Q: I have built up a 1970 Monte Carlo with a 350. I want to run the car at the track, so I’m thinking I need to scrap my 1 5/8-inch primary headers for something bigger to help the engine make some extra top-end power. However, I don’t want them to be too free-breathing–I want to keep enough backpressure to prevent valve float. Right now, the engine makes about 400 to 420 horsepower with the stock valvetrain (except the springs, which are double coil). Should I go with 1 3/4-inch primaries, or go all the way up to 1 7/8 inches?
A: In this case, we’d recommend the 1 3/4-inch primary headers. Like you mentioned, going any larger on the primary tube size will be too free-breathing, which reduces the scavenging effect in the combustion chambers and could cost you some torque.