Carb CFM

[gregbr]

How do you know which carb to buy? Are our factory Rochesters 750 cfm & would you look for the same CFM in other brand carbs?

[Ogier]

Originally Posted by gregbr

How do you know which carb to buy? Are our factory Rochesters 750 cfm & would you look for the same CFM in other brand carbs?

It depends on the engine. I have seen 850cfm Rochesters out of the factory on several occasions.

Do you look for the same CFM in a different brand, no.

Rochesters and Predetor carbs are more alike than they are different in that they are both variable in how much CFM is actually reaching the engine at a given time. Whereas Holleys, Carters, and Edelbrocks are not.

Having built something around a quarter of a million horsepower in my lifetime in automotive engines, I would never ever consider putting a non-variable venturi carb larger than 650cfm on a street driven vehicle unless the driver planned on turning the RPMs up over 5000.

For example, on the street, even when racing light to light, a Chevy 350 will not turn over 4500-5000rpm. To do this, you only need a 550cfm carb.

If you are planning on running it up to 8000rpm, you will need an 850cfm carb.

Please keep in mind that to run 8000rpm reliably, you need to do a LOT more than just bolt on some parts and go for a drive. It can be done, but, as my teacher used to tell me:

Speed costs money, how fast do you want to go?

All that said, if you use a variable venturi carb like the Rochester or the Predetor, you can run 1000cfm if you like, but the carb will only open as much as the engine actually needs...

Good luck.

[gregbr]

I'm running a mid 70's 350 with a mild cam and T350 trans.....if that helps. I currently have a Edelbrock carb, but the electric choke butterflies/linkages are missing. So I need to get another edelbrock, or choke parts, or swap back to a rochester.

[Old Yeller 1970]

You should also consider choosing carb size based on any mods you've made to the exhaust and intake. What kind of exhaust system (headers vs stock manifolds?) and intake (stock or aftermarket?) do you have?

[gregbr]

I have headers and full duals with turbo mufflers, edelbrock intake & carb, mild street cam (specs unknown). Engine work done before I got the truck. Will probably go back to stock engine & exhaust manifolds with duals soon. Greg

[Old Yeller 1970]

Before my current engine rebuild, I ran headers, Edelbrock Performer intake and a 625 Road Demon on a stock 350. Before the Edelbrock and Road Demon I had the stock intake and Q-jet. The difference in power was substantial from idle all the way to redline.
I'd recommend a squarebore carb in the 600-625 range if you stay with your curent setup. Or you can go back with a stock 750CFM Q-jet but probably won't have the power of a Holley or Demon.

[67chevemall]

ya I run both a holley 600 and a 78 Qjet and the Holley has way more pep
but the qjet is way better on fuel and you never have to fiddle with the qjet