idea of running individual intake runners to each carb venturie on a tunnel ram intake to reduce plenum area, or to weld in a partition in the plenum and thus increase the rapid response to throttle changes comes up fairly often,but it won,t work unless all the carburetor venturies , both flow equal quantities of fuel/air mix all the time at the same fuel/air ratio, and all the throttle blades open at exactly the same rate. which is only something a few carbs are capable of doing and most of those are Holley dominator based designs designed for that application. Id also point out that the plenum under the carbs allows a great deal of flow sharing that allows you to feed a single intake runners from more than a single carb venturie, thats critical, because in almost all cases a single four barrel carbs single venturie is too small in diameter to effectively supply the needs of an engines cylinders.
If you do the research youll quickly find out about port stall speeds, and effective port cross sectional areas.




http://www.highperformancepontiac.com/t ... index.html

http://www.highperformancepontiac.com/t ... index.html


Determining Port Velocity and Volume
Determining air velocity is a useful part of testing. It is calculated as follows:
V = 1096.7* √¯H/d
V= Velocity in feet per minute
H= Pressure drop across test piece in inches of water (28” of water column being a standard)
d= density of air in pounds per cubic foot ( .075 pounds per cubic foot at standard conditions )
This represents the highest speed of the air in the flow path, at or near the section of minimum area (i.e. through the valve seat at low values of L/D, through the pushrod pinch, or minimum intake cross section, which ever is smaller).
Once velocity has been calculated, the volume can be calculated by multiplying the velocity by the minimum cross section area. Since we measure flow in cubic feet per minute, we use the first formula and divide by the cubic feet of cross section area: If we have an average cross section of 2.1 square inches:
Volume = Velocity (21,190 fpm) x Area (2.1 sqin x 1 sqft/144 sqin - converts sqin to sqft) = 309 cfm @ 28”
Since we use feet per second in the flow measurement of ports the formula above the equation becomes:
V = 1096.7/60 (18.278) * √¯H/d
V = 353 feet per second (fps)
This should be the highest velocity measured in a port flow tested at 28” of water column.

Mean Piston speed in linear ft/sec from stroke in inches & RPM (David Vizard)

We want to find the average speed of a piston traveling up or down between TDC and BDC. Though it should be emphasized the piston speed peaks much faster than the value we will find, and it slows to a stop and perhaps reverses slightly at TDC & BDC, this mean (average) velocity figure is highly useful as a comparator. It's also useful to note for later that because of the secondary-linkage effect of the connecting rod, the piston spends more time in the lower 180* of the rotation.

Conversion of measurement units: stroke in inches divided by 12 to get stroke in feet : s/12 = stroke in feet

then take the stroke times two because the piston travels
the stroke's distance twice in a revolution, and by convention we measure rotational speed by full revolutions : ( S/12 ) x 2 or,

by taking the 2 inside the brackets, we get ( S /6 )

Then, since by convention, we measure rotational speed in automotives in a per minute format,
and we use a per second format for gas velocity, we need the revolutions each second, i.e. per
one sixtieth of a minute ,
so : ( S/6 ) x RPM/60 or cleaning up the formula by basic algebra :

S/360 x rpm = average speed of a piston traveling up or down between TDC and BDC
carb air flow depends on the VACUUM of the PLENUM under the carbs venturies, 4 barrel carbs are factory rated at a vacuum reading equal to 1.5" of mercury
change the plenum vacuum and the flow changes

your never going to get 1200 cfm flow on any dual quad set-up but thats NEVER an issue
its the carbs job to supply a totally consistent well atomized fuel/air ratio you control through careful tuning the flow rate is MEANINGLESS.

lets look at it this way, lets say you put that intake that has 2 600 cfm rated 4 barrel carbs on a 350 sbc that spins a max of 6700 rpm
350 cubic inches of displacement x 6700 rpm/2 (every other stroke an intake stroke)=1172500 cubic inches /1728 (number of cubic inches in a cubic foot)=678.53 cubic ft of potential flow at 100% fill efficiency , thus theres no chance the engine could ever pull the rated 1200 cfm flow
but again thats meaningless the only thing that matters is that the carbs can supply a consistent and dependable and repeatable fuel/air ratio of about 12.6:1 where youll get max power

Mean Gas speed from piston speed (above) and "area/cross section quotient":

We want to find the average gas velocity over a stroke of the piston- a half revolution of the crank, assuming we have the cross-sectional area of a header tube, or of an intake or exhaust port. The latter can be approximated by measuring centerline length and port volume, and subsequently converting the units of measure to match the units for the motor bore area, in this case, square inches.

There are several ways of accumulating the measurements required, but the simplest starts with bore area which we are likely able to calculate easily, either working from bore diameter (‘B’) taken down to radius, & squared & multiplied by Pi, or from engine total volume divided by stroke and number of cylinders.

So: ('B' area/ 'P' area), where p can stand for either pipe or port

So we now have an existing area relationship mathematically described. To use some examples, if the piston had 6 inches area and the header pipe 2, we would have a factor of 3... and a design error on our hands: practice has proven that the relationship on the exhaust side should be between approximately 4.5, for high RPM 4-stroke modern motorcycle engines and dedicated auto racing power plants, to as little as 9 for those of us working with older low-RPM-oriented production power plants, and between 4 and 8 on the intake side.
This relationship is far better described/defined by what we are working toward a calculation of: gas velocity at RPM.

Gas velocity at RPM

We can calculate that by combining the two formulas. When we had bore area calculated above and looked at its relationship to port area, we could envision the gas achieving some unknown- but already apparent to be high- velocity as it exited through the port, or arrived through the port. What we didn't include was quantification of the volume being transferred each cycle: how deep the piston traveled to reach BDC, or how many times per second did it come back to TDC?

Again, our first formula has linear information, which combined with area gives us volume, and it has rate. Thus:

( S/360 ) x RPM x ( B area / P area )

let’s try some examples:
We spoke of an imaginary engine having pistons of 6 sqin and an exhaust header pipe of 2 sqin (you could also evaluate the intake side by using intake runner size). That's a pretty small bore diameter- 2.75" or 70mm - more motorcycle range than automotive, so lets say it's a 1000cc (61ci) four cylinder sport bike. The stroke is then calcuable to be 2.54", and we are doing calculations like this with power in mind, so let’s say that the RPM we wish to examine is 10,000rpm. The port area used was 2- we referred to the header pipe area, but that's an extension of the port; we'll start ignoring the difference.
So, we have:

( 2.54/360 ) x 10,000 x ( 6 / 2 ) = 212 ft/sec

and, as noted, that is too low a figure. In practice rules of thumb have developed saying that at peak power, the ft/sec figure should be somewhere within 280-380 exh and 240-355 intake.

Let’s take another example- a 350 cid v8 with a 1.5 OD diameter header pipe and a relatively mild state of tune that leaves us interested in its 5,000 rpm statistics. It has a 4.000" bore (area=12.57 in2) and 3.48" stroke, and the ID of the header pipe is approx 1.39" (1.52 in2) as in a typical 1.5" OD pipe with wall thickness deducted for accurate calculation.

( 3.48 / 360 ) x 5 000 x ( 12.57 / 1.52 ) = 399.7 ft/sec

This result implies the motor is being revved slightly beyond its power peak and/or that larger headers would likely give more power at this and any higher rpms. As always, a trade-off exists, as the 1.5" pipes will have produced a greater power up close to this rpm, and may make it unwise to take that power advantage which has "accumulated" as the motor rpm's have risen under load, and trade it off for greater peak power.
The next available tubing size up - 1 5/8th's- is an 18-19% jump, putting the ft/sec figure @ 325, which is actually perfect for 5000, but progressively tending to be wrong as you look at increments of lower RPM.

The first header intend is for a motor with 3.64" bore, 3.5" stroke, being shifted at 4900rpm but with the emphasis on low end and midrange, so calculate using the rpm figure given, but emphasize the low and mid range by using higher ft/sec figure than I would otherwise.

3.50" / 360 x 4900rpm x 10.41" / 360ft/sec = 1.378 in2

So where does that leave us in picking a tube size? That figure is in square inches, so we divide by Pi, take the square root, multiply by two, & arrive at a dimension for ID. I add .049" x 2 for the walls to get the OD, and find it to be 1.42". We already figured I'd use 1.375" OD for several inches coming off the ports and do the remainder in 1.5". Now we’re less worried that the 1.5 need to be mandrel-bent, though we know the disadvantage of 'crush bent' is less related to simple loss-of-area type restriction and more the energy used in speeding up and slowing down the gases as they pass through cross section changes. Since this area of pipe figure is notably smaller than the stock port area as measured at the opening (~1.8 in2), it confirms I am on the right track in filling the port floor.

Since it's becoming apparent that the variable we are most likely to want to solve for is the port area, we need to reformat our equation to solve for port cross-sectional area when we have all the other figures. This requires setting the formula up with an adjustable ft/sec position, because depending on the character of the motor's use you will have different aims.

Starting from:
( S/ 360 ) x RPM x ( B area / P area) = ft/sec

writing it on one line, since we are fortunate that order of operations is irrelevant here

S / 360 x RPM x B area / P area = ft/sec

It then holds that:

S/360 x RPM x B area / ft/sec = P area

Putting the formula to yet another use, you can measure the ports you have to see what RPM band they suit. Take soft wire, like solder, and measure the top length (beside guide) and bottom length, averaging them to get centerline length, then CC the port. You can now calculate average port cross section: take the CC's and divide out the length. Remember 16.39cc=1ci, convert the port volume into cubic inches before dividing by the length, assuming you used inches to measure centerline length.

The derivation of the formula I am providing for this requires you to find the reciprocal, but scientific calculators have that function

S / 360 x Barea / (ft/sec x Parea )= 1/rpm

So, some rules of thumb as far as desired gas velocities.

Highly developed ports such as those found in race specific castings can accept 10-20% higher velocities, due to the lack of flow differentials. In other words, they use the full port area, and to not have "hot spots": areas where the flow chooses to concentrate and which could become turbulent or supersonic if pushed beyond a certain speed. Also such engines are likely to have high compression which somewhat changes the dynamics of cylinder filling and emptying.

Figures shown are average velocity throughout the port, and references to effects are in the RPM range of peak horsepower.

240 ft/sec - intake - ram effect faint
- exhaust- scavenge faint

260 ft/sec - intake - ram effect moderate
- exhaust- scavenge weak to moderate

280 ft/sec - intake - substantial ram
- exhaust - scavenge moderate

300 ft/sec - intake - * ideal ram
- exhaust - substantial scavenge

320 ft/sec - intake - possible loss
- exhaust - * ideal scavenge

340 ft/sec - intake - likely loss
- exhaust - possible loss

Correcting Runner Length

Q: We have just got a sbc built and the engine builder had a blonde moment and has installed a sheet metal intake with way to large of runners on it. Needless to say the motor does not make the power we are looking for. My question would be if we are able to install a sort of insert and get the runner volume under control will we see a good gain in power even though the plenum volume is still large. It is a 366 SBC with Brodix canted valve heads 290cc 13.5 :1 comp. The total runner length is 12.4 "and needs to run at 6000 to 6500 steady WOT rpm. The runner volume right now is 600 cc and I figure that it should be about 300 cc.

A: With a 12.4" long intake tract (port and runner total length) you are tuned from 6800 to 8000rpm. This is well above your engines intended operational rpm range. This means your operating the engine at an RPM where the intake tract pressure pulse is either neutral or slightly negative. You may be neutral in this area so reversion might not to bad but it’s far from an optimum situation, now for the good news.

Depending on the manifold design you have, you can either increase air speed to band-aid the runner length or make spacers to put in the plenum to lengthen the runners. You really need .750 thousands to bring the manifold back into tune but that will take up a lot of plenum volume. If you’re out of tune the runner volume is secondary to air speed and wave tuning. I have done this many times and actually do it quite often. In many cases the results are astounding. Our 622 engine uses the Profiler 2/4 tunnel ram intake.

heres what a SIMILAR custom sheet aluminum, tunnel ram intake plenum looks like

This manifold is tuned from 7400rpm to 8700rpm. The larger engines all operate from 6000rpm to 7500rpm and some as high as 8000rpm. So the engine is trying to make peak TQ at 5500rpm and peak power at 7500rpm. Using this manifold causes a hole in the mid range of the power curve because it’s "out of tune" there. I fill the runners to increase air speed as much as possible without hurting top end power. The result is a 45ft/lbs increase in torque and a 60hp increase in power through the middle of the power curve. The engine accelerates much faster and recovers on the gear change better. If I where able to put plates in the plenum to increase runner length instead of increasing air speed the result would be even more power and a broader power curve because I would not only have the proper runner volume but the proper tuned length as well.

The course of action we take as well as the end result will depend on the manifold design you have to work with. Sometimes I can band-aid them and sometimes I can’t. If you have measurements and pictures of the manifold it would help a great deal. I can talk you through it if you want.

Darin Morgan
R&D-Cylinder Head Dept.
Reher-Morrison Racing Engines

heres a different tunnel ram but it shows the tunnel ram easily produced better hp than a good single plane intake




yes they are still available new, yes they work rather well once the engines above about 3500rpm if properly set up and tuned
and Ive used them in several engine combos






BTW heres the cam I usually use with a CROSS RAM APPLICATION
http://www.cranecams.com/product/cart.p ... il&p=23968
the cross ram intake REQUIRES an engine with about 11:1-12:1 compression,good headers with a low restriction exhaust, a manual transmission and 3.90:1-4.56:1 rear gears to work correctly, Ive used them mostly on 377 and 406 and 383 displacements with two identical 600cfm carbs, but I know other success full cases using two 450 cfm carbs , just be aware this intakes designed to run in the 3500rpm-7000rpm power band all the time, on a engine with open or low restriction exhaust and a solid lifter or roller cam and heads that flow a minimum of about 250cfm at .500 lift
The Engineering Truth..

The Cross Ram was never intended, nor was it developed (tuned) to be used with the Smog Equipment. The only reason it appeared in the Car Life article (and others) was to satisfy the Corporate mandate to pre-empt any negative connotations derived from the article that might imply Chevrolet was not in sync, or worse yet, ran counter to the Government mandated SMOG laws. It was a risk that Chevrolet could not afford to take for political reasons.

The true intent by Chevrolet was to have the tuner build the Cross Ram equipped 302 in the "recommended" tuned state it was developed for using Chevrolets Tubular Headers with specific timing and tuning considerations - never with the Smog Pump since it was counter indicated for performance which was implied and understood by all race tuners and street racers back in the day.


The SMOG Pump is actually a centrifugal AIR pump that forces compressed air into the exhaust manifolds to decrease the hydrocarbons by DILUTION. A totally ineffective smog reducer and supreme power waster that adds extra weight. In fact, you could make the argument that it actually increases pollution due to its inefficiency. No one was fooled by it back in the day.

The Smog Pump will cost you 15 important hp in the lower rpm ranges that you will not want to lose
Use of the equipment prohibits optimum jetting and timing and therefore;
The optimum power/torque distribution curve is not achievable and at best you will end up with a de-tuned 302

Bottom Line - The only reason the 302 appeared in some publications with the Smog Equipment was to appease Corporate Management and placate the Government. Understanding that, if you go with-out the pump as intended, you will benefit from maximum power which is consistent with Chevrolets true intent for the Cross Ram and from a pure nostalgic point of view be in sync with what everyone actually did back in the day. That is the true spirit of the Cross Ram equipped 302.

I'M always amazed at guys who overlook checking the simple stuff, frank stopped by with a friend who has a 1967 fair-lane with a 428 ford engine with dual quads that looks fairly close to this picture I found on the internet, except his cars a burgundy color.


they had installed two carbs that were not ideal, but carbs they had gotten locally

they had spent a couple hours installing the intake manifold and swapping jets on two 470cfm rated Holley carbs they had installed om a swap meet dual quad intake, it was barely running so knowing I'm a big fan of dual quads they decided to come by.
the first obvious issue was the linkage was set up incorrectly and the linkage contacted the air cleaner assembly restricting the movement of the linkage .




well the first thing I suggested was getting the linkage binding issue resolved and then verifying the ignition timing and fuel pressure, once I checked those it was easily apparent that the float levels were not set correctly.once that was corrected I started checking the linkage which they had set up so both carbs opened exactly at the same time and rate rather than progressively.
checking that and adjusting the linkage resulted in finding the both the front and rear carbs throttle bores intake gasket was binding the throttle blades , so a trip to the local napa store to get new carb to intake gaskets the gaskets came off and got replaced as they were not correct. once that was fixed its was time to make a test run and it was obvious the jetting was a bit too lean, naturally NAPA didn,t have a large selection of holley jets, so I had to send them out to get several sets, after an hours wait we swapped jets up two sizes and the engine seemed to pick up 50-70 hp and the car idled far smoother, but after a few minutes I found the power brake booster vacuum line leaked, once that was replaced the car ran much better.

related thread





http://www.vintagemusclecarparts.com/pa ... sram1.html


http://guinns-engineering.com/Cross Ram FAQ's.htm

http://www.superchevy.com/technical/eng ... index.html

http://www.carcraft.com/techarticles/03 ... index.html
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Holley now has available a mid rise dual quad intake for the LS series of engines, , it was tested on a 5.3 LS engine that had a stroker 4" crank and trickflow 215cc genx heads installed.

the 383 displacement LS , had a MSD ignition installed,(part# 6010), set for 30 degrees full advance, 1 3/4" full length headers, ,that had a cam designed to be run with a fuel injected induction ,in a previous combo, not with the two 390cfm Holley 4160 carburetor s ,on that engine, its was tested on page 48-50 of the April 2012 super Chevy magazine vs a single plane LS carburetor intake with a 750 cfm Holley, acting as the base line to test against, the results were rather interesting
the single plane intake produced 527 peak hp at 6000rpm and 484 ft lbs of peak torque at 5100rpm
the dual quad ,dual plane intake produced 526 peak hp at 6200rpm and 490 ft lbs of peak torque at 4700rpm

carefully done, port work on the intake runners and plenum can produce significant flow rate improvements

but much of the lower rpm torque curve was no contest, the dual quad dual plane intake produced more than 50 extra ft lbs of torque at 3200rpm, making the power curve much more street friendly , and knowing how POORLY most of those magazine guys can tune dual quads and knowing that the intake was not ported before it was installed theres ZERO doubt in my mind that a properly ported and tuned dual quad intake with a cam designed to match the application could significantly improve on those power numbers, especially with two 500-600cfm carbs installed vs the twin 390cfm carbs used,keep in mind EFI cams generally have a wide 112-116 LSA and carburetors generally work better with a tighter 106-110 LSA , as it increases the cylinder scavenging efficiency a bit, and increases the working dynamic compression, but makes efi sensors read flawed data.
and a couple 390cfm carbs are a bit small for a 383 running up to 6200rpm on a dual quad intake.

Id also point out that no carb spacers or extensive tuning was done to find or maximize the true power potential.
some carefully done plenum and runner port work and a couple carb spacers can in some applications add an easy 15-20 more horsepower


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Awesome Reading Grumpy .
Thanks for taking time to write up in the past.
Was researching on my own & found what I was looking for.
Big B was a Dual Quad guy like you.
He did amazing stuff on the street with them Tuned.



Staff member
some related info

http://www.diamondbackengines.com/techn ... ual-quads/

http://www.chevyhiperformance.com/howto ... ewall.html

http://www.edelbrock.com/automotive_new ... vids.shtml





HERES A GOOD EXAMPLE, of a MAGAZINE TEST, Ive seen repeated several times and I,m rather surprised at the outcome simply because in my experience its rare for the normal edelbrock air gap ,dual quad set up to not come very close to matching or even exceeding the single quad if properly tuned after a mild bit of port matching and plenum clean-up, and a correctly set-up tunnel ram almost always leaves a single quad looking at tail lights if its on a serious race engine set up to use that extra air flow, and yes Ive worked on dozens of SBC and several big block chevy engines with a wide verity oh heads and cams and displacements, using dual quad and tunnel ram intakes
406 Small-Block Edelbrock Performer RPM Air Gap Dual Quad
Who wouldn't be happy with a street engine that pumps out an honest 1 hp per cubic inch, especially when the motor displaces 406ci? Equipped with a set of Pro Comp aluminum heads, a healthy hydraulic roller cam, and the new Edelbrock Performer RPM Air Gap Dual-Quad intake (with 500-cfm carbs), the 406 produced 406 hp and 470 lb-ft of torque. Torque production from the big-inch small-block exceeded 450 lb-ft from 2900 rpm to 4600 rpm. Add to this power production the cool factor that comes only with multiple carburetion and you have an impressive rat rod powerplant.
406 Chevy Small Block Carburetor Dyno Test

406 Small-Block Edelbrock Performer RPM Air Gap Single Quad
This combination clearly demonstrates why dual-plane intakes combined with a single four-barrel carb rule the streets. Equipped with the Edelbrock Performer RPM Air Gap and 750-cfm carb, this 406 produced 444 hp and 505 lb-ft of torque. In all honesty, it is tough to beat the Air Gap for average power production. This combo netted torque reading that exceeded 450 lb-ft from 2500 rpm to 5300 rpm.

And A One, And A Two...
The results of our comparison show the single four-barrel to be the hot setup, however, as the old advertising escape clause goes, "individual results may vary." An Edelbrock spokesman says its testing has shown the two systems used here to be very similar in power output, and that he was surprised by the 39hp deficit we witnessed for the dual quad. In hindsight, maybe we should have swapped induction systems to see if there were any variances in the two 406s. Still, given our compression ratio, cam, and redline, our trusty old carb-picking formula says 750-cfm should be just about ideal for this engine.

http://www.superchevy.com/technical/eng ... ewall.html

http://www.hotrod.com/techarticles/engi ... e_systems/

http://www.carcraft.com/techarticles/03 ... _contrast/

http://www.hotrod.com/techarticles/hrdp ... he_street/
Tunnel-Rams On The Street
Wrenchin' @ Random
By David Freiburge
Hot Rod Magazine, March, 2013





Tunnel Rams On The Street Edelbrock Tr1y Ram
We tested this Edelbrock TR1Y ram and 650 Demons on a rowdy 350 Chevy, comparing it with a

As antiquated as it may be, we love a good tunnel-ram and dual quads atop a muscle car V8. As a result, we've probably tested more of them than most guys have, so we rarely fail to disagree when we see stuff like this on forums: “Tunnel-rams are great for racing, but that is all. I laugh when I see people with tunnel-rams on the street, because they are so impractical. Those intakes have absolutely no bottom end, and they don’t come alive until 4,000 rpm.” That’s a direct quote that we haven’t attributed because it sums up all the commonly held beliefs.

We have half a dozen tests on small- and big-block Chevys and Mopars that show tunnel-rams with significantly more torque production—even at the lower end of the scale—than with a dual-plane intake. In some cases, the rams didn't improve peak torque figures, but they always made better top-end power. That said, it tends to be the camshaft that dictates the rpm at peak power, and intake manifolds only wiggle that by a couple hundred rpm at the most. An engine that makes peak horsepower at 6,200 rpm with a high-rise dual-plane will probably make peak power at 6,200–6,400 with a tunnel-ram; if the peak-power rpm jumps more than that, then the engine was seriously strangled by induction to begin with. The rpm at peak torque may rise, but typically, the torque at the old rpm point will still be higher than it was.

This isn't to say that any tunnel-ram is great on the street. The race-style, low-rise tunnel-rams do tend to sacrifice low-end torque; it’s the high-rise units like the Edelbrock TR1Y (shown) and the Weiand Hi-Ram series that are street-oriented with longer runners. Long runners make torque; short runners make high-end power.

The other problem with tunnel-rams on the street is that cold starts can be a hassle. Guys claim that tunnel-rams bog, but that’s only when the carbs are too large or not tuned properly. A tall ram and dual quads on a midrange street mill can make for incredibly snappy power.

Read more: http://www.hotrod.com/techarticles/hrdp ... z2eG3YUUmg
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The only problem with a Tunnel Ram on the Street is that that Cops 5 'O Take notice.
Especially when its as tall as the roofline of the car.
Heavy Rain Downpours spoil your fun too.

Ultra Rare Mickey Thompson Tunnel Ram Dual Quad the best ever made for a Pontiac V8.
Magnesium. Bathtub intake.
8 Originals made.
Know a guy that owns 2 in Illinois.
$10k value each.


Staff member
you can watch these videos, as they are interesting, but Id point out a good deal of these videos seems to indicate the guys making them in some cases don,t have a great deal of tuning skills, and I certainly don,t know or recommend,or have any reason to not recommend or know anything about these guys personally

https://www.youtube.com/watch?v=eYIGIMb ... 6BC2A41CE1

https://www.youtube.com/watch?v=rcPBQR0 ... 1&index=10





I will have to watch the U-Tube videos today while turning wrenches.
Wheel bearing replacement job.
Hydraulic press out style. Not fun. Remove steering knuckle.

I spotted Split Dominator Race Carbs.
Not run of the mill.
$10k Pro Stock Material . Win the Race at all costs.

Also spotted your old Pontiac V8 Mickey Thompson Crossram.
1959-64 Head to intake Bolt pattern.
455 1ST Invented by Rodders & Racers.
Well before 1970.
4.300 welded & offset ground early 389 Forged Cranks.
Castillo. An old Master.

Brian R.


solid fixture here in the forum
I found a Tunnel Ram this morning Grumpy.
Price is right.
Just $300.00.
Offy Olds 425-455.
Not sure if I really need it.
Sure Is Kool..

Not sure if it will fit under the hood of the 63 Pontiac.

Think it was off a Jet Boat Olds 455 powered.


The Grumpy Grease Monkey mechanical engineer.
Staff member
its almost certain to not fit under an un-modified hood, but just as certain to ,
if properly tuned to provide a noticeable boost in mid and upper rpm power


solid fixture here in the forum
its almost certain to not fit under an un-modified hood, but just as certain to ,
if properly tuned to provide a noticeable boost in mid and upper rpm power
Yes What I am thinking too.
Probably Pass on it.

Offy Dual Port intake has been very good.
Looking for that extra punch.

Like to put an Offy Super Sonic on it.

Offy 360 I found. Kinda pricey.
I don't think much better than Olds Big Block Dual Port I have now.

China Intakes just No way.
MAGA guy I am .

Edelbrock...? Quality not like it used to be reading.


Well-Known Member
Whats up Maniac ? I like the idea of 2 throttle bodies on top of a tunnel ram.kicked that idea around when I bought the 78 for shits & giggles. Hows the fingers ?