A common method of determining properly sized cylinder heads for a given displacement and rpm range is port volume, but it's not necessarily the most precise method. "Port volume doesn't mean anything," says Judson. "All that matters is cross section, because you have to compare like ports. You can't compare 23-degree heads to 18-degree heads since the longer runners in an 18-degree head means it has more port volume for any given cross section." Generally, port volume is just a substitute for measuring cross section, and the larger the cross section, the higher the rpm the motor must turn. Here's the industry standard formula for determining the proper average cross section of a port:port speed = piston speed x (bore area port average cross section)
Short-Side Radius & 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 airflow 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 around 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."
Shaping A Port
Small- and big-block Chevys feature a variety of rectangle, oval, and even cathedral ports, but not all are equal when it comes to airflow. "In terms of cross-sectional area to flow, an oval port is the most efficient because it has no sharp edges," explains Judson. "Air travels in the path of least resistance, and eliminating sharp edges minimizes resistance. If there's enough meat on the head, ports should be oval." Some rectangle ports can be reshaped into ovals by filling in the corners with epoxy. Extending this concept into the intake manifold, runners that taper from the plenum into the port makes incoming air less sensitive to the changes in the contours of the port. This effect is more pronounced with longer intake runners, where taper helps out even more. "Air likes consistency, not a lot of changes."