Angle-Milling Heads For Power
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," Judson says. "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.
One of the more controversial areas of cylinder head theory is swirl, but it's a concept that packs more hype than substance. Swirl proponents say it contributes to a more homogenized air/fuel mixture, which eliminates lean pockets and reduces the potential of detonation. However, Judson isn't too convinced of its merits. "Experts can't even agree on how to accurately measure swirl, and every device that measures swirl affects it somehow," he says. "Promoting swirl adversely affects inertia and flow. While it may help with gas mileage, emissions, and to some degree power, optimizing quench is a much more effective method of homogenizing the air/fuel mixture without the adverse effects in airflow associated with swirl." In other words, quench trumps swirl every day of the week.
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."
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," Judson says. "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."
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."