Is it possible to build a motor with high compression 88 pistons and a max shaved head? Would the valves hit? Is there enough clearance? What would the HP TQ numbers be on a motor like this? Would a motor like this be legal, or too much compression? Would that be above the 10.5 to 1 compression?
Too many questions!!

My last car was a 924s with 88 motor. At the 2010 Nationals it was tested on the machine to determine combustion volume that was then converted into compression ratio.
It was determined that that motor averaged out at 10.5:1 compression.
It tested over once, under once and at 10.5:1.
I do not remember the exact thickness of the head but it was deemed to thin after the head thickness rule came into effect.
The was no issues with the valves touching the head. And that was with new valve seats. The motor was built by Rusikof in Ca. He had done many of the motors for the POC guys in 944.
At Miller I believe that the car chassis dynoed out at 138 RWHP and 140 TQ.
I do not know what HP and TQ numbers the DA system that NASA was using at Nationals was.
But I am sure that Ryan has that info.
That particular car has tested as high as 145 RWHP at Buttonwillow in Ca. With the same configuration.

Dave,

So your Nationals motor had a 6R head?
If so, they're obviously capable of making good HP.

Ken,

There is a different max shave spec for low compressin, and high compression cars in the rules. A head shaved to the max by the low compression piston limit, would results in a compression of well over 10.5:1 if put on a motor with high compression pistons.

The more you shave the head, the closer the valves com to the piston, regardless of wether it's a 9.5 or 10.2 piston. The bigger dish on the 9.5 pistons is not in a place that it adds valve clearance.

Putting new valve seat in, as Charlie mentions, can make the valves stick out more into the combustion chamber as well, but that is a separate issue from shaving the head.

A max shaved head for a LC pistons, with new valves seats would cause the most risk for piston to valve contact.

BJ, here is a link to the dyno results from the best motors at Nationals.

944 Spec Dynos

The identifiers are cryptic to keep them anonymous. If you click on the "chart 1" tab, it will take you to a plot of the dynos from 4K RPM. Any dyno that has an "H" by it is a 10.2 motor car. "L" identifies the 9.5:1 piston cars. The "merge" graphs are all of the HC, and LC cars averaged, respectively. Chart 2 compares the averages of the best two LC cars, with best two HC cars.

My car, is the "0L" line (since it's mine, I'm free to disclose that) It's a little busy, but you can see it's the second highest point at 4K RPM, but drops off dramatically at high RPM. The other LC cars had 8R heads, mine had a 6R head.

This would seem to support your theory, however, Dave Dirk's results would not be consistant with that. I'll let Dave fill in the details of his results, if he feels comfortable with that.

Had I put my car on the dyno before Nationals, I would have spent some time seeing if I could tune up this curve, but I did not. Behind the wheel my car ran fine, and certainly turned in very competitive lap times, though it did seem to give up something on the top end, but then we all feel the other guy has a little more motor, right?

RacerX wrote:


The 88 motor is 10.2:1 from the factory. The rules allow all motors to be raised to 10.5:1 max, regardless of where they started. There is a different minimum thickness when using 88 pistons vs. low compression pistons.

-bj

edit: Just saw that Kuhns answered this already.