Quote:
Originally Posted by eddie c
I recently did some work on a guys car,its a 68 Camaro with a 350ci
engine,700r4 trans,and a 4.10:1 rear gear. The engine has a mild cam,
not exactly sure of lift and duration. Anyway when the guy had the engine built a few years ago it had a 780 cfm Holley. At that time the guy was very happy with the performance of the car,now the car saw about three different people work on it over the first few years. Each time ,things were modified or changed,I dont really know what was done,however one of the guys that previously worked on the car changed out the carburetor, he put a Quadra Jet with an adapter plate on it and after that the car really performed poorly. Now I come into the picture,and suggest that we go back to a Holley because the intake,Elderbrock Scorpion, is designed for that type of base.Using the carb size calculator,I take the engine size and max RPM which is 6500 and determine that a 650 cfm Holley would be the best choice. After changing jets acc.pump nozzles and spark plugs
the car starts running really good,unfortunately no et slips,just seat of
the pants. Well long story short the guy says he wants to go back to a
780 cfm carb. I have explained to him that the carb size recomendation
was within the correct limits. What do you guys think? Will this car
run better if we go to a 780 cfm carb? Thanks for reading such a long post. ed
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Most of the "carb size calculators" are intended to yield a carburetor size that will run fairly well and still be driveable on the street for the average basic tuner on the average car with a less than perfect combination. So they tend to suggest a carburetor around 100CFM smaller than could be run by a good tuner on a fairly well matched combination.
The average tuner is going to make 1-2 jet changes, might change a power valve, and maybe even a squirter. A real serious tuner will go so far as to change or modify metering blocks, sort through accelerator pump cams, and change diaphragm springs on the vacuum secondaries.
A smaller carburetor (relative to the biggest that can be run effectively) will create pumping losses, as it will create enough restriction at WOT at the RPM where peak HP is made to generate well over 1.5" of vacuum. Most often, the carburetor the "calculators" will suggest will make 2.0" or more. It does hurt HP. You can get down to around 1.0" of vacuum on a properly tuned carburetor on a good combination.
If a car is not over cammed, and has good gearing, you can run a carburetor 100CFM larger than those calculators suggest. So, yes, you can run a 780 vacuum secondary on it. Remember, the Holley 780 vacuum secondary was the carburetor used on the 302 Z-28 engines, and the 350 LT-1.
If I were going to buy one, it would be the Quick Fuel 780 just like we use on the Stock and Super Stock cars. It is really easy to tune, you can wring a ton out of it if you're willing to work on it. It will normally beat an out of the box Holley with relative ease, as it comes out of the box. You'll get great support from the guys at Quick Fuel, too.
By the way, make absolutely certain that the bottom of the carburetor throttle lever has the correct geometry to make the throttle valve cable on the 700 R4 work absolutely right. Most often, a 4.10 is too much gear for a car with a 700 R4, especially if it is relatively light. You can't shift it fast enough, and the car is going too slow when it goes to second gear. A 3.73 works better for 7-8 out of every 10 cars. I know, I had a 4.10 behind a 700 R4 in a car that would pull to 6500.