Which Makes More Power?
"Unless rules require it, you don't want to run an iron head. The only advantage is the lower cost. All this hoopla that heat escapes too quickly out of the chambers with aluminum heads compared to iron heads is pure B.S. If you're that worried about heat dissipating too quickly, just move the water jacket farther out. How hard is that? Iron is more prone to cracking and much harder to repair; you have a whole lot better chance of salvaging an aluminum head if it's damaged. You can also port aluminum twice as fast. I remember the days of spitting up black stuff for two full days after porting a set of iron heads. Another major problem with iron is that you can't weld and add metal to it, which takes away from a skilled porter's creativity in reshaping ports and combustion chambers. The weight difference isn't too bad with typical small-block wedge heads, but the penalty is significant with big-block or canted-valve heads, or any time you raise the runners. Some circle-track guys say you need an extra 35 hp on a motor with iron heads just to make up for the additional mass, since the weight is so high up off the ground and so far forward in the chassis."
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 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 com-bustion 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.
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."