Valve-Seat Angle
"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."
Reshaping Combustion Chambers
Porters typically don't pay enough attention to the combustion chambers. The basic idea is for the chambers to be an extension of the valve job all the way into the cylinder. Following this principle, with wedge heads, a heart-shaped combustion chamber is ideal. Judson tells us, "If you perform a valve job, you have to work on chambers. The goal is to keep velocity even all around the valve." If the chambers aren't optimized, the penalties can be severe. "On one of our race heads, we cut 1-2 cc of material out of the chambers to get some extra piston clearance for the aluminum rods we were running. Our flow dropped from 410 to 385 cfm. It just goes to show you every little thing on today's heads is so much more critical than on the junk heads we had 15 years ago."
Port Velocity & Flow
"I tell my students they'll spend the rest of their careers trying to find the right balance between flow and velocity. A simple way to look at it is if you increase the cross-sectional area of a port and pick up flow, then you haven't hurt velocity. On the other hand, if you open up a port and don't pick up flow, you've hurt velocity. It's a delicate balancing act, and air velocity is not uniform throughout a port. There's the average velocity and localized velocities, and air moves faster toward the center of the port, where friction from port walls doesn't affect it as much. The trick is mini-mizing the differences between localized velocities. If air moves too fast, it won't want to make the turn at the short-side radius, which makes a big difference between localized velocities in that part of the port and hurts flow. Although there are people who swear by high-velocity ports, it isn't nearly as important in a high-winding motor. The lower the motor's rpm range, the more velocity you need, but you can't make runners big enough if you want to turn high rpm in a race motor."
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.