Thanks, @Martin's Eleven (UK). Given what you've said, I'd agree-- the exhaust seems a bit big for the engine.
@Martin's Eleven (UK) posted:The engine has got Dellorto DRLA 40s on it, but I haven’t removed them yet to see the inlet manifold size. I’m not looking for extra power, just reasonable fun up to 70-odd mph and less anti-social noise from the exhaust. The exhaust is a bit of a free flow monster - when I take it off to give it a clean (and to see if I can get a quieter muffler), I’ll take some more pics. But as you can see here, the final output exhaust from the muffler is 63mm/2.5”.
You seem like you might be a perfect candidate for a Vintage Speed canister exhaust system.
I (personally) like what you've got better, but given what you know about your engine and what you've conveyed in the highlighted portion-- something like this might be what you are looking for. New to the Vintage Speed site is a chart for their exhausts. I think ALB's chart is better for those of us interested in power above all else, but Vintage Speed seems to understand that sound and packaging is more important to a lot of people than getting everything possible from their engines.
Given this chart, I think I'd go with a 143mm in a 38 mm primary (1-1/2").
As an aside, a guy can get really, really deep into exhaust theory. An excellent resource is A.G. Bell's book, 4-Stroke Performance Tuning, particularly the chapter on exhaust design.
... but at the end of the day, the exhaust needs to meet 3 different criteria:
- It needs to physically fit. This is a lot harder with a rear-mounted engine horizontally opposed cylinders than it would be in an inline 4 or V8 mounted conventionally in the front.
- It needs to perform well. Packaging limits what is possible here.
- It needs to sound good. Short tail-pipes on 4/1"extractor" type headers are loud. Packaging longer tail-pipes is tough.
The Vintage Speed canister fits points 1 and 3 perfectly, and are probably completely adequate for a sub 2L engine as it pertains to point number 2.
Everybody has to live with an exhaust that does no more than 2 things really, really well. If it does 2 things well and the other adequately, that's as good as it's likely to get.
Good luck.
