Ross,


Most current .91 engines, the ST-90 (and 75) included, are bored and stroked .61 engines.

This means they breathe through .61 size, or smaller passages, bypasses and ports.


This in turn means they will be biased toward lower RPM ranges than the .61, into its frame they were built.


Specifically to the ST-90, please read [link=http://www.supertigre.com/engines/supg0235-man.html]this engine review[/link], for the right info.

With a suitable tuned-pipe, it can make the open-exhaust HP figure, 3,000 RPM lower.
No real attempt was made to get more HP at the open-exhaust RPM, since the porting is pretty much up to flow capacity...

You can, however, get more flight speed through higher pitch props - not high RPM.