[toobroketoretire]

Correctly Sizing A Carburetor For Street Use

I have a 1980 Corvette with an L48 engine and a TH350 transmission. I would like to install and Edelbrock Performer manifold (part no.2101) and an Edledbrock Perfomer 750 carburetor (part no. 1407). Will the factory accelerator linkage hook to this carbutetor or will modifications need to be made?

For starters you would be making a huge mistake to put a 750 cfm carburetor on an L48 because it is such a weak engine that operates at a very low rpm.

A 350" engine running at 100% volumetric efficiency can only swallow 608 cfm at 6000 rpm. About the only way you can achieve 100% volumetric efficiency is with a tunnel ram, a very long duration cam, and running open long-tube hedders at engine speeds of over 6000 rpm. Most street engines operate at about 75% volumetric efficiency but some high-performance engines can reach 80% to 85% with long duration camshafts, high-rise intake manifolds, and low-restriction exhausts. But even then the 80-85% efficiency comes at the 6000 to 6500 rpm levels; levels far above what your L48 operates at.

With it's "emission" camshaft and small-port cylinder heads your L48 will only make about 75% volumetric efficiency so you start by taking that 608 cfm and multiplying it by .75 which equals 456 cfm. Then as your L48 will only spin 5000 rpm for a very brief time in 1st and 2nd gears multiply that 456 cfm by 5/6 (or 83%) and that equals 378 cfm.

Sorry but the math doesn't lie as 378 cfm is the real-world amount your engine can swallow at it's fastest rpm. So I suggest buying the little 500 cfm version as that would still be big enough to handle a better cam if you ever decide to go that way.

As your L48 is street driven and has high rear end gears I would suggest the 500 cfm #1801 Edelbrock as it has an electric choke and is an "AVS" (Air Valve Secondary) so the secondaries will only open when the air demand is high enough for them to open. With an air valve secondary your engine can never bog under a full throttle as it operates under the same principal as the very successful Rochester QuadraJet. Which brings up a point and that is why do you want to replace your present Quadrajet when it is one of the very best carburetors ever made? With it's very small primaries and huge AVS secondaries the QuadraJet will work very well on the little 6-cylinders all the way up to the big 500" Cadillacs as it'll flow 800 cfm (if needed).

 

[toobroketoretire]

A Sobering Experience

When you're contemplating changing carburetors I suggest visiting Holley's website and follow their formula which asks you how big your engine is, what maximum rpm will it be operated at, and what volumetric efficiency it has. The results are very sobering to say the least as the recommended size ends up being a LOT smaller than you ever expected.

I'm running a 750 Holley (#3310-2) on my cammed and high-rise 454" and even then it's too big as it'll only swallow 750 cfm once it exceeds 5500 rpm; an rpm that my engine never reaches as it makes it's shifts at 4800-5000 rpm.

 

[toobroketoretire]

Doing The Math

With the duration and lobe centers of that cam, I think I would run a 750 cfm also.

I'll "do the math" for you unbelievers so you can see first hand what an engine requires:

1. First of all a 4 stroke V8 engine takes two complete revolutions to fire off all 8 cylinders

2. Assuming a 100% volumetric efficiency (which is impossible to reach in any street engine) a 350" engine will only swallow 175 cubic inches in one revolution

3. To calculate what a 350" engine will swallow at 6000 rpm multiply 175 X 6000 and get 1050000 cubic inches

4. Now divide that 1050000 cubic inches by 1728 (the number of cubic inches in a cubic foot) and get 607.63 cubic feet @ 6000 rpm

5. As 750 cfm is 1.233 times 607 cfm multiply 6000 rpm by 1.233 and get 7401 rpm. So in order to consume that 750 cfm you would have to spin your engine clear up to 7401 rpm or well beyond what your 350's valve train and connecting rods can stand

6. That's it guys. Unless you're actually spinning your corked-up (mufflers) 350" engine to 7401 rpm ON THE STREET a 750 cfm carburetor would be a gross overkill

This is why I never recommend anything larger than a 600 cfm for any 350" with a TH350 running on the street as the full-throttle shift points of a TH350 seldom exceed 5000 rpm...................and 5000 rpm only requires 506 cfm at a 100% volumetric efficiency. As street engines can only achieve about an 85% volumetric efficiency that 506 cfm falls to 430 cfm which would make a 500 cfm carburetor the very best choice. There is no point whatsoever to install a carburetor that is so big it's secondaries will barely open at a maximum load when a smaller carburetor will easily do the job and still have plenty of reserve capacity.

 

[toobroketoretire]

Carburetor Sizing

The famous 370hp LT-1's came with a 780 cfm #3310 Holley. Why so big? Well, the intended usage for an LT-1 (with it's manual 4-speed and 3.73 rear axle) was for drag or track racing where it could be run at 6500+ rpm.

So ask yourself this question. Just how fast am I REALLY going to spin my street engine? And be honest. About 5000 rpm? Maybe 5500 rpm? It makes a huge difference when selecting a carburetor for a street engine as street engines never spin as fast as drag or track engines because street engines require street-legal exhaust systems which reduces their volumetric efficiency by 15-20%. Which means their carburetors should be sized 15-20% smaller.

People often have the mistaken idea their engines are giant "vacuum cleaners" that are able to suck in enormous quantities of air like a jet engine. Uh uh. Far from it. So when selecting a carburetor select one based on the ACTUAL amount of air your engine will swallow; not based on what your buddy recommends because your buddy most likely doesn't have the math skills to calculate what your engine needs. And..........your buddy most likely got his information from another buddy who also doesn't know anything about selecting carburetors because he got his information from another buddy who doesn't know how.

With my 45 years of experience here's what I recommend: For cars with a stock rear end ratio figure on 500 cfm for the TH350's and 600 cfm for the 4-speeds.

 

[toobroketoretire]

and all that is absolutely correct Toobroke, but remember, carbs are rated at 1 in hg not 0 in hg and a dual plane intake does allow slightly larger carb and as you well know, a vacuum secondary carb will only flow what the engine can pull at WOT...the Q-jet is a 750cfm vac sec. and was used on millions of GM 350 s .....I agree a 600-670 cfm would be better choice but ,since he has the carb, use it...

As the size of the carburetor's primary bores increase with the cfm rating the primaries of a 750 cfm carburetor are almost as large as 2-barrels bores. Yes, an AVS carburetor will only deliver what the engine demands but the excessively large primary bores reduce fuel mileage. I myself would much rather use a smaller 4-barrel in which it's secondaries are almost fully open at maximum load rather than only 10% open.

When G.M. selected a carburetor for an engine they sized it for what the engine was intended for (not talking about the multi-purpose Quadrajet AVS style that can be used on virtually everything). So the 780 cfm Holleys only went onto the 370hp LT-1's and 375hp 396's that would see 6500 rpm or more.

Most people have the mistaken idea that their engine is somehow going to be able to use all the air that a large carburetor can deliver but they don't take into account their actual highest rpm nor their engine's volumetric efficiency.

When a 350" engine tops out at 5000 rpm and has a volumetric efficiency of 80% a 350" engine will only consume 405 cfm which is almost HALF of a 750 cfm carburetor. So what's the point of using a carburetor that can flow almost double what you really need?