[Grady Clay]

Luke,

You said:

“Grady,
You mention 2.8RSR pistons, I assume to run these I'd need to severely modify the heads to fit these pistons? I suppose with all the effort I'm going to i could make this a 2.8.......”

I assume you have Bruce Anderson’s book, 2nd Ed. Chapter five and particularly Pages 138-139 give some explanation. If you can tolerate (with race fuel) 11.5:1 and higher it is relatively easy with GE80 or 935 cams. The issue with RSR pistons is to get the CR lower and back in the realm of fuel available at the pump. With 98 octane, twin plugs, and proper cooling, I think 10.5 to 10.8 is pushing the envelope with S cams. Now the issue becomes how to maintain a proper combustion chamber, particularly the piston-to-head clearance.

There is not a lot more effort and machining to use 92 mm RSR than 90 mm RS, it is just more critical. In neither case are the heads “severely” modified. The difference between 8.5:1 and 8.7:1 may make some more power but the difference between 10.7:1 and 10.9:1 may make the difference between the engine living or not on pump fuel.

I would only go this high using twin plug electronic ignition with data acquisition and knock sensing feedback.

In a relatively light 911 or 914-6 this 2.8S has neck snapping performance with MFI.

Best,
Grady