A Bpipe outlet diameter is 2" od, 1.825" id. Powerfactor pipe is 1.625" id. A reduction of about 26% in area. Of the few books and papers on expansion pipe design, they all recommend a stinger diameter of 60-70% of the headpipe diameter. This is a bit difficult to measure considering this is a 2-into-1 exhuaust. My guess is the powerfactor's smaller stinger size is still on the safe side, considering it's used for just about any engine from 701-1200cc. Probably not good for long periods of wide open throttle, but should suit my riding style. It seems to be a smaller stinger diameter will increase the pipes back pressure and raise power everywhere at the expense of higher engine/piston temperatures. I'll be turning some 1-1/2" schedule 40 aluminum to slip inside the existing stinger and use a second hose clamp to keep it secured in place for testing.