engine guys, what cfm carb do I need?

I have my 302 chevy for my willys finished (finally), I am trying to decide what cfm carb to go with. I am wanting a holley double pumper, 600,700, or 750 cfm?? The engine is built pretty hot, I don't know the horsepower, but it is running around 11.5-1 compression, 461 1.94 camel humps, tm1 tarantula intake, full floating rods, roller tip rockers, 280 comp cam. I know the heads are a little small, But i had the fresh set laying around. I plan to go with bigger heads W/full roller rockers and girdle and a big solid roller cam later on. With the compression I am running I prefer to run a little fat instead of lean, that will help keep the cylinder temp down a little. Any suggestions?

 

750 Double pumper will do you just fine You'll do better using the appropriate size rather than a bigger carb.

http://www.thedirtforum.com/cfmcalc.htm

 

The Factory 302 Z28 engines used a Holley #3310 which was a 780 cfm unit.

 

I'll go against the grain here..
I would run a 650 dual feed with mechanical secondaries..

The smaller CFM will usually increase your low end power over the larger CFM carbs..
You *may* sacrifice a little top end but we tend to spend most of the time at the lower rpm's unless your rig is a full out race mill and spends most of the time above 4000 rpm or higher.
I completely understand running it a touch 'fat' to keep cyl temp's down too, but with a properly tuned carb you can still run it a touch rich if need be..
I ran mine a touch fat as well.
I built a small journal 327, 12.1 slugs, Brodix aluminum street heads, i think they were just under 200cc intake runners, 2.02 valve's, dual pattern solid stick with exhaust lift just under 550, and duration of around 336 (can't remember exactly sorry) roller rockers etc...
It ran BETTER with a set up 650 than a 750..
It felt like it had more low end torque (and with a small journal mill, thats always lacking) and never fell on it's nose even above 6500..

This is only my 2 cents though..so take it for what it's worth..

Tony

 

Here's another link with a formula you can play with..

http://www.mortec.com/carbtip1.htm

 

I'm with Tony on this one, 650 should do it pretty good, unless it's all out race in a lighter car..by your name i'm going to say no. if you are really wanting to get it perfect you can click the link in my sig and ask a Tech from Barry Grant, no registration required. He can get you the perfect carb size for you intended application.

 

I'd run a 650...unless you're running at the drags or driving at 7G's all the time...

 

The engine is going in my willys truck g***er, 2400 pounds at the most. I will be driving it on the street some and at the track. No I won't be driving it at 7000 ALL the time, just some of the time. LOL

 

600 is plenty,vacuum secondaries would be my choice.

Bottom end will be soft with a single plane intake.

 

On a combination like this a 650 is going to be your best choice. If you're going to use a manual transmission go with a mechanical secondary carburetor, if you're using an automatic (with less than a 3000 stall) use a vacuum secondary carburetor. A larger carburetor will give you a little more top end power, but the smaller carburetor will idle better, give smoother acceleration and drivability.

 

I'm leaning in this direction also. I made over 400 horse with a slightly altered 600 cfm holley ( it actually more than 600 but...). But you probably don't want to or shouldn't alter one so the 650 should do you just fine.

Bigger isn't always better, the mill will be more crisp with the smaller carb. I probably wouldn't go over 700.

 

I'd put a 3310 on it! chevy knew what they were doing.

and being vacuum secondary it won't be "overcarbed"

 

When it comes to streetin' it - smaller is generally better.

Having said that - a vac secondary carb set is going to use only as much air as will maintain the vacuum signal. Quadrajets were used on some pretty small motors with good results!

I'd calculate your engine based on 7000 RPM redline, 90% volumetric efficiency - your engine will 'pull' somewhere around 550 CFM at redline. Oversizing by about 10% yields a 600 CFM carbie. The closer you stick to the actual CFM rating of the motor for carb selection - the better your throttle response.

 

Just wondering but what would make you choose a mechanical secondary over a vac***e. Wouldnt a large cam engine produce very little vac***e.

 

Yeah... in 1970.

Listen to the BG guy. He is a carb tech guru... he suggests carbs for a living... Whether you use BG, Holley, Edelbrock, Carter... 650 is the best option for street.