Carburetor CFM selection question
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Laughing Gas
Joined: Aug 2010
Posts: 982
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From: Northern Kentucky
Carburetor CFM selection question
I have a carbed 351W with C-6 trans (4x4), shorty headers, Edelbrock manifold and DS2.
CFM calculators show that for max RPM of 5500, I need a carb. of less than 500 CFM. For practical street use in a truck used as a truck, but not for heavy hauling/towing, what are the advantages or disadvantages of a 500 CFM versus 600 CFM carb.?
CFM calculators show that for max RPM of 5500, I need a carb. of less than 500 CFM. For practical street use in a truck used as a truck, but not for heavy hauling/towing, what are the advantages or disadvantages of a 500 CFM versus 600 CFM carb.?
Post Fiend
Joined: Apr 2002
Posts: 19,311
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From: Waterloo, Iowa
Depends on what you want to do. Big carbs "sweet spot" is with the engine wound out. A lot of fun, but not practical for just tooling around. Tailor the carb size for your intended use. It's all about proper atomization of the fuel. A big carb at low speeds or rpms often doesn't run well on the street. OEM tended toward the small side, at least on paper. They idle smoother though and have crisp low end torque just off idle esp. with heavy loads. So there was a method to their madness. I doubt you'd notice much difference. Vacuum secondaries are what you probably want for a street motor.
FTE Legend
Joined: Jul 2010
Posts: 32,875
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From: Northeast, OK
Theory says the smaller the carb the closer to max CFM you normally run, and the more accurate the metering can be. Way too big of a carb means low % CFM and low vacuum from the venturi, so poor metering.
I ran an Eddy 1406 600 CFM on Rusty's 351M and it worked very well. So I'm not sure I'd say go with the 500.
I ran an Eddy 1406 600 CFM on Rusty's 351M and it worked very well. So I'm not sure I'd say go with the 500.
Cargo Master
Joined: Oct 2005
Posts: 2,295
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From: NJ
500 CFM is pretty borderline for a stock 351w, the downside is sacrificing top end. If it were mine and I intended to keep it stock I'd use a 600 CFM Holley 4160 vacuum secondary.
Too large a carb can result in a mushy bottom end or not as crisp and responsive throttle response off idle, but that's not likely to be a problem on your engine until a larger than 650 CFM carb.
Too large a carb can result in a mushy bottom end or not as crisp and responsive throttle response off idle, but that's not likely to be a problem on your engine until a larger than 650 CFM carb.
FTE Legend
Joined: Jul 2010
Posts: 32,875
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From: Northeast, OK
Also, there's a difference between carburetor brands on the CFM ratio between primaries and secondaries. Some have it about 50/50 and some have it more like 33/67. In other words, some brands will have the primaries much smaller than others for the same CFM rating. But, any of the 600 and down CFM carbs should give crisp part-throttle acceleration as the primaries aren't really all that big.
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Laughing Gas
Joined: Aug 2010
Posts: 982
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From: Northern Kentucky
I appreciate the comments. I am most interested in low end torque and crisp throttle response, so opinions in that direction are very helpful. The shorty JBA headers and dual exhaust with no cats seems to have helped in that direction.
It will stay stock for right now, although I keep dreaming about a better set of heads (currently D8 heads-Ford's not so better idea) and a torque cam. It has too many other things to address before that though, like body and suspension work.
It will stay stock for right now, although I keep dreaming about a better set of heads (currently D8 heads-Ford's not so better idea) and a torque cam. It has too many other things to address before that though, like body and suspension work.
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