What diameter of exhaust for 1000ds engine?

[lechef]

I know this is an impossible question to answear but what is your take on 40mm-45mm-50mm fulle exhaust on 1000ds engine?

Im looking to get my self an exhaust for my 1000 ds projekt which has been tuned a bit 1123cc big bore, hc pistons, cams, porting etc.

Today i have a 2-1 termi full exhaust with 37mm inner dia en aprox 40mm outer dia, that is 3mm smaller then the exhaust valve. Im thinking that that is a bit small and restricting the engine.

I have found a spagetti style header from termi that is 50mm for a good price but im thinking that it might be to big and ill be loosing torque with it. Its the same dia as many 9** exhaust.

Kaemna uses 45mm for there deamon bikes but that doesnt really tell me anything.

Drop ideas impressions etc.

[CrimsonCloak]

On most of the track/race Ds1000 bikes that I've seen, 45mm is used. I have seen a few 50mm exhausts on bikes with big valves, porting, 1080cc overbore, high compression, nemesis etc. The 50mm systems seem to work well on bikes with the bigger displacement, high compression, and bigger valves with more efficient ports.

We tried putting a larger system on one of the milder bikes (1080 kit and cams only), and it made no more power than the 45, with some rideability lost in the mid range. We also tried a 45 on one of the built bikes (1080kit, big valves, cams, porting) and it did lose a small amount of power at the top of the range. It wasn't a large difference, and only came in to play in the last thousand rpm, but the motor just didn't seem as "happy". I think more than making more power, it allowed the peak to hold on longer into the over-rev which was nice.

I'd say if you have an overbore and cams, a 50mm system isn't really required. Add porting and oversized valves to the equation, and it seems like a good idea.