Simple, a piston ring bends up and down every time it changes direction. We run our race .21's to 40,000 rpm. Thats too much for a ring to handle. Now if we had bigger slower 4 stroke thumpers........Theoreticly, I don't see how our race engines can hold up at all at speeds like that.
The piston in a RB WS9 goes up and down and back to up (where it started) 733 TIMES EVERY SECOND.
Now I'm not sure if the practical RPM fiqures for these engines actually happens on the track but they're doing at least 20,000 rpm and even that is moveing.
Even just taking it easy tooling down pit lane that engine is cycleing up and down at least 166 times a second.
A better question would be, when should we start trying 4 stroke ringed .51's
Less clutch wear, more torque, usually better traction, better fuel mileage. And they really aren't much heavier. Usually in airplanes the high-end Saito four strokes are actually lighter than the 2 stroke mills.
Would work real nice in truggies, not sure if it would have an advantage in buggy.