Hi Bob and thanks for the reply.

I honestly don't know of many modern engines that use SPI due to the fact that induction systems work so much better than years gone by and so I was some what surprised that your successful range does!

And I can well see that the reciprocating mass would be minimised by having the lighest piston possible and this would in turn make the engine smoother and easier to balance.

I was under the impression that the use of any enclosed exhaust system would greatly effect how SPI in that area would work, by this I mean that if indeed the low pressure area below the piston can induct then it can only breathe in what is available to it and that surely would be a spent charge.

This is what puzzles me and why most engine constructors who have to use mufflers or exhaust pipes steer clear of it like the plague. They state that they rely on the intake port entirely for that function and that SPIis big 'no no' because it dilutes the incoming fuel/ air charge in the worst possible way and somewhat takes away the functionality of the carburettor.

But obviously it works for you guys!
I am left thinking that either there is no sub piston inducting going on at all or the little that would be apparent at high revolutions within the milliseconds of such a small port dwell time would be inconsequential, exhaust fumes or not.

Cheers.