Anyone able to confirm different pistons and compression ratio?


From Dec 2002 Excellence Magazine

Thanks for the letter. The Present DME has the capability to store data in a cache that is refreshed repeatedly to keep the data used closer to the cpu. This particular method is called a stack memory. If you are repeatedly running the car in a fairly normal driving configuration, most of the curve and enrichment data for that driving will e in the stack memory. The rest of the data is stored in e-prom and is used only when that type of driving or driving condition is encountered. Then that data will remain in the stack memory.

So what happens is that the computer cpu does not have to go to the stored memory information which takes longer to fetch the data it is looking for. It also does not have to clear the previous data from the cache memory in the stack which the engine must run through until the information it is looking for for the changed conditions appears. Once used, it remains in the stack until displaced by other information for a different condition. The memory is divided so that specfic informaton (like ignition advance) is put into a given memory area (address). The computer then looks into specific areas of the stack for information based on what the engine sensors are telling it are happening.

The X51 is more than a DME change. Your car is quite special as it has higher compression (different pistons) and a much different enrichment profile and advance curve. The Porsche technicians lso have a different fitting and assembly procedure so as to optimize the components. Essentially, all components are weighed and fitted per spec and the lowest weight rods and pistons are matched and balanced. WHile it may seem that your engine is still ordinary, it is not. In 1973 I purchased a 911S from a dealer that had this type of prep. It was only $450 in those days. But the car was much quicker than other 911S' and had the suspension tuned as well. I showed and did some autocrossing with the car for 16 years. The present owner is a collector and was very impressed at the performance compared to teh other 911 S cars he had looked at.

I would leave the engine alone. Higher compression engines do perform better at low-end as well. Information on how much advance or retard is set for the vario cam and the intake runners is something Porsche will not divulge. I have figured that they are doing a different type of testing during engine development than any dealer or independent tuner can accomplish. The intake balance system has to be programed to match the adance curve and enrichment. In addition, the cams and timing must be held to a specific curve to accomplish adequate performance.

Despite your misgivings about driving it in Metro New York, it will not affect the behavior of your engine. We have the same kind of traffic problems in the San Francisco area as New York does. The real problem for the engines is the heat from prolonged driving in traffic. That is also what sets the memory map to a see data from a less-than-aggressive driving style.

At this point the best info I can give you is that you should just drive the car as you normally would and enjoy an option that most other 996 owners will envy you for. You may have to adjust your driving style as I have for a Tiptronic model.

Jim Pasha