ptuomov

Attempted a tiny bit of engineering math on this piston ring width thing. I first calibrated a simple model of the 928 S4 engine at the upper limits of the Mahle piston book. Then, I kept the bore wall temperature and the total heat flow thru the piston constant. The result that I got was +21C/38F higher piston top ring temperature when going from 928 S4 1.5/1.75/3 ring pack to Subaru’s 1.2/1.2/2 ting pack.

Going after higher rpms and/or longer stroke will require lighter pistons and thinner rings. Those thinner rings will in turn increase the piston temps significantly, making the pistons a lot weaker. This is right in the range where Mahle piston book says that their M124 (analogous to 4032) and M-SP25 (analogous to 2618) alloys lose tensile strength and yield strength dramatically with every degree of temperature increase.