Most likely you will be doing some modifying
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?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?
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.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).
Please pardon me, I should have told everyone that the engine was not entirley stock.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
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!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
With the duration and lobe centers of that cam, I think I would run a 750 cfm also.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.
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...
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..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?