I must have misunderstood your remarks:
"id recently purchased and fitted an untested road exhaust when i tested it for the first time at the mallorry pre season test day its stinger diameter prooved too small for the power output of my race motor "
and the later comment: "stingers are pretty basic in standard form but sonic nozzles add more complexity,the stinger was fully welded in and not exposed until i cut it open "
I took this to mean you were using off the shelf Lambretta pipes which had sonic stingers, which is why I questioned it. I suppose because this is a Lambretta & Vespa based forum, and the thread is about a Lambretta engine.
Didn't realise you were referring to non-Lambretta applications, i'll try to keep up in future.
