exhaust diameter ?

[Peter the blacksmith]

I have a question about the importance of the diameter of a single-cylinder diesel exhaustpipe - isn't it better the wider the pipe is ?
I just managed to buy myself a neat bike, a Honda -87 CMX Rebel former 250cc, now sporting a single cylinder diesel of 418cc. The diameter of the exhaust pipe is about 30mm (outside measured). I have no experience of diesels at all, but if this was a fourstroke petrol engine, putting on a narrow pipe is truly strangeling the engine So, my plan is to change the exhaust pipe to a 50mm steel pipe, and build a new, wider muffler.
Any comments appreciated, thanks !

[tappy]

Yep - the general concensus for these engines seems to be to give them as large a bore as possible to reduce pressure loss.
An abrupt change in diameter can increase losses even more however. The ideal is a gradual increase from the port size up to your intended flow size, with a taper of about 14 degrees.

If your engine's exhaust port is rectangular, you could try taking a round tube and squishing it at one end to make it meet. For a given perimeter a circle has more area, so if you do the squishing over a decent length, then you'll get from the port flow area up to the circular pipe area immediately after the exhaust port, and can then carry on with round pipe as normal.

[Peter the blacksmith]

THANKS a lot !