[chevymudd54]

I am well into a sbc rebuild for my '79 4x4 truck. Its a 355. Has GM performance parts fast burn heads with valve job done. Has upgraded springs and I put scorpion roller rockers 1.6 lift. 270 duration comp cam, edebrock rpm air gap intake. I know I want a vacuum sec. carb. Kinda tossed between quick fuel or demon carb. Open to suggestions for sure. Dont know what would be the proper cfm for my carb too. Heard mixed info from 600cfm to 750 to 800cfm. Any comments would be greatly appreciated. THANKS!!

[70CHEVYSHORTY]

I have not ever dealt with the quick fuel but here they are good. The demon is a good carb forsure. And i would say 700 cfm would probly work. not much info on the motor but i'm sure 700 would be enough.

[chevymudd54]

10:1 compression. Hypernautretic pistons, gonna just run high octane gas, stock crank. will be over 400hp when done. (had a friend do similar rebuild) any other info u might need?

[schovil69]

This has to be the most common question I see on these boards. The following is a quick rundown for how to figure out what you need for a carb. I posted this on another site for a guy doing a 402. Just put your info where it needs to be and grab a calculator.

Originally Posted by SCHOVIL69

An 850 on a 402 is a little too much. If you spin your engine to 7000 rpm and it had 100% volumetric efficiency(VE) you would need just over an 800 cfm carb. A very well designed engine intended for racing only may reach 95% VE, let's see how the 402 fares:

cfm = (RPM X VE X CID)/3456 = (7000 X 0.95 X 402)/3456 = 773.52 cfm

You can see here that under the best conditions and at high rpm's, your carb is too much for your engine. I suggest getting a smaller carb or severely downsizing the jets in the one you have.

By the way, at 6000 rpm's, you would only need a 663 cfm carb. Play with that formula to determine what your needs are.

[chevymudd54]

That is awesome man! thanks a lot for that info! I'm definatly gonna write that one down!

[chevymudd54]

that equation comes to 585.5 at 6000rpm...if my math is correct thats a pretty low cfm. So if I go with a 600cfm carb i'd be good?

[Schoust]

I'm doing a similar build and had the very same question I think it depends on what the engines purpose is? What RPM range and what you plan to do with it? I am using my engine as an every day driver and think a 650-700 street avenger is a good size. If I was drag racing I would go with a Double pumper.

[chevymudd54]

Its a 4x4 that i will drive and might find its way to some mud from time to time...any sugestions on demon or quick fuel?

[schovil69]

Originally Posted by chevymudd54

that equation comes to 585.5 at 6000rpm...if my math is correct thats a pretty low cfm. So if I go with a 600cfm carb i'd be good?

Yeah, that's right. The equation works out to about 737cfm for me. I use a 750 DP on my 383 spinning to 7000 rpms.

[Schoust]

Sorry should have read the whole thread. The more I read up on the small block Chevy the more I would suggest that the carb be on the conservative side as far a CFM especially in the case of a 4x4 truck. If you have ever built an engine or had a vehicle that was over carbed it is a pain. Smaller is better IMHO.......

[ken konoff]

Have an similar motor 355 in my 84 C-10, and I have an 600 holley with vacuum secondary's, and engine has great throttle response. I too believe smaller carb is better than one too big. I keep my rpm's at a max of 6,000.