Thread: What Size Carb To Use

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Son has 72 chevelle, 327 30 over samll 268 cam. I was thinking about putting
a 500 edelbroc carb on it. Is this too small?

Here's a handy-dandy formula for sizing carbs - plagerized by me from other sources:
"One general rule of thumb uses a formula to determine the CFM requirements of your engine. It goes like this: You need to know the CUBIC INCHES of the motor. You also need the maximum RPMs the motor will be spun to. Finally you also need the VOLUMETRIC EFFICIENCY PERCENTAGE (VE%) of the engine. The first two items (CUBIC INCHES and RPMs), are relatively easy to determine. The engine VE% is another matter. If an engine could use all of the air it ingested, it would have a VE% of 100%. Many performance engines reach this level. Certain race engines can actually exceed this and reach a VE% of over 100% at certain points in their RPM range. Most production engines and most street performance engines have VE levels below 100%. In fact, stock, production, low performance motors will fall around 75%-85% volumetric efficiency. "

The math formula is:

CARB CFM = CID x RPM/3456 x VE

Therefore: 331x6000/3456 x .90 = 517CFM (or rounded up, a 550/600)

The 3456 is a constant - but darned if I know from where

The 3456 is a constant - but darned if I know from where

Man I had to think about that one for a while.

one cubic foot = 12x12x12 = 1728 cubic inches per cubic foot.

One would think that is the conversion factor needed but -

At any given rpm only 1/2 the displacement is actually intake flow.
so you have to divide the total flow by 2 before converting CIM to CFM.

therefore -

CIDxRPM/(12x12x12)2 = CIDxRPM/3456

Originally Posted by rdobbs

Son has 72 chevelle, 327 30 over samll 268 cam. I was thinking about putting
a 500 edelbroc carb on it. Is this too small?

All depends on what you want the motor to do. I'll pass on Denny's own formula for maximum power, cubic inches times 2.

If you're just looking for a good cruise carb, follow the formulas given above.

ussually my formula for a good cruise with a little get go is the closest size down from dennys power method... say a 351x2 is 702 so i'd run a 650... thats ussually a good way to get something that'll cruise well tune in easily but still pick-up when you want it to.... if you went with a 700 on that formula, it'd be a lil more work to dial it in, but it'd run at top RPMs a lot better

Originally Posted by thesals

.... if you went with a 700 on that formula, it'd be a lil more work to dial it in, but it'd run at top RPMs a lot better

This is fine - if you have a car that runs only on weekends, and a quarter mile at a time. Most folks run their car to work, to the store with stop and go driving. This means compromise for driveability, fuel mileage and just plain driving enjoyment. A mild 331 CID Chevy(or any other engine) with a flush toilet size carb is NOT fun to drive. Bigger is not always better. Been there - done that with 750/780CFM 3310s and 950 CFM three barrels on BB Fords to name a couple of toilet sized carbs

Formula?

I just look in the carburetor cupbard and pick one that looks like it'll do the job....

Originally Posted by C9x

Formula?

I just look in the carburetor cupbard and pick one that looks like it'll do the job....

Sugar coated, granola, fruit flavored, artificially colored and flavored or.....

Originally Posted by Irelands child

Here's a handy-dandy formula for sizing carbs - plagerized by me from other sources:
"One general rule of thumb uses a formula to determine the CFM requirements of your engine. It goes like this: You need to know the CUBIC INCHES of the motor. You also need the maximum RPMs the motor will be spun to. Finally you also need the VOLUMETRIC EFFICIENCY PERCENTAGE (VE%) of the engine. The first two items (CUBIC INCHES and RPMs), are relatively easy to determine. The engine VE% is another matter. If an engine could use all of the air it ingested, it would have a VE% of 100%. Many performance engines reach this level. Certain race engines can actually exceed this and reach a VE% of over 100% at certain points in their RPM range. Most production engines and most street performance engines have VE levels below 100%. In fact, stock, production, low performance motors will fall around 75%-85% volumetric efficiency. "

The math formula is:

CARB CFM = CID x RPM/3456 x VE

Therefore: 331x6000/3456 x .90 = 517CFM (or rounded up, a 550/600)

The 3456 is a constant - but darned if I know from where

Ding,ding,ding we have a winner.
In other words a 500 cfm carb will work great.

My opinion (humble as it is ) would say that a 1406 Edelbrock 600cfm electric choke with tuning kit work would be optimum with a 500cfm part 1403 or 4 as a good second choice for all around driving. When I recall that my old F350/460CID had an approximate 500 cfm Holley on it, ran pretty good.

Originally Posted by DennyW

Yes, the 500 will work fine, but, he hasn't commented on what he really plans, or how he wants to drive it. With not knowing, I figure it as what if ?

Vacuum secondaries, and that 4 barrel will still be very good, unless he puts his foot in it. I wouldn't want to see him go lean on the top end. I'm just talking ingeneral here guys. I like to make sure I have that extra punch if I need it. I run a little 390 cfm on my 2.8. Get good mileage, but have the punch when I need it. And the engine can only draw so much depending on cubic inches anyway.

Denny,he won't go lean he will tend to go rich or the engine will go flat on hp and torque and fall off if it runs out of air.

Car will be a daily driver, no racing..rdobbs

Have to use the formula with the RPMS if your building it like that "302" thread LOL. What was it? 12,000 rpm 302 cid? how many cfms would that be?

cubic inches x max rpm / 3456

Originally Posted by DADNOVA

cubic inches x max rpm / 3456

Yup....That formula has worked for many years, bet it still does, huh????