Holley accelerator pumps/cams


Staff member
heres a needle and seat fuel valve

Here's a picture, Double Pumper on the left, Vacuum secondary on the right... Note on the upper left of each carb, the DP has linkage attached to the base plate/throttle shaft that actuates the secondary pump lever... The Vacuum carb has the large vacuum pod on the upper left side of the carb...


if your running great but seeing a momentary bog as the throttles snapped from idle to wot, I think youll find its a momentary lean condition, your going to easily fix that momentary hic-cup, just swap to a richer cam on the acceleration pump and or a #37 squirt-er , or you may need the 50cc accelerator pump upgrade should make the response damn near instantaneous, during the throttle transition stage
be aware the diaphragm must be the newer off green/blue version if your local fuel contains ethanol, as the older black diaphragm in the acceleration pumps eventually degrade and leak badly (FIRE HAZARD)


If you get a momentary bog in first gear on acceleration, the sudden increase in inertial load on the fuel pump , due to the weight of the fuel its pushing slamming back into the fuel pump, might stall some electric fuel pumps for a couple seconds , or it may be a problem with the carbs acceleration fuel pump not covering the fuel demands , allowing a momentary lean-out in the fuel/air ratio.
swapping to a BLUE Holley Accelerator Pump Cam, (and maybe a 500 cc accelerator pump vs a stock 35cc pump) might help


http://www.youtube.com/watch?v=k-Nx5HEz ... _embedded#!

http://www.youtube.com/watch?feature=pl ... -Nx5HEzvlY

http://www.youtube.com/watch?v=eiRIfcrc ... _embedded#



http://www.stockcarracing.com/techartic ... index.html


http://www.quickfueltechnology.com/tech ... -continued







http://store.summitracing.com/partdetai ... toview=sku

http://store.summitracing.com/partdetai ... toview=sku
http://store.summitracing.com/partdetai ... toview=sku
http://store.summitracing.com/partdetai ... toview=sku




Holley Accelerator Pump Cam Chart (above)


Holley Accelerator Pump Cam Chart , INDEPENDENT TESTING DONE, notice its NOT an exact match to what HOLLEY tech guys posted
don,t forget the accelerator pump itself can be upgraded from 35cc to a 50cc version on many holley carbs

http://www.stockcarracing.com/techartic ... index.html

http://www.holley.com/types/Accelerator ... 0Parts.asp

http://www.holley.com/types/Accelerator ... 0Parts.asp

This chart gives the pump shot volume in CCs for each cam in each position.

Position 1 Position 2
White 17 19.5
Blue 18 20
Red 18.5 20
Orange 19 24
Black 19 18
Green 24 30
Pink 30 37.5
Brown 36
http://store.summitracing.com/partdetai ... toview=sku

don,t forget the squirters or Accelerator Pump Discharge Nozzles
They come in 3 different types. Straight - anti pullover - and tube types. and different sizes



anti pullover


tube types.


the accelerator pumps shot should hit the lower edge of the booster nozzle for better atomization, so the squirter you've selected should in part depend on the type of booster, accelerator pump volume and the distance from squirter to that booster, the type of squirter used should also depend on the pump volume and carburetor vacuum readings as exceptionally high plenum vacuum can draw fuel from the squirters. the anti pull over design is just needed for special applications like using a 2 barrel or a small 4 barrel on a larger displacement V-8 where the plenum vacuum is strong enough at WOT suck fuel from the squirter , occasionally causing a over rich condition



http://store.summitracing.com/egnsearch ... toview=sku

http://store.summitracing.com/egnsearch ... toview=sku

http://holley.com/data/TechService/Tech ... 20Info.pdf

http://www.automedia.com/Holley_Carbure ... 000718hm/1

http://www.nationaltbucketalliance.com/ ... /carbs.asp

You generally select a power valve thats 1/2 the vacuum at idle , example if your pulling 17" at idle you select a 8.5-or 9.5 power valve, if your pulling 12" you select a 5.5 -6.5 power valve, your running about 5" so youll want a 2.5 -3.5 power valve AS A START POINT THEN TUNE UP OR DOWN IN F?A RATIO AS REQUIRED READING PLUGS OR USING A F/A RATIO METER ON THE EXHAUST


yes its a P.I.T.A. but reading thru the links holds a lot of info you'll miss if you don,t take the time
this is a power valve its screwed into a metering plate between the carb fuel bowl and the carb body in many carbs, it restricts
additional fuel flow until the vacuum drops to a known level stamped on the power valve rim,

keep in mind the HIGHER the number the earlier in the transition process from idle to full throttle, the power valve allows fuel to flow

The power enrichment system supplies additional fuel to the
main system during heavy load or full power situations.
carburetors utilize a vacuum operated power enrichment system
and a selection of power valves is available to “time” this sys-
tem’s operation to your specific needs. Each Holley power
valve is stamped with a number to indicate the vacuum opening
point. For example, the number “65” indicates that the power
valve will open when the engine vacuum drops to 6.5" or
below. An accurate vacuum gauge, such as Holley P/N
should be used when determining the correct power
valve to use. A
competition or race engine which has a long
duration high overlap camshaft will have low manifold vacuum
at idle speeds. If the vehicle has a manual transmission, take
the vacuum reading with the engine thoroughly warmed up and
at idle. If the vehicle is equipped with an automatic transmis-
sion, take the vacuum reading with the engine thoroughly
warmed up and idling in gear. In either case, the power valve
selected should be 1/2 the intake manifold vacuum reading
taken. EXAMPLE: 13” Hg vacuum reading divided by 2 = 6.5
power valve. If your reading divided by 2 lands on an even
number you should select the next lowest power valve. EXAM-
PLE: 8” Hg vacuum reading divided by 2 = 4 power valve.
Since there is no #4 power valve you should use a 3.5.
Most of the popular Holley “Street Legal” and “Street
Performance” carburetors incorporate a power valve blow-out
protection system. A special check valve is located in the throt-
tle body expressly for this purpose. This check valve is
designed to be normally open but will quickly seat to close off
the internal vacuum passage when a backfire occurs. Once
closed, the check valve interrupts the pressure wave caused by
the backfire, thus protecting the power valve.
If you have a carburetor older than 1992 (or you have experi-
enced an extreme backfire) and expect a blown power valve,
use this simple test. TEST: At idle turn your idle mixture screws
(found on the side of the metering block) all the way in. If your
engine dies the power valve is not blown

keep in mind theres lots of small passages that can get clogged and theres several different fuel bowl gaskets, these vary by manufacturer and carb model, so be sure you use the correct gasket on the meter block and fuel bowl, if you leave debris in the fuel bowl or use the wrong gaskets they can block passages and the carb won,t function




keep in mind a few basic concepts, you can select a 30cc or 500 cc accelerator pump volume ,
the accelerator pump cam will control how fast the carbs linkager starts to cover the transition between idle and fully open venturies when the engine could go lean, remember that a larger squirter size does not give you more total fuel, it only allows more volume to move through the squirter faster, but this only tends to result in a shorter duration squirt, of fuel befor and during the transition,as the power valves and secondary jetting add fuel and air flow, THE Only way to truly know how the combo of power valve,booster type, accelerator pump cam ,squirter, accelerator pump size and carb jets will work on your combo, is to try a couple different combos and see how the 60' times and the engine feels as you accelerate, and both experience and knowledge of how to read spark plugs and change ignition advance curves will help a great deal.






If your tuning a holley carb having the 50cc accelerator pump kit and a selection of accelerator pump cams, a set of jets, squirters,and power valves in your tool box and a decent vacuum gauge, to tune with is a big help at times
Ive generally found the BLUE ACCELERATOR PUMP cam to be the best choice in most applications once the jets and power valves are correct, but obviously each application will need to be carefully tested

bits of reference material.
Id try to get as close to the basic listed parameters below as a start point as you can, and the slight hesitation on acceleration might require a larger accelerator pump cam curve , or a power valve that kicks in at a bit higher vacuum level, example swapping from a 7.5 to and 8.5, make only a single change at a time and keep good detailed notes on results, and don,t discard parts, label them, keep in zip lock bags with labels
yeah I might not be up on the latest electronic data loggers but I have a good idea what an engine wants as far as a fuel/air ratio and combustion conditions and ignition advance curves and can read spark plugs, and while each engines a bit different you may be amazed at what consistent condition to start from does to help isolate any tuning problems



set the float levels and verify the fuel pressure consistent at about 5 psi entering the carb inlet port.
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
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
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
this is only a 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


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

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Staff member
http://www.summitracing.com/parts/hly-135-7/overview/ 50CC-GAS


http://www.summitracing.com/parts/hly-135-9/overview/ 50CC-GAS ALCOHOL


blue accelerator pump diaphragms are alcohol tolerant
they come in 30 cc and 50cc sizes
obviously you need to select the size you require
https://www.speedwaymotors.com/Holley-135-10-30cc-Accelerator-Pump-Diaphragm-Alcohol,9043.html?sku=42713510&msclkid=abed1e09704516c905e17e6b4bff9f84&utm_source=bing&utm_medium=cpc&utm_campaign=SMI - Shopping (CSE) (Bing)&utm_term=4577404347376845&utm_content=All Products (Feb28_2020)

http://www.summitracing.com/parts/hly-135-5/overview/ 30CC-GAS

http://www.summitracing.com/parts/hly-135-10/overview/ 30CC-GAS ALCOHOL

glad to help when AND WHERE I CAN.
blue diaphragms are alcohol tolerant
they come in 30 cc and 50cc sizes
obviously you need to select the size you require
https://www.speedwaymotors.com/Holley-135-10-30cc-Accelerator-Pump-Diaphragm-Alcohol,9043.html?sku=42713510&msclkid=abed1e09704516c905e17e6b4bff9f84&utm_source=bing&utm_medium=cpc&utm_campaign=SMI - Shopping (CSE) (Bing)&utm_term=4577404347376845&utm_content=All Products (Feb28_2020)

Q: I can’t get the “Bog” out of my mechanical secondary carburetor what’s wrong and how can I fix it?

The sole purpose of the accelerator pump system is to provide a combustible air/fuel ratio during hard acceleration, especially from idle, for the time it takes the main metering system, via the booster venturi, to respond to the increase in air velocity. When you mash the throttle opening the throttle plates, air responds instantaneously to the pressure drop (vacuum) generated by the engine through the intake manifold. Fuel on the other hand is too heavy and cannot respond as the air does. Assisting the engine during this transition carburetor engineers developed a hydraulic fuel delivery system to push fuel into the slug of incoming air to make a combustible mixture.

The accelerator pump system is simply a mechanical fuel injector that provides enough fuel for a brief period of time for the engine to respond smoothly to the heavy wide open throttle demand. Its purpose is oh so simple and its function is oh so critical. And, when it isn’t working right, the dreaded “Bog” and the always dreaded backfire through the carburetor are effects of an accelerator pump system that isn’t equal to the task. The “Bog” simply means the engine is not receiving enough fuel when you go to wide open throttle. While there are other issues that may cause similar reactions within the carburetor, often the problem can be traced back to the accelerator pump system.

Incorrect adjustment is the typical culprit. The accelerator pump function, as mentioned, is based on hydraulic principles. Therefore, it is essential that the accelerator pump always has tension at your normal idle position. Any air gap here will delay the actuation of the accelerator pump system allowing air to rush into the engine without fuel.

The common misconception is that it is necessary to adjust the pump arm to obtain an air gap. This is correct; however that air gap is only at Wide Open Throttle (W.O.T.) not at idle. The air gap is simply to ensure additional travel in the pump arm so it does not bottom out on the fuel bowl. If it does bottom out eventually it could loosen the accelerator pump housing causing a fuel leak. Fuel leaks are something we all want to avoid.

Assuming the first adjustment is correct. Then the next step in the process is to add more fuel to the incoming air which is accomplished by increasing the size of the pump discharge nozzle or squirter. This will quite often correct the “Bog” or hesitation by richening the air/fuel mixture when the pump system is activated. If the carburetor works well everywhere else on the track then a bigger squirter might be the only change necessary.

Pump squirters are available in a wide range of sizes, the bigger the number (size) the more fuel when you accelerate. Simply remove the pump nozzle screw, lift off the old nozzle and install the new. Be sure both gaskets above and below the nozzle are replaced. If the gaskets are a different thickness the thicker goes under the nozzle and thinner one on top.

Q. How do I know if I need a bigger pump, i.e. a 50cc versus a 30cc?

A larger capacity accelerator pump achieves one thing, it extends the duration of the pump shot. A larger accelerator pump is usually required when the carburetor, intake manifold and/or cylinder head is too large for the engine and the resulting weaker signal at low engine RPMs requires more time for the booster venturi to pull fuel through the main jet. Generally, the telltale sign is good initial throttle response, but shortly thereafter the engine bogs down or flat lines and as the RPM climbs it recovers accelerating normally. That bog or flat spot is where the pump shot duration ran out and the booster venturi fuel flow alone is not enough to keep the engine running efficiently. In this case, the larger pump capacity helps the transition. While this is a more popular change among foot brake cars, the 50cc is also sometimes a necessity with large plenum 2 X 4 tunnel ram applications especially when racing on a .400 Pro Tree.

By the way for you curious types, the 30 and 50cc capacity is the volume of fuel displaced from the accelerator pump in 10 complete strokes of the pump arm. So the actual capacity is 1/10 that number or roughly 3 and 5cc respectively when opening the throttle from idle to wide open.

The larger capacity 50cc pump is easily recognizable by the large pump arm with the encapsulated spring. In addition, the 50cc pump also uses a larger pump cam either a brown color (conventional carbs) or a yellow color (model 4500).

A 50cc pump will not deliver the 50cc rated volume if any other cam is installed.


these pictures may be helpful

















http://www.4secondsflat.com/Carburetor Sizing and Series Charts.htm

http://www.amazon.com/Holley-Carburetor ... lley+carbs









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

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

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





IF you can read spark plugs , USE A TIMING LIGHT, and use an INFRARED TEMP GUN AND VACUUM GAUGE
http://www.harborfreight.com/cpi/ctaf/d ... mber=93547

http://www.harborfreight.com/5-in-1-dig ... 98674.html




infrared thermometers are a very useful tool to track down issues with tuning, or mal functioning sensors


IF you can read spark plugs , USE A TIMING LIGHT, and use an INFRARED TEMP GUN AND VACUUM GAUGE







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The Grumpy Grease Monkey mechanical engineer.
Staff member

speedtigger said:
The accelerator pumps are an interesting topic. If you look through the carb database, you will see a huge range of setups. It made me think about the factors that contribute to accelerator pump needs for a given engine.

Here are my thoughts:

Displacement to intake volume ratio:
A relatively large engine with a small intake tract volume will not suffer as much drop in velocity through the carburetors venturis as a small engine with a large intake tract. Therefor the larger engine will not need as large or as long of a duration of accelerator pump enrichment because the drop in velocity through the carburetors venturi will be less. This means the boosters will be able to supply a more consistent fuel supply negating the need for alternative enrichment. A smaller engine with a larger intake tract such as high volume ported heads and an intakemanifold with large ports will suffer a greater velocity drop through the carbs venturis creating the need for more alternative enrichment.

Carburetor size:
Larger carb venturis suffer greater velocity drops at low RPM WOT in a given engine. Therefor the signal suffers and more alternative enrichment is needed. This is why General Motors adopted the spread bore Rochester Quadrajet carburetor. This carburetor incorporated very small primary venturis and very large secondary venturis. This allowed for excellent low speed metering.

Camshaft timing/duration:
Longer period (read larger) duration camshafts are less efficient at low RPM which decreases their signal to the carburetor at low RPM. This reduces velocity through the booster resulting in the need for alternative enrichment.

Stall converter and/or gearing:
Vehicles with low stall converters and/or stick shifts with low numerical final drive ratios will have a longer dwell time in the lower RPMs when launched at low speeds with full throttle. Conversely, a vehicle with high stall converter and a high numerical final drive ratio will not dwell for much time at all at low RPMs when going to full throttle at low speeds. It is in these low RPM situations where velocity through the carburetor is low and alternative enrichment is needed.

Weight of vehicle:
Heavy vehicles take longer to get rolling, thereby longer to get into the higher RPMs where the signal to the carburetor is stronger. This means that, in general, heavier vehicles will require alternate enrichment for a longer duration of time.

If you take the above into consideration, it become quite obvious why my big heavy Buick with ported heads, ported intake, 950 Holley and a relatively low stall for my camshaft car would want more accelerator pump size and duration than your light weight, near stock engine Datsun.

I also want to point out something about the 50cc pump for people considering the upgrade. I have seen a lot of guys install this pump and never change their pump actuator cam on the throttle shaft. 50cc pumps use much bigger cams with taller ramps. If you do not change this pump cam, adding a larger volume pump will do nothing.








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Question for you today Grumpy.
When Verifying you have the correct power valve # , do you check engine vacuum with the car in park (Auto Trans Turbo 400) or do you check in Drive with foot on brake ?
It seems to me checking in Drive with engine idling and foot on the brake is most accurate.
The 1963 Grand Prix has the Factory console mounted vacuum gauge and it works & accurate.
I see 16 -18 inches with engine idling in park typical.
In drive 13-14 inches.
When its real humid out & Air temps are 90-95 F Engine vacuum is lower slight I have noticed.
When its cool out at night & DA is much better engine vacuum picks up slight.
I can feel Torque & HP increases, exactly whats supposed to happen in theory when DA is better.

Spark plugs have nice burn tan.
Slight bit rich but OK.
No exhaust yet, open manifolds still.
2,000 miles now no exhaust.
I ride right by 5.0 & they don't care here. Careful to just idle by & not romp on it hard & blow their ear drums out.

Slight stumble off idle sometimes when Hot punching it hard & fast from a dead stop.
Think I need to re calibrate the accelerate pump cam. Maybe a 50 cc pump kit. Read you must use the BROWN CAM WITH IT.
Maybe change pump shot nozzle size too.

Remember I am running a Vertex OAC magneto on the Olds 425.
I am happy with current mechanical advance in it.
11 degrees & 22 actual at the crankshaft.
Starts up immediate every time.
Overall drive abilty is very good.

7 psi on the Autometer fuel pressure gauge
I have it mounted next to my oil pressure gauge & watch it constant.

When DA is good. no stumble ever.


Looks like the Holley 6.5 power valve is perfect match for my engine .


The Grumpy Grease Monkey mechanical engineer.
Staff member
Brian, your more than skilled and experienced enough to know that your going to be a bit more consistent, taking the reading
" in Drive with engine idling and foot on the brake"
is how I,ve always found the at idle vacuum, reading I use but I doubt how you find the start point for the power valve reading is notably less important that your ability to read the plugs, and make the changes required as you see are required.

and get a feel for how the engine responds to the changes in jetting,
float levels (if required)
ignition advance curves
,accelerator pump cams,
squirters, venturi booster types
,vacuum springs
and fuel pressure changes,
you,ll use to get the engine responding as you intend it too run.
one commonly over looked issue is exhaust back pressure , not one guy in ten seems to bother testing,
that change in exhaust back-pressure at every 1000 rpm under full throttle launches and runs up through the gears,
and you can waste a great deal of time scratching your head wondering why the engine won,t respond the way you expect it too, if the exhaust is restricting exhaust gas,flow






I will look at today's Links you provided Grumpy.

In this case I definite 100% positive there is no back pressure issue.
Open 425 Starfire exhaust manifolds I am running.
The Boy Nicky gets scared.
Thee Wifee' says its too Loud.
I like it Loud.
Reminds me of Dirt Track Racing...No Mufflers.

I am seeing back pressure issues doing Tech on Max myself.

The Company Torque Tech I bought the True 3.00" inch Mandrel bent exhaust that is still on my 1970-1/2 TA RAIV system is gone.
They were from Valodossa Georgia.
Near everyone has 2-1/2" on their Pontiacs.

To get 3.00, 3-1/2, 4.00, & 5.00" inch duals you must fabricate all from scratch on your own.
And the custom mandrel benders you & I knew have become super rare.


OK Grumpy.
Losing Flow Velocity when I need it running Open exhaust manifolds on the Olds 425 engine.
Its amazing how well it runs now as is.

Have to earn extra $ to build fabricate the one off exhaust system for the 63 Grand Prix.



I found this info posted else ware but its good info
Back pressure: The myth and why it's wrong.

I. Introduction

One of the most misunderstood concepts in exhaust theory is backpressure. People love to talk about backpressure on message boards with no real understanding of what it is and what it's consequences are. I'm sure many of you have heard or read the phrase "Hondas need backpressure" when discussing exhaust upgrades. That phrase is in fact completely inaccurate and a wholly misguided notion.

II. Some basic exhaust theory

Your exhaust system is designed to evacuate gases from the combustion chamber quickly and efficently. Exhaust gases are not produced in a smooth stream; exhaust gases originate in pulses. A 4 cylinder motor will have 4 distinct pulses per complete engine cycle, a 6 cylinder has 6 pules and so on. The more pulses that are produced, the more continuous the exhaust flow. Backpressure can be loosely defined as the resistance to positive flow - in this case, the resistance to positive flow of the exhaust stream.

III. Backpressure and velocity

Some people operate under the misguided notion that wider pipes are more effective at clearing the combustion chamber than narrower pipes. It's not hard to see how this misconception is appealing - wider pipes have the capability to flow more than narrower pipes. So if they have the ability to flow more, why isn't "wider is better" a good rule of thumb for exhaust upgrading? In a word - VELOCITY. I'm sure that all of you have at one time used a garden hose w/o a spray nozzle on it. If you let the water just run unrestricted out of the house it flows at a rather slow rate. However, if you take your finger and cover part of the opening, the water will flow out at a much much faster rate.

The astute exhaust designer knows that you must balance flow capacity with velocity. You want the exhaust gases to exit the chamber and speed along at the highest velocity possible - you want a FAST exhaust stream. If you have two exhaust pulses of equal volume, one in a 2" pipe and one in a 3" pipe, the pulse in the 2" pipe will be traveling considerably FASTER than the pulse in the 3" pipe. While it is true that the narrower the pipe, the higher the velocity of the exiting gases, you want make sure the pipe is wide enough so that there is as little backpressure as possible while maintaining suitable exhaust gas velocity. Backpressure in it's most extreme form can lead to reversion of the exhaust stream - that is to say the exhaust flows backwards, which is not good. The trick is to have a pipe that that is as narrow as possible while having as close to zero backpressure as possible at the RPM range you want your power band to be located at. Exhaust pipe diameters are best suited to a particular RPM range. A smaller pipe diameter will produce higher exhaust velocities at a lower RPM but create unacceptably high amounts of backpressure at high rpm. Thus if your powerband is located 2-3000 RPM you'd want a narrower pipe than if your powerband is located at 8-9000RPM.

Many engineers try to work around the RPM specific nature of pipe diameters by using setups that are capable of creating a similar effect as a change in pipe diameter on the fly. The most advanced is Ferrari's which consists of two exhaust paths after the header - at low RPM only one path is open to maintain exhaust velocity, but as RPM climbs and exhaust volume increases, the second path is opened to curb backpressure - since there is greater exhaust volume there is no loss in flow velocity. BMW and Nissan use a simpler and less effective method - there is a single exhaust path to the muffler; the muffler has two paths; one path is closed at low RPM but both are open at high RPM.

IV. So how did this myth come to be?

I often wonder how the myth "Hondas need backpressure" came to be. Mostly I believe it is a misunderstanding of what is going on with the exhaust stream as pipe diameters change. For instance, someone with a civic decides he's going to uprade his exhaust with a 3" diameter piping. Once it's installed the owner notices that he seems to have lost a good bit of power throughout the powerband. He makes the connections in the following manner: "My wider exhaust eliminated all backpressure but I lost power, therefore the motor must need some backpressure in order to make power." What he did not realize is that he killed off all his flow velocity by using such a ridiculously wide pipe. It would have been possible for him to achieve close to zero backpressure with a much narrower pipe - in that way he would not have lost all his flow velocity.

V. So why is exhaust velocity so important?

The faster an exhaust pulse moves, the better it can scavenge out all of the spent gasses during valve overlap. The guiding principles of exhaust pulse scavenging are a bit beyond the scope of this doc but the general idea is a fast moving pulse creates a low pressure area behind it. This low pressure area acts as a vacuum and draws along the air behind it. A similar example would be a vehicle traveling at a high rate of speed on a dusty road. There is a low pressure area immediately behind the moving vehicle - dust particles get sucked into this low pressure area causing it to collect on the back of the vehicle. This effect is most noticeable on vans and hatchbacks which tend to create large trailing low pressure areas - giving rise to the numerous "wash me please" messages written in the thickly collected dust on the rear door(s).

VI. Conclusion.

SO it turns out that Hondas don't need backpressure, they need as high a flow velocity as possible with as little backpressure as possible.

Turbo Exhaust Systems:

Some of you asked for a better explanation about restriction in a turbo exhaust, so here 'goes. To sum it all up, on a turbo car, the tighter and more restricted the exhaust housing of the turbo is, the faster you're going to spool your turbo... because the restricted gasses escape through the exhaust housing with more velocity (much like the garden hose description quoted above)... but with this restriction comes the downside. Less exhaust volume will be able to fit through that turbine housing once the turbo is spooled and starts squishing more air through the intake.

This is where the wastegate comes into play. The wastegate is actuated BEFORE the exhaust wheel in the exhaust housing. When it opens, whatever the diameter of your wastegate is gets added to your exhaust piping. Effectively increasing the capacity of the exhaust provided that everything downstream is large enough in diameter to handle it the extra flow. The main reason it does this is to prevent over-spooling the turbo. Once the turbo reaches its efficiency, it doesn't need to flow all the extra gasses through the turbine wheel, so the wastegate allows you to route the exhaust around the turbo... if it can't route enough exhaust around the turbo (restricted wastegate) then too much exhaust gas will be forced by the exhaust wheel and BOOST CREEP will occur as your turbo over-spools.

One way to prevent boost creep is to port the wastegate housing (if you have an internally gated turbo) or to replace the wastegate with a bigger unit. If that still doesn't work, then the problem is likely going to be a restriction in the exhaust downstream from the wastegate. Many overboosted car owners prefer to vent their wastegate dumps to the atmosphere. Not only is it illegal to bypass your catalytic converter, but it's loud as hell, gets your engine bay filthy, and clogs up your K&N like nobody's business; however, it nearly eliminates boost creep. It's a cheap and easy solution that fixes boost creep on a race car.

The best solution to boost creep is to route your wastegate dump past the catalytic converter and back into the exhaust. It will be a custom setup. Nobody makes this. Make it look clean and you'll pass emissions because they don't run your car hard enough to open the wastegate when doing emissions testing. You have to reach full boost for the wastegate to open, and since the exhaust runs through the cat until the wastegate opens, it all gets "cleaned" before it reaches the sniffer. With this setup, the wastegate will also be much quieter because it still runs through the muffler, and you won't trash your engine bay with black caustic funk.

Putting a separate catalytic converter on your wastegate dump is a stupid idea because you'll never get it hot enough to "light off" and start converting the carbon monoxide... so don't get any crazy ideas and create unnecessary exhaust restrictions.

Once compressed air comes into the mix with an engine, exhaust tuning has much less to do with making power. So what if you can milk another 3-5 hp out of a car with a tuned exhaust... the benefits of making an engine sustain an insane final compression ratio (boosted air x compression in the combustion chamber) has much more affect on making power if you can just get rid of the extra gasses it produces. Bigger is almost always better on a turbo setup. The only place where it isn't good is on the exhaust wheel where too big can = no chance of spooling your oversized turbo any time this year.

http://www.uucmotorwerks.com/html_produ ... uemyth.htm

ITS NOT RARE TO LOOSE 20% or more of your potential hp to a restricted cat on your exhaust, or restrictive mufflers or exhaust pipes that are too small and restrictive, no header can effectively scavenge your engine unless its got less than about 2 psi of back pressure at the peak rpm range, on most muscle cars youll want a 2.5"-3" diam. exhaust system with an (X) pipe as close to the header collectors as clearances under the car allow as a MINIMUM size exhaust and its been my experience that is RARE for a 3" dual exhaust with an (X) pipe to NOT allow a hot combo with over 400hp to produce some mid and upper rpm power gains.
if the exhaust pipes too small in cross sectional area the pipe acts like a long rather restrictive collector on the headers, this may be good for low rpm cylinder scavenging, but it usually kills peak hp potential, its best to tune the engines intake and exhaust to match the gearing and cam timing power curve. in an ideal set up the exhaust system beyond the header collectors has a very low restriction to flow, and there will be an (X) pipe to effectively double the exhaust cross sectional area to significantly lower resistance to exhaust flow restriction still further, to increase the header,s ability to scavenge the cylinders efficiently in the intended power & rpm band, keep in mind your goal is or should be in most cases to maximize the torque in the intended rpm range that your engine combo produces
Id highly suggest adding one of these 3" (X) to most exhaust systems

looking thru an (X) pipe


Not sure if I can use an X- pipe layout easy on 63 GP.
It sits very low to the ground.
All 63 Grand Prixs do.
Part of exhaust with X may have to be Nascar style oval tubing.
There is a guy on Fleabay that specializes in Pontiacs from Chicago area.
I think its Ram Air Restorations.
Seen Oval 3-1/2 & 4.00 ".



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Details about 67-81 Camaro & Trans Am 3.5" Oval Exhaust System

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Made for my Trans Am Grumpy.
Pretty expensive.

Something to look at & get ideas to Fab up my own .


Had to run an errand a bit ago & took the 63 Grand Prix Grumpy.
Cooler out today & lower humidity.
About 73F.
Felt the Torque & HP increase.
Obliviated all on the highway. Passed 100-110 like they were standing still.

I think maybe the fuel is heating up before entering the carburetor fuel bowls also.
Be best to run stainless braided Earls fuel feed line & Race AN fittings.
Nice aluminum BLP or AED Holley double pumper fuel log style.
Current Holley chrome D.P. Fuel carb lines too close to the T-stat housing & I can not bend enough to maintain a 3-4' inch air gap.

Have my Moroso Race Fuel Cool Can I can mount & fill with ice - water-denatured alcohol on real hot 90-100 F days here too.
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solid fixture here in the forum
The Holley Street Avenger 4150 series is very good.
The Emulsion circuits are dialed in nice for street performance cams.
The Accelerator pump circuit is lacking I have found when used on Big Blocks 425 cubes and bigger.
Do not waste your time with Accelerator pump cams.
Go right for the Holley 50 cc pump kit.
One cam supplied is for the 4500 Dominator. Set that aside.
The other is for 4150 series.
Install it.
Use a #37 squiter nozzel.
Olds 425s V8 wants a Huge Pump shot of Gasoline off idle.
Big Block Chevy will be the same.
When jetting is right Driveability will Rival modern EFI.
Tap the Gas at any speed and instant throttle response will be there.