Throat Diameter

A critical aspect of maximizing cylinder head flow is establishing the proper throat diameter. Going too big or too small can seriously impede airflow, but getting it right is pretty clear cut. "The rule of thumb is that the throat diameter should be roughly 90 percent of the valve diameter," says Judson. On race valve jobs, since the seats are moved farther down near the valve, the guidelines change slightly depending on the specific valve–seat angle. With a 45–degree valve seat, the throat diameter should be 0.200 inch smaller than the valve, and with a 50–degree seat, it should be 0.180 inch smaller. Stepping up to a 55–degree seat requires a throat diameter 0.160 inch smaller than the valve.

Angle–milling cylinder heads is always beneficial to airflow. "Removing more deck material off of the exhaust side than the intake side reduces the combustion chambers, raises the runners, and helps deshroud the valves by moving them closer to the center of the combustion chamber," says Judson. It also allows more material to be milled before hitting the valve seats. However, there is a practical limit to how much a head can be angle–milled. Rolling over a 23–degree head to an 18–degree valve angle just isn’t practical or feasible. On a small–block, "cutting 0.017 inch off of the exhaust side per inch of cylinder head width reduces the valve angle by one degree, and 1.5 to 2 degrees is the absolute max. If you get the chambers and seats optimized to go along with an angle–mill, the procedure can net a solid 15–cfm increase in a head that flows 300 cfm." The only downside is that the minute change in geometry that results from angle–milling may require some creative installation when fitting the intake manifold and headers.