Originally Posted by JustinL
Did some more investigating into pistons. It turns out that the S2 pistons are not forged, but the 968 pistons are and have the same dish as the S2 pistons. Problem is that the 968 pistons I have seem to be for a slight overbore. They say 104 on them, but seem tighter than they should at the top of the bore. I'll have to measure them more carefully.
Pistons are not round. They are 'cam ground' as, due to there being more aluminum across the pin bosses the expand differently. We're it not so early, I would remember where you measure them, but I don't want to steer you wrong first thing on a Monday ... I believe it is across the pin bosses.
Anyway, there is more wear half way down the bore as there is more side loading because that is where the rod is at the greatest angle (a longer rod means less angle, and less friction, but more mass ... That is the trade off). Really, 2mm is not going to affect it much ... Much. Truly, the bigger issue I could see is if Porsche designed in some 'quench' ... An Internet search will explain that in more detail ... Essentially, it is turbulence created within the combustion chamber to quell detonation before it can happen. It relies on areas of close interaction between the piston crown and the cylinder head. If this is lost, detonation can still be avoided by lowering the compression ratio further, but at the cost of response and efficiency.
Another generality is that engines with dished pistons tend to make more power than flat tops or domed (all else, including compression ratio,being equal). it has to do with flame propagation.
I believe you are right in choosing a compression ratio in the 9 to 9.5:1 range with a reasonable boost level. Less compression would theoretically allow more boost but off boost performance suffers terribly. I remember a story about the Skylines running in Austrailia.... The sanctioning body, in a effort to slow the cars down, reduced the allowed boost level. Nissan found that they were able to raise the compression to 9.5:1; thereby regaining much of the lost HP and, ultimately, making the cars faster.