When trying to match a camshaft to a set of heads like this, it is important to know the amount of flow differential between the intake and exhaust ports. For example, let's say you have a traditional Yates head that flows 400 cfm in the intakes and 250 on the exhaust. If you divide the exhaust number by the intake's flow, you get 62.5. Exhaust flow is 62.5 percent of the intake flow, so we call that a 62.5 percent head. Chevrolet's 18-degree head has a real good exhaust port, so in most applications, you would not have much more exhaust lobe duration on the cam than you would on intake duration. It is usually around 4 to 6 degrees, and no more than 8. Sometimes, you can even run a straight pattern on a cylinder head such as this.

It's exactly the opposite with a lot of stock Ford castings that don't flow much better than 50 to 65 percent exhaust-to-intake. On something like that, in order to get the exhaust out of the cylinder, you might need the exhaust lobe duration to be 8 to 12 degrees longer than the intake.

Of course, a high-flow exhaust port isn't the primary determining factor when it comes to building power in a race engine. Compare the 18-degree Chevy cylinder head to a Yates Ford head when it first came out. The Chevy head's intake port isn't as good, but its exhaust is far superior. On the Chevy, the durations are very close to each other-typically 260/264 (intake-to-exhaust) on a low-rpm engine and 270/274 on a higher-rpm application. The splits are usually 2 to 6 degrees. The traditional Yates head needs about 12 more degrees on the exhaust than the intake. The exhaust port doesn't flow nearly as well, but you can make up for that somewhat with more exhaust lobe duration. But the results on the track proved that the Yates head was superior, and Chevrolet was forced to redesign its cylinder heads and come up with the SB2.

A lot of people say that a 75 percent cylinder head is the perfect balance. That's a good guideline but not necessarily true for all applications. Generally, an 85 percent head needs 6 degrees more exhaust than intake. As that number increases to 90 percent, the split drops down to nothing. If the percentage drops to 50, you wind up with about 20 degrees more exhaust lobe duration than intake. Your OE Ford heads are in the 60 percent range. The R-type heads are in the mid-to-high 70s, and your Yates head is back down in the 60s. Those numbers don't necessarily determine engine power, but they do help you predict what cam you will need to make them run best. The best heads on the market are clearly developed around the intake port. When the intake is perfected, the best possible exhaust port is designed with what is left.Billy GodboldCompetition Cams800/999-0853www.compcams.com

Torque Talk
If you're running a steel-ring gasket that doesn't have much crush, you may want to adjust how you torque your head bolts. This is especially true if you are running cast-iron heads. The new gaskets with steel rings have very little to no crush, and that can lead to cracks in the head if too much torque is applied. The inner row of head bolts will need more torque than the outer row, or the bolts closest to the exhaust. I cannot give you exact figures because there are too many variables involved, but you want at least a 15 ft-lb difference between the intake row of head bolts and the exhaust row. Without this difference, you can bend the exhaust flange of the head, which will result in a crack. That split in torque numbers might not need to be as big if you are running a cheaper head bolt, but on premium ARP-type bolts you want at least that much difference. Because Fords only have 10 head bolts, you probably will want to run a premium fastener anyway.Jeff KlaverFord Racing 586/468-1356www.fordracing.com/performanceparts