I started with 650's. Too much carb, switched to 600's. Too much carb, went to the 500's. Runs much better. 2 X 500 = 1000, that's a lot of cfm's. The engine is mild, only bored 30 over to clean up the block, and mild cam. Stock iron heads, stock exhaust manifolds, factory Weiand intake. I built it to drive, not to race, or win trophy's or burn out contest.