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 the 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 we had 15 years ago." Port Velocity and Port 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 the port walls doesn’t affect it as much. The trick is minimizing 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."