Originally Posted by ptuomov

Another conclusion that we've come to, at least for our use cases, is that usable rpm band that ends at 8000rpm requires 1.2mm or 1mm rings even with stock S4 stroke. With longer than S4 stock stroke, you'll be looking at 1mm rings (since there isn't that good of selection of sub 1mm width rings yet). The physics of the ring issue are in principle simple, the intertia of the ring can't overcome the gas load. Working it thru, each stroke and rpm combination has a maximum ring width assuming conventional materials.

The next issue is that if you are going to make any sort of power with the 1mm rings in a 100mm (or larger) piston, there's a piston cooling problem. Our conclusion is that high rpm engines that need thin rings also need piston oil squirters. John Kuhn fabricated the tools to drill the piston oil squirter housings to any S4 (etc.) block.

The squirters create two interesting issues. First, in order to get light but stable pistons, one needs "Evotec" or box-in-box style pistons with very short and stocky pins. The stock 928 squirter angles aren't ideal for those, so one needs adjustable squirter location and angle tooling. Second, one needs to control windage very well at 8000rpm. One key to windage control that we've found is that the cylinders (4&8) next to the bell housing have to be bored in a certain way with a fixture, otherwise the end cylinders distort when the head bolts are torqued. Another issue is that there's a conflict between the piston to bore clearance and piston ring end gap objectives, high rpms and heat want them loose, the windage control wants them tight.

It's an endless string of little things impacting other little things with an 8000rpm 928 engine. The variable cost for another such engine isn't a stratospheric, but the required R&D amortized over a small number of engines tends to be.

Originally Posted by GregBBRD

Although this is a bit of a rhetorical question....
At what rpm do you think the 928 oil pump cavitates?

Originally Posted by Alan

And alternators - there's a lot of spinning rotor winding that doesn't want too many excess RPM's before it will lose its shape & self destruct against the stator. Yet we need it to spin fast enough to make decent power at idle.