someone was wondering what size carb to use but I can find the post now that I have found the equation. You are suppose to take your (engine cubic inch size x the max rpm) then divide that by 3456 = optimum carb size

example my race motor. 355cu x 7200rpm=2556000. now divide 2556000 by 3456 = 739.58 so basically I went with a holley 750dp and it works great for me

a typical small block 350 with a max rpm range of 5800rpm would only require a carb of 600cfm and that would be a bit big

I know, I know it is hard to buy the small stuff, we are all top fulers at heart.

I hope this helps someone.

 

yup thats right

the equation is simply <CFM = CID * RPM * VE * 1/3456>



so mine would be.

355 x 6500 = 2307500. 2307500 / 3456 = 667cfm

so i just round that up to 670cfm.

works like a charm, you will most likely have to play with jets and metering needles and such to get it perfect.

good post

 

Please don't take this as Im bashing the equation BUT..... I run a 355 roller and turn it 7200. I started with a 750 holley... went to 850 chuck nuytten and picked up a tenth ...... went to a 950 race demon and picked up another tenth.

This was all in very similar conditions and at the same track.

Now this could be from carb quality and not size but it if works then run it. IMO give the engine what it likes not what "they"say it needs.

Ray

My .02 cents

 

I have to side with Ray on this. That sizing equation is okay if we are talking about a mild street motor, but it does not take into consideration modern head castings which flow a lot better than the old stuff ever did, or modern cam profiles with quicker ramps.

Engines today can make much more power at say 6500RPM than they did 20 years ago. You need to feed that power with fuel and airflow. I ran a box-stock 750 vac. secondary Holley carb this summer on my Z. By milling off the airhorn and gaining 45cfm (to 795cfm) my car instantly picked up 5 mph in the 1/4, and with more tuning and tweaking over the summer, eventually a total of 7 MPH. That is roughly a 70HP gain from more airflow, which indicates the carb was actually too small for the application. Since my engine makes peak power at around 5500-5600 RPM, this indicates to me the engine would prefer a bit larger carb yet.

I once got into an argument with a bud who owns a speed shop about carb sizing. He had been on me about upsizing the carb on my engine, and I had dismissed that as being too much (he said to go with an 850 over my 750). Then he pulls out a dyno chart of a 383 engine he built for a customer. This engine was run on the dyno with a 750 Holley dbl. pumper and made in the area of 475HP. Then they swapped carbs for an 850 and the engine made 530HP. You could see across the RPM range on the dyno sheet the engine was picking up torque and HP over almost the entire RPM range.

A smaller carb *is* better for aspects such as driveability, but could be the limitation for unlocking some great HP gains if it is too conservative.

 

yes but what he is talking about is more of a day driver, but a drag racer or anything like that, just the base carboration that u should start at.

but i do see where u coming from, but i just wish i had the money to upgrade from an ol holley to a nice new one