RE: Is An OS 55AX The Right Engine?
Spinner,
A 13x6, or a 14x4W prop, on a normal, muffler equipped .91, is below the optimal load.
The engine would be 'over the hill', as far as its HP curve is concerned...
The engine would be spinning at higher RPM, but actual power output would be diminished.
The engines would spin beyond the RPM, where the engine makes its greatest output.
If you go to the Jett site and examine [link=http://www.jettengineering.com/engines/bse40.html]the BSE .76L[/link], you'll see it usually comes with a non-tuned muffler.
It is set to spin this prop at a rather low RPM of 10,500.
The OS.91FX/ST .91G, are not nearly as 'plugged up' as that Jett, so they are capable of spinning 13-14K effectively, with a normal muffler, or without any muffler.
But ultimate power would be achieved, with a tuned exhaust system, on a larger prop, spinning much more slowly.
If you check out the results in the [link=http://www.supertigre.com/engines/supg0235-man.html]ST .91 review[/link], you'll see the best pipe could only get maximum power equal to the open-exhaust number, proving induction does not permit the engine to make any more power.
The OS is maxed-out with a JettStream or UltraThrust, both high RPM tuned mufflers, spinning a 12x10 prop (roughly equal in load to a 13x8).
Both engines are limited by their induction, having a large displacement crammed into a rather small frame.