Heres what I have gotten out of mine

O.S. 1.40 RX with Karl Mueller header 2 5/8 rise. Our own homemade c.f. 3 baffle pipe. Total pipe length is about 28 in. and over all length is about 36 in. from engine exhaust.

15% Cool Power, 17x10N APC, O.S. "F" plug netting 8700rpms and 92.5 db at 800' above sea level and 85 degrees.
Good transition and unlimited vertical in a 10.5 lb airplane.

I have a 17x12N APC but havn't tried it yet so looking forward.
Dont have the particulars on length to first baffle at this time.
But so far it seems like a good setup.