Valve-Seat Angle
"Generally, a valve-seat angle greater than 45 degrees will make more power," says Judson. "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."
Porting ToolsThe easiest way to begin is by picking up a porting kit from Standard Abrasives that includes most of the abrasive materials necessary for head porting. Either an air or electric grinder capable of at least 10,000 rpm is required, and the abrasives can safely handle 18,000-20,000. Judson prefers using a Milwaukee 5196 electric unit. Additional necessities are a selection of carbide cutters, radius and telescoping gauges, a protractor, dial calipers, scribes, different-length mandrels, cartridge rolls for finishing, and lights with magnetic bases to help illuminate the ports.
Short-Side Radius & PowerAs 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."
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 make 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. Says Judson: "Air likes consistency, not a lot of changes."
Port Volume
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).