Short-Side Radius and Power As air moves through the intake port, its naturally tendency is to continue moving forward rather than transition downward toward the valve, which causes turbulence and impedes flow. "What you’re trying to do as a porter is stick as much air as possible to the short side to help it make the turn toward the valve," says Judson. "Many times the short side matches the angle of the port roof, and laying it back farther away from perpendicular to the valve guide generally improves flow. The trick is laying the short–side radius back as far as possible without going so far that you hurt flow." CFM vs. Horsepower For most street/strip and mild race motors that generate up to 1.7 hp per cubic inch, improvements in air flow translate directly into increases in horsepower. However, at anything beyond that level of specific output, the flow bench is no longer the Holy Grail of predicting power. "Once you hit 2 hp per cubic inch, things start getting crazy," says Judson. "Take a Pro Stock engine, for instance, that puts out right about 2.7 hp per cube. On a motor like that, it’s a given that you have a serious cylinder head that can fill a cylinder quite well. So when the exhaust valve opens, residual pressure reaching hundreds of psi escapes out of the cylinder. The velocity of the exhaust coming out the head is a bazillion times higher than what can be replicated on a flow bench." To put it succinctly, experts can find no correlation between airflow and power on the exhaust side beyond 2 hp per cubic inch. "There are things going on at that power level in a motor we just don’t understand. You can have one head that’s down 50 cfm to another head on the exhaust side that makes just as much power. Don’t get me wrong, the flow bench has contributed more to horsepower than any other part or tool in the last 30–40 years, but after a certain point flow numbers don’t mean squat."