Mildly interesting ! I started using a two into one ppe on my short stroke triumph years a go in an attempt to get more torque and s top it from being insane. I found that the outlet of the collector had t o be the same crossectional area as the sum of the two header pipes or I lost a lot of top end. The motor was fitted w ith big base circle cams with about 370 deg. duration. I found that if I opened the exhaust before 85 deg. BBDC, the noise got louder and the bike went slower. I also opened the inlet about 80 deg. BTDC, and power cut in with a big bang at 6000 rpm. Fitting the pipe allowed me to get decent lap times because it fattened up the mid-range and only limitted the revs to 9,500 instead of 10,500.
When I built the seeley 850, I used two header pipes intended for a kawasaki 900, and a good long tail pipe. Because I was rebuilding the motor I decided to compensate for the exhaust pipe by advancing the cam 15 degrees, which would cause the exhaust to open at 72 de g. BBDC. I had the cam reground and thought I was getting the standard combat profile. It t urned out to be the standard 850 grind, so the exhaust closing point came t o 27 de g. ATDC, which I felt was a bit t oo early. I'd expected it to be around 40 deg. ATDC. However the torque characteristics of the motor are excellent, and I need to run overall gearing so high that using a CR gearbox with a high first is very difficult at the start of races. So I bought the 6 speed TTI box (yet to be tried in anger).
I recently bought a standard combat cam from Brett Wolfie on this f orum. I'll wait unt il I can afford JS pistons and rods, and I will rebuild the motor again.
I had a look at the Z900 Kawasaki cam timings on Ivan Tighe's web site. The race bikes use four into one pipes with the same length header pipes that my commando pipe uses. The track cams on Ivan's site give timings similar to what the standard combat cam will give if advanced 15 degrees. Considering that the Z900 is a short stroke top end motor, using the combat cam the way I intend to use it in my 89mm stroke 850 commando engine, is quite radical.
I feel that every thing I've done to make my bike go faster has been the reverse of what Norton did to make their standard bikes tractable and user friendly in traffic. I've tapered the inlet port, filled the hole in the flywheel to change the balance factor, rigidly mounted the motor, and advanced the camshaft to where a racing motor would normally have it, and fitted a racing exhaust .
