Holley carburetor question

[Alan Roehrich]

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

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.

[Geerhead55]

Alan,, Do you think he should stay with the Scorpion? As I was reading this, and seeing a max RPM of 6500, I was thinking use an Air-Gap. We're not given any info on cylinder heads being used, so if they're nothing outrageous, perhaps the dual-plane would work better with this combo,,,just thinking out loud.
Paul,, I've loved following your Chevy II build, as I had one myself years ago. Yours looks great, and I hope you have a lot of fun with it.
Danny Durham

[Alan Roehrich]

Originally Posted by Geerhead55

Alan,, Do you think he should stay with the Scorpion? As I was reading this, and seeing a max RPM of 6500, I was thinking use an Air-Gap. We're not given any info on cylinder heads being used, so if they're nothing outrageous, perhaps the dual-plane would work better with this combo,,,just thinking out loud.
Paul,, I've loved following your Chevy II build, as I had one myself years ago. Yours looks great, and I hope you have a lot of fun with it.
Danny Durham

Danny, I don't think the intake is a really big deal here. It's a light car, with a ton of gear. A 3.06 low gear with a 4.10 rear, in a car that weighs maybe 3300 or so. The only problem the car has is the drop between 1st and 2nd with a low vehicle/wheel speed.

Certainly, if I were choosing an intake because it did not have one, I'd lean more toward a Performer RPM Air Gap. Or maybe a Holley 300-25, if I were wanting to move the power band up by running a single plane. It might be different if the car were heavier (say 3600), or had more cam (big), or less gear (3.08 or 2.93). If one of those were the case, then I'd say an intake change would be a good idea. Even more so if two of those were the case.

We also have to realize it is probably a fun street car, and he's probably not hunting that last hundredth.

[Paul Merolla]

Absolutely right on the 4.10 / 700R4 combo, Alan. Just cruising around town, mine shifts into second in about 20 feet from a stop, and there's way too much rpm drop.
Danny - thanks for the comment on the Nova...it's close!

[Alan Roehrich]

Originally Posted by Paul Merolla

Absolutely right on the 4.10 / 700R4 combo, Alan. Just cruising around town, mine shifts into second in about 20 feet from a stop, and there's way too much rpm drop.
Danny - thanks for the comment on the Nova...it's close!

The gear ratios in a 200 4R are better, as it has a 2.75 low. But you need an expensive forward input drum to make it live, the shaft breaks on the 2-3 shift. You can reinforce the forward input drum and upgrade the sun gear shell in a 700 R4 way cheaper.

They actually use the rear planetary from a 700 R4 in a 350 to make a 2.75 low gear. I've considered using the TH350 rear planetary in a 700 R4, but never gotten around to it. A low gear of 2.75 or there about would make the 700 R4 a much better transmission for performance use, giving it a much closer ratio spread between 1st and 2nd, and a higher wheel/vehicle speed at the shift.

[eddie c]

Thanks much for all the input on carb size,I really appreciate everyones help. Allen R, from the tel con we had a few months ago you probably
remember that I am an old stocker racer. Since when can you run an
aftermarket carb in stock and s/s. I believe you said something about a quick fuel unit ,just like you use in todays class racing? ed

[Alan Roehrich]

Eddie, Quick Fuel has had at least 3 of their street/OEM replacement carburetors approved for Stock and Super Stock. Much the same as some of the Weber/Edelbrock Q-Jets were approved a few years ago.