Working Runners vs. Working Bowls

On production cylinder heads, the bowls require much more attention than the runners. "The average backyard hack needs to stay away from the port, gasket–matching it at most, and work on the bowl," says Judson. "It’s hard to mess up the bowls, and using common sense will improve airflow. The goal is to create a nice transition from the bowl into the valve job." However, just the opposite is true with a quality aftermarket casting. "If you buy a good aftermarket head, the bowls are already 90–percent there, and the average person is more likely to mess it up than improve it."

"Generally, a valve–seat angle greater than 45 degrees will make more power. There are some trade–offs, though. As the seat angle increases, durability decreases. That’s why lower angles are common in many production motors, and just about all diesels have 30–degree seats. With 50–55 degrees you’ll lose 10–15 percent of flow from 0.200– to 0.400–inch lift that you can’t get back. However, you can’t sacrifice high–lift flow for low–and mid–lift flow because that’s not where you make power. Some of the top engine–builders in the country don’t even turn the flow bench on until 0.300– to 0.400–inch lift. And the improved high–lift flow of bigger angles lets you open up the venturi–at that point the venturi becomes the restriction. You lose a ton of energy when air exits from the port into the cylinder, so a bigger venturi helps maintain that energy. Porting is all about area relationships, and you want to maintain the valve area as the restriction, not the port. In other words, you don’t want a weak port with 50–to 55–degree seats. A weak port with a valve seat area that flows well creates lots of turbulence, which hurts flow. The more you know what you’re doing, the less you lose down low by going with a higher seat angle."