dual quad

alex · Dec 5, 2015

Hello,i have a low profile single plane offenhauser 5186 dual quad intake manifold with 2 holley 600 list 1850's vacuum secondaries ,i was wondering am i getting the full 1200 cfm,or is there a formula to figure out the actual cfm,thank you for your help.

Grumpy · Dec 6, 2015

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
http://users.erols.com/srweiss/calcdchg.htm
http://garage.grumpysperformance.com/index.php?threads/dual-quads.444/#post-36004
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
http://www.mustangandfords.com/proj...rs-testing-part-1/?cx_navSource=related-right

http://www.mustangandfords.com/how-to/engine/mdmp-1207-dual-quads-twin-win-or-double-trouble-part-2/

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

Grumpy · Dec 6, 2015

as most intake manifolds come out of the box they are hardly ideal nor do they exactly match or correctly fit or function on your particular engine,most will require plenum clean-up, port matching and various stages of runner clean-up, gains of 10-20 plus hp are not that un-common through taking the effort to match the intake to the application.
your selection of the carburator(S) that will be used and carefully tuning the carb(S) to maximize the potential is another area where the experienced tuner can gain a noticeable advantage. and obviously additional enhancements like low restriction air filters, cold air induction anti-reversion plates, plenum spaces and matching the cam timing and exhaust scavenging can easily make or brake the combos efficiency, get it all correct and gains in excess of 60-80 plus hp over slapping a stock intake and cam, and exhaust manifold on an engine are very common.
simple additions like adding two insulating spacers that add plenum volume can make a noticeable difference, effectively tuning dual quad intakes is mostly science, some art and requires some experience to do correctly but the skills can be learned if your observant
ID certainly point out that a common 4 barrel carbs rated flow is meassured on a standard industry vacuum reading of 1.5" or mercury , and a change in vacuum changes the effective air flow!
think about that....
that single 600 cfm rated carburetor , you were using ,might see 17 inches of plenum vacume on a rpm air gap single carb intake!
put the same carb, on the same engine, with an identical second carb, bolted on a dual quad manifold, and youve effectively doubled the venturie surface area, effectively reducing the plenum vacuum at W.O.T. so theres no question that the carbs will need to be re-tuned from the way they were set-up to function correctly

http://wallaceracing.com/intakecfm.php

http://www.wallaceracing.com/runnertorquecalc.php

http://dnserrorassist.att.net/search/?q=http://fiftechefi.com/&r=&t=0

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.

The Carburetor Shop LLC

http://garage.grumpysperformance.com/index.php?threads/cooler-denser-air.8961/

http://garage.grumpysperformance.co...-the-holley-4150-and-4160-series-carbs.10736/

http://garage.grumpysperformance.com/index.php?threads/holley-carb-emmulsion-tube-tuning-info.11557/

http://garage.grumpysperformance.com/index.php?threads/carb-intake-manifold-engine-flow-rates.10194/

http://garage.grumpysperformance.co...-guys-that-just-slap-on-factory-headers.3155/

http://garage.grumpysperformance.com/index.php?threads/holley-accelerator-pumps-cams.1790/

alex · Dec 8, 2015

What would be a good baseline jetting size to start with and spring color.

Grumpy · Dec 8, 2015

Id start with 5-to 5.5 psi psi of fuel pressure at the carb inlet

(BE DARN SURE TO CORRECTLY SET THE FLOAT LEVELS ONCE THE ENGINES UP TO OPERATIONAL TEMPS AND ADJUST THE VALVES)
(YES YOU NEED A FUEL PRESSURE REGULATOR, GUESSING AT THE FUEL PRESSURE NEVER WORKS AND JUST BECAUSE ITS 5 PSI AT 1000RPM IS NO GUARANTEE ITS STILL 5 PSI AT 6000 rpm,
UNLESS ITS VERIFIED AND CONTROLLED WITH A FUEL PRESSURE REGULATOR AND A PROPERLY MATCHED FUEL SUPPLY SYSTEM)
a fuel pump rated at 120-135 GPH at 7-12 PSI and a HIGH QUALITY RETURN STYLE FUEL PRESSURE REGULATOR FUEL SYSTEM WITH 1/2" ID lines can prevent most fuel supply issues

As you make changes take accurate notes of the results, achieved,
you should see a pattern develop as you make changes that you can use as a partial guide
don,t forget accelerator pump cam changes may be helpful

http://garage.grumpysperformance.com/index.php?threads/holley-jet-chart-size-and-area-of-jets.15869/

http://garage.grumpysperformance.com/index.php?threads/holley-accelerator-pumps-cams.1790/

http://garage.grumpysperformance.com/index.php?threads/holley-jet-chart-size-and-area-of-jets.15869/

even changes to the throttle linkage ratios and spring tension may help
http://garage.grumpysperformance.com/index.php?threads/holley-jet-chart-size-and-area-of-jets.15869/

changes to accelerator fuel pumps and boosters and squirters?
http://garage.grumpysperformance.com/index.php?threads/holley-annular-vs-down-leg-boosters.5229/

http://garage.grumpysperformance.com/index.php?threads/holley-jet-chart-size-and-area-of-jets.15869/

Id start with 6.5 power valve but obviously, if changes are required, you'll be smart to make those changes.
http://garage.grumpysperformance.com/index.php?threads/holley-carb-power-valves.1639/

and Id start with 68 jets all around
but obviously, if changes are required, you'll be smart to make those changes.

and plain or brown secondary spring
http://garage.grumpysperformance.com/index.php?threads/holley-carb-secondary-springs.8126/

blue accelerator pump cam
http://garage.grumpysperformance.com/index.php?threads/holley-accelerator-pumps-cams.1790/

LEARN TO READ PLUGS
VERIFY THE ALTERNATOR VOLTAGE OUTPUT
(use of a 200 amp alternator and multiple good electrical grounds can prevent many mysterious intermittent ignition issues)
DON,T IGNORE OR SKIP READING LINKS AND SUB LINKS
VERIFY EXHAUST BACK PRESSURE
VERIFY IGNITION WIRE RESISTANCE OHMS
VERIFY TIMING CURVE
GET AN INFRARED THERMOMETER FOR TUNING
Ive found its a whole lot faster to use a quality IR temp gun, to locate a individual cylinder that's mis-firing as it tends to run significantly cooler than adjacent cylinders, or hotter if its a vacuum leak at times, as lean F/A mixes tend to run hotter
(Ive used this one for years)

infrared thermometers are a very useful tool to track down issues with tuning, or mal functioning sensors , without verified facts your guessing.
this is the most consistently accurate I.R temp gun I've used for testing[/img]

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

http://www.professionalequipment.com/ex ... ermometer/
Wide temperature range from -58 to 1832Â°F (-50 to 1000Â°C)

http://garage.grumpysperformance.com/index.php?threads/spark-plug-info.202/

http://garage.grumpysperformance.com/index.php?threads/edelbrock-carb-rebuild-related.15406/

http://garage.grumpysperformance.com/index.php?threads/verifying-your-real-advance-curve.4683/

http://garage.grumpysperformance.com/index.php?threads/reading-plugs.5428/

http://garage.grumpysperformance.co...conductive-grease-for-ignition-modules.10748/

http://garage.grumpysperformance.com/index.php?threads/why-use-anti-seize-paste.9424/

http://garage.grumpysperformance.co...ition-wires-getting-the-header-clearance.840/

http://garage.grumpysperformance.com/index.php?threads/is-backpressure-hurting-your-combo.495/

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

http://www.efisystempro.com/efi-pro-hangout/building-twin-4bbl-throttle-body-engine

http://fitechefi.com/products/30061/

http://www.superchevy.com/how-to/en...-into-the-latest-efi-systems-on-market-today/

http://www.hotrod.com/articles/car-crafts-giant-efi-test/

http://www.hotrod.com/articles/self-tuning-efi-systems/

http://www.enginelabs.com/news/entry-level-self-tuning-efi-showdown/

http://www.youngsperformance.com/

http://www.bgsoflex.com/megasquirt.html

http://members.ozemail.com.au/~davodos/Megasquirt.html

http://www.glensgarage.com/catalog/index.php

http://megatunix.sourceforge.net/

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

mitchk · Dec 9, 2015

well that certainly gives you a base to start from,
and having seen Grumpy work on my 1974 BBC 454 corvette several times,
I know he has a good experienced grasp on whats required!

dual quad

alex

Member

Grumpy

The Grumpy Grease Monkey mechanical engineer.

Grumpy

The Grumpy Grease Monkey mechanical engineer.

alex

Member

Grumpy

The Grumpy Grease Monkey mechanical engineer.

mitchk

Member