[cnassiri]

Here are some solid facts that will help you with your decision: There is a formula that most carb guys have used for years, it's not exact, but it will give you a good approximation of what you need:

Carb CFM = (Engine Displacement * Max RPM) / 3456

In your case 305ci *6000 rpm / 3456 = 530 cfm.

This formula shows you the minimum carb size you'll need given a certain rpm - 6000 rpm in this case. If you never plan to rev your engine past 5500 rpm then the formula gives you 485 cfm.

So in you're case, you're right in between. The 500 cfm carb will do OK for your engine, but at 6000 rpm it will be choking your engine badly, and costing horsepower. In this case I would error on the side of going too big.

I'm sure you've been told not to get a carb that is too big, and that a big carb will cause you all kinds of problems, but stories about big carbs ruining a motor have more to do with guys slapping Race prepped Holley 750 double pumpers on a stock 302. Clearly the engine is way over-carbed.

In your case, a 600 is slightly large, but it won't hurt performance or driveability at all, on the other hand, a 500 will hurt performance, and it doesn't leave any room for performance improvements. Plus, if you later drop in a 350 or 383, you'll definitely have to get a new carb, the 500 will suck the life out of a bigger engine, but the 600 will do just fine.

It's your call, either way, you won't be disapointed, but for my money, I'd go for the 600.

Another thing to think of: If you ever go to sell the carb, the 500 won't be worth anything (not much market for them), on the other hand, if the 600 is still clean and funtioning, you'll get top dollar for it.

Cyrus
Owner - Torque Vendors Racing