heads for a 331

Is it a street motor or mostly race? Just wondering because the 331 can only ingest so many CFM. At 6800 RPM a 331 cubic inch motor can take in a maximum of around 650 CFM. And that's at 100% volumetric efficiency. Most of our well built motors are in the 80 something percent area. Like 83-85%. So that means 550 CFM or so is all that 331 can flow. The general rule is figure out the CFM your motor needs and get the next carb size bigger. So a 600 or 650 Holley would be a good bet. Peak to peak both will make around the same power but the 750 will probably be somewhat less responsive and less efficient at low RPM.

I think the only difference the cam will make as far as the carb is concerned is in the engines V.E. percentage. If it makes your engine flow more at higher RPM your CFM requirement goes up.

Also I am also running a carb that's too big for my 6500 max RPM 302. I have a Holley 650 DP. With some tuning we got it running great. But you run what you have and that's what I have. Holley DPs with smaller than 650 CFM ratings are scarce around here.

 

Street but also Auto Crossing. It will be just what you said, less responsive down low. You are right on the money, the difference between a 650 and a 750 i was told by several is 10-15 hp on top with my setup. My revs should be 4000+ for the track, anything less and i'm in the wrong gear. Not the best for street but with a stick I was told it could work. The custom cam has a mild lift and more duration than OTS cams according to the guy I called on. I did do my research when it came to picking a person with knowledge about SBF's and cam specs.

 

Sounds like a cool set up. Good luck with it

I can't find the issue but Engine Masters did the definitive article on carb sizing. They even used a ford motor. I believe it was a Windsor stroker (383 I think) with the usual upgrades like heads, cam and other stuff. They ran all Holley mechanical secondary carbs. A 650, 750, 850, and 1050 if I remember right. They tuned all of them to achieve as close to optimum air fuel ratio as possible and also measured vacuum and fuel consumption. Any way. There was less than a 10 peak HP difference between 1050 and the 650. The 750 made about 3 more peak HP at 6200 RPM or something like that. The bigger carbs had to run different jets and consumed more fuel at low rpm and the bigger the carb the more problems it had keeping the fuel atomized at low RPM.

I have been looking for the mag for a year. I know I didn't throw it out. :16suspect