Reworking Stock Two-Valve Cylinder Heads

When the Dearborn boys started putting the 4.6 two-cam motor in the '96 Mustang, our collective hot-rodding hearts sank. On paper, the trade-off from the 5.0 pushrod mill, to the 4.6 overhead-cam modular motor didn't seem to be that bad--we lost only 20 ci and we gained less valvetrain weight. But when the horsepower potential didn't seem to be there--there were no high performance parts (head, cams, manifolds, and so on) available--the future seemed bleak.

Ford SVO has jumped into the 4.6 scene with some really cool parts: a supercharger, high-flow intake manifold, exhaust head-ers, and cylinder heads. But what about the parts that make up the stock motor?

Well, maybe there's hope. In looking at the individual parts, it appears that the cylinder heads are the key to more power. By normal high-performance standards, the 4.6 heads are a sorry lot. The intake valve diameter is only 1.750 inches, while the exhaust is an anemic 1.340 inches. The ports are just as small--1.5-inches by 1.3-inches for the intake, and a 1.35-inch-round exhaust port. With a cylinder bore of 3.55 inches, there isn't a whole lot of room for larger valves.

The more we looked at these heads, the more we thought the valves and ports appeared to belong on a motorcycle. So to find out if there was any hope for them, we took them to Jerry Branch of Branch Flowmetrics; he has more experience in reworking motorcycle cylinder heads than just about anyone, so if anybody could help them, he could. After looking over the cylinder heads, Jerry said, "The best place to start is to flow-test the heads for a baseline and then saw one up so that we know what we have to work with."

The flow bench pointed up the head's shortcomings. The intake port flowed 208 cfm at .450 inch of valve lift, and the exhaust 140 cfm at .450 inch of valve lift; the measurements were taken at 28 inches of water. The valve lift was limited to .450 inch because that is the amount of lift for the stock camshafts. The valve springs had a seat pressure of 60 pounds and an open pressure of 150 pounds at .450 inch; these low pressures are acceptable due to the low weight of the valvetrain.

The cut-up head showed that any porting of the heads would have to be done very carefully. But there was a bright spot in the combustion chamber, in the form of a turbulence vane next to the intake valve. Jerry believed that if the vane was removed, it would unshroud the intake valve which would improve air flow into the combustion chamber.

After all the work was done, Jerry flowed the cylinder heads again. The intake flowed 250 cfm at .450 inch of valve lift, for a 20-percent improvement, and the exhaust flowed 158 cfm at .450 inch of valve lift, a 12-percent increase in flow. This should equate to about 25 hp. The power increase would be even higher on a motor that uses a blower. And since the aftermarket is finally starting to offer up some camshafts, there should be even more potential in the stock heads.