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Discussion Starter · #1 ·
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?
 

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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).
 

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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.
 

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There are lots of opinions out there just wondering why you want to change intake and carb? Q jets are very good carbs but difficult for some folks to build and or tune you can get decent mileage and performance from them. CFM range from 600 to 870 or so, Holley is very easy to tune and set up and build they have very good performance but economy will be less than a similar sized q jet. Personally I dis like the edelbrocks/AFB, They can have outstanding performance but setup is harder than a holley and require much more attention on daily drivers. primary metering is held by vacuum on metering rods through a piston setup which can carbon up causing it to run rich at idle and low rpm. Or lean up high depending where the piston gets stuck. but there are folks who have great success with them. If you are not planning on upgrading cam, exhaust, maybe heads why change carb and intake? sit down and plan your mods, where do you want the vehicle to be/do. Daily driver, auto cross, street strip or all out race, then plan mods. Personally I feel I get more from a good exhaust on a stock or mild engine than carb and intake. Again just an opinion, what do the rest of yall think?
 

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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?
What carburetor do you have on the engine now?
 

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Discussion Starter · #9 ·
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).
My L48 has significant upgrades to the camshaft, heads, and rotating assembly. The company that performed these modifications dyno'd the engine with long tube headers, an Edelbrock Perfomer intake manifold and a 750 Edelbrock Perfomer carburetor. Long tube headers have already been installed. Future plans include Hooker headers and side pipes. The current carburetor is a 670Holley. Current manifold is stock. The camshaft has considerable duration and overlap. Since the cam has overlap, vacuum at idle is very low. I understand that the dual plane manifold will help with my vacuum problem and will flow much better than stock. As for the 750 carburetor, I happened to have one on another car. Seems as though everyone I ask about the carburetor has a difference of opinion. Some say it's probably too much and others say it seems about right. Will the 670 bolt to this manifold or will I need an adaptor plate.
 

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theres a difference of opinion because you left out that important info in your first post.. you simply stated L48..we all assumed "stock" ..... the Dual plane manifold will improve low -mid-range performance but will not improve low idle vacuum much.. how much vacuum at idle do you have now?it takes 10-12 inhg to operate brake booster.. also the Dual plane will allow for slightly bigger CFM carb, because it increases velocity at low speeds.... what did the engine dyno? ... the stock manifold is spreadbore type, but that Edelbrock manifold is made to take either spreadbore or squarebore carbs,no adapter necessary...what are the cam specs? if its as big as you say, you might benefit more from an Air-Gap or even an RPM Air -Gap manifold
 

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Picking your manifold is fairly straight forward. Do you know your rpm range? What cam you have, lift duration centerline overlap all comes in to play. I’m sure you matched this with your trans, axel ratio and tier size. To fit how you drive most street, strip, autocross. Performer is good to 5500 rpm Dual plain low rise. (Spread bore normally but you can find similar square bores), performer rpm is said to be good 1500-6500 rpm Dual plain high rise square boar. Not sure if I’ve ever seen a spread bore RPM style (other company’s may offer them IE Holly or Jeggs) your 670 Holley will feed the engine may need to be rejeted. You will most likely need a lower power valve and a stiffer secondary spring. (its not a double pumper is it? if so you may want to get a vac secondary) some on here can give much better advise on setting up carb. I have to read my books every time I do one.
 

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Discussion Starter · #12 ·
theres a difference of opinion because you left out that important info in your first post.. you simply stated L48..we all assumed "stock" ..... the Dual plane manifold will improve low -mid-range performance but will not improve low idle vacuum much.. how much vacuum at idle do you have now?it takes 10-12 inhg to operate brake booster.. also the Dual plane will allow for slightly bigger CFM carb, because it increases velocity at low speeds.... what did the engine dyno? ... the stock manifold is spreadbore type, but that Edelbrock manifold is made to take either spreadbore or squarebore carbs,no adapter necessary...what are the cam specs? if its as big as you say, you might benefit more from an Air-Gap or even an RPM Air -Gap manifold
Please pardon me, I should have told everyone that the engine was not entirley stock.

It's a Class II Jasper Performance Products 350. 350hp @ 5500 rpm, 389 lb. ft. of torque @ 4200 rpm, 9:1 compression, 67 cc combustion chambers, 2.02 intake and 1.60 exhaust valves. Cam lift is .320 intake, .320 exhaust, valve lift is .480 intake and .480 exhaust. .050 duration: 232 degrees intake, 232 degrees exhuast. Advertised duration 288 degrees intake, 288 degrees exhaust. Lobe center is 106 degrees intake and 110 degrees exhaust. The rear end gear is stock a 3.07 gear. Jasper recommends a 750 carb but suggests rejetting. Vacuum is 9" Hg at idle. I considered an air gap but can't afford and L-88 or high rise hood to clear the taller manifold.
 

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thats a respectable engine....an Air Gap will clear hood with a drop base style air cleaner ...with only 9 in hg vacuum, your brakes must be iffey at best....My cam is considerably larger @ .576 lift and 254* duration , and I ve only got about 5-7 inhg vacuum.. no brake boost at all. I had to go to an electric vacuum pump to boost brakes ..I run an RPM Air Gap..dyno d @ 525HP .....either the performer or an Air Gap will improve performance over the stock iron manifold.[ which is 35# of crap]. the 750cfm will work as long as its vacuum secondaries ....I run a Quickfuel SS 750 CFM Mechanical Secondary carb on mine...anyway, good luck, let us know what you go with and the results
 

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Discussion Starter · #14 ·
thats a respectable engine....an Air Gap will clear hood with a drop base style air cleaner ...with only 9 in hg vacuum, your brakes must be iffey at best....My cam is considerably larger @ .576 lift and 254* duration , and I ve only got about 5-7 inhg vacuum.. no brake boost at all. I had to go to an electric vacuum pump to boost brakes ..I run an RPM Air Gap..dyno d @ 525HP .....either the performer or an Air Gap will improve performance over the stock iron manifold.[ which is 35# of crap]. the 750cfm will work as long as its vacuum secondaries ....I run a Quickfuel SS 750 CFM Mechanical Secondary carb on mine...anyway, good luck, let us know what you go with and the results
Thanks everyone for the advise! Hope you all had a happy holiday. I'll post updates and pics! Now I'm in the market for an air gap!
 

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Please pardon me, I should have told everyone that the engine was not entirley stock.

It's a Class II Jasper Performance Products 350. 350hp @ 5500 rpm, 389 lb. ft. of torque @ 4200 rpm, 9:1 compression, 67 cc combustion chambers, 2.02 intake and 1.60 exhaust valves. Cam lift is .320 intake, .320 exhaust, valve lift is .480 intake and .480 exhaust. .050 duration: 232 degrees intake, 232 degrees exhuast. Advertised duration 288 degrees intake, 288 degrees exhaust. Lobe center is 106 degrees intake and 110 degrees exhaust. The rear end gear is stock a 3.07 gear. Jasper recommends a 750 carb but suggests rejetting. Vacuum is 9" Hg at idle. I considered an air gap but can't afford and L-88 or high rise hood to clear the taller manifold.
With the duration and lobe centers of that cam, I think I would run a 750 cfm also.
 

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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.
 

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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...
 

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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.
 

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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?
 

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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?
no point at all, except that he already has it and with the vac. sec. it will work .. I agree completely , Steve, that a smaller CFM carb would be better, I even said so in my first post.......just for kicks,..I just did the Holley website Interactive Carb Calulator,.. for my set-up it recomends a Holley 750 CFM double pumper Mechanical . Secondaries...with the exception of mine being a Quickfuel instead of Holley, thats exactly what I run [ and what produced the highest HP/tq on the dyno] ... I put in all his info [ estimating 5800rpm redline,with his cam specs] and Holley says a 600 or 650 cfm vac sec is what should be used... but again, since he has the 750 w/vac sec, , it will work...and yes, it won t have any power benefit and may even lose a little mpg..
 
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