I think I've figured out what you Poms might be up to!
I was looking at pictures on SOS of early Marston outboards. In particular the rather curious 'parsnip' exhaust pipe. This gave rise to an investigation. I back engineered / computed the exhaust section profiles & length & made an interesting discovery.
The exhaust pipe forms an almost perfectly tuned expansion chamber design when the engine exceeds 35,000 rpm.
Rex NZ