CMW 911 cylinder heads

These ‘secret weapon’ race heads may be the hot ticket for the street

By Greg Raven
VW&Porsche magazine exclusive
November-December 1987, page 94

You would think that after twenty-some years of building 911s for the street, in addition to nearly that many years’ worth of race engine development with the 911, Porsche would be getting just about as much out of their cylinder heads as could be gotten. According to Bob Cousimano, however, this simply isn’t the case. And as the patent holder for the D-shaped exhaust port, Cousimano is someone who knows a thing or two about cylinder heads.

Cousimano’s heads start life as a billet of 6061 aluminum. Compared with the aluminum castings used by Porsche, the billet has no porosity, is stronger, and has much more even cooling. This means that head flex, cracking, and hot and cold spots are a thing of the past.

The billet is then machined on a four-axis computer numerical controlled (CNC) mill. This makes it possible for Cousimano to manufacture heads that are virtually identical to one another, a big difference from the Porsche heads. In fact, during the two years Cousimano was developing this head, he came to find that Porsche factory heads are almost as different as fingerprints. Valve angle, spark plug angle, spark plug index, and other important aspects of the heads fluctuated so greatly from head to head that it was impossible to guess what value was correct. Having the blueprints was no good, according to Cousimano, because few of the heads even came close to the specified dimensions.

As you might expect, Cousimano’s Porsche heads do incorporate his D-port design. Because of the much greater flow potential of this port shape, Cousimano also spent a great deal of time optimizing the intake port to match. The result is a head that flows much better than even some well-known extensively reworked Porsche heads from a leading Porsche race shop. Rumor has it that a set of these heads has found its way onto Al Holbert’s 962. The reported increase in horsepower was enough to prompt at least one engine builder to immediately begin negotiations for exclusive rights to use the head.

At this time, these heads are so new that there are no good performance numbers available for publication. The people who know aren’t talking, and the people who talk tell of figures that, if true, are boggling. To his credit, Cousimano refused to give any credence to dyno results that he had not personally supervised. He did say, however, that his heads flow more air at a higher velocity than Andial Porsche’s hottest race head.

In the course of developing this head, Cousimano claims to have explored some other possibilities, such as multiple valves per cylinder. As with other Porsche heads, Cousimano’s heads can be fitted with twin spark plugs, and molybdenum coating of the valves and combustion chamber is available as an option.

Currently Cousimano has developed a head only for the 3.0 and 3.2 liter 911. The heads work with either normal aspiration or in turbocharger applications, although in a normally aspirated car you would want to do some work on the intake manifold to squeeze the full potential from the heads.

Depending on the outcome of negotiations, these heads may or may not come on the market. If and when they do, however, the cost is certain to be high. With 80 to 100 hours of machine work and flow testing on each head, a set of six might run as high as $15,000.

Whatever the price turns out to be, it appears that a new plateau has been created for those who must have the fastest Porsche money can buy.