ah k, Ive only been playing with hillbourne alky and we are only doing 8.0's and 7.25's. I didnt realise that they had banned it, thought it was simply because it was going to be too low a cost reward ratio to make it worth it

one reason Jaysen that BB's will only turn to to about 8 is valve actuation issues start to arise. the rotating mass is not that much of an issue rather than getting the air in and out with the right timing. I'll stand corrected here but a mechanical spring has a ceiling limit of about 12-13250 rpm. Think from memory the highest recorded rev limit was by a Williams Renault, somewhere around 13258rpm. They then went pneumatic and presto a regulated limit imposed of 23500! think highest recorded is about 23851 or thereabouts!

Im sure that a CBR 250 runs mechanical springs, and they go to approx 20,000RPM from memory, but your right, the valve trains are a let down due to mass, but rocipitating mass in a BB is still huge. If I recall correctly, last meeting there was a guy in our pit talking of a test done with BB Hemi piston pins failing @ 110,000lbs, There must be some serious piston G's going on in there.

Yeah I've heard that one too, but am thinking urban myth. I have also heard of a honda gp bike doing those sorts of rpms but the engine wasn't using poppet valves but rather a reed or disc valve which just doesn't have the same issues & don't seal as well as a poppet. The issue with poppets is reaction time of springs and the valve bouncing on the seat-they have a very definite physical limit