This is a follow up to a post a couple of days ago.

We currently have three S4 engines under construction.

An alternative to porting S4 heads is to use a version, such as we did on Chris Clarke's engine, that has port sizes, valve size, cam profile, etc., identical to the RS4 engine. We have sold about five sets of these heads and can only get one more set (at this time).

If you tried to match S4 head intake ports with the RS4 manifold, you would see that there is a considerable amount of material to be removed. The stock S4 manifold does not leave much room for porting...it will get pretty thin...even if you did port the S4 head. The RS4 head intake port is about 30% larger.

When you are building a performance engine, one thing that you certainly want is a smooth flow from the intake manifold into the intake port of the cylinder head. That is more important than just having a large port at the head. We have used RS4 intake manifolds and full RS4 intake pieces with stock S4 heads and saw zero power increases...actually we might have lost power because the ports are so mismatched.

The exhaust cam profile of the stock S4 head is the same profile as the RS4....great!, but...the S4 intake cam is a lower lift and less duration than the RS4, hence, the cylinders do not fill as well with air. The heads we use on customer's performance engines are the larger ones and give the same power benefits as the RS4.

We really don't try porting the S4 heads because of the lack of material on the stock S4 manifold...the small porting you can do really isn't worth the time...especially when you can bolt on the larger RS4 style heads and then sell your stock heads for a pretty good price.

We have cc'd the combustion chamber of the heads versus the S4 and the difference is only about 2%.

We are about 5 weeks away from delivery of 81.5mm pistons we are having built for the S4s. We have seen S4 engines with scored cylinder walls. The repair choices were:
1...scrap the engine
2....overbore, sleeve, and then rebore to stock size with new stock pistons
3...or overbore once and install new, larger pistons.

The overbored engines would be about a 2.75. A nice feature of the slightly larger engine is a slight drop in the compression ratio...just right for a performance engine to run more boost without fitting a thicker head gasket.

We have contracted with Speed Channel for an in car camera for eight out of the next nine Grand-Am Cup races. We had great footage and action shots at the Daytona race. Our next televised race will be Phoenix in April.

Go Randy Pobst this weekend in Miami in my friend Micahel Levitas's TPC Racing Porsche Cup car in the Rolex Grand-Am race!! Randy will race at Sebring in a few weeks in the Champion Racing RS6 to defend his title, and then race the following weekend back at Miami with us in our #4 car.

Don Istook
Istook/Aines Motorsports