Hey Bill
Looking for some advice from someone that knows I purchased a WM 60 size Extra that weighs in at slightly over 8 pounds... I am trying to decide on a two stroke engine for it and it looks like it is coming down to a .90FX now that the "problems" seem to be resolved... Can you tell me what the peak torque rpm is on the .91 ?.. Also what prop (APC) would keep the engine in this range and produce the most torque ... Seems like I read somewhere the peak torque band is around 9K on this engine ?... I looked at the 1.20 but that really seems like overkill for this application Although..........
Thanks
IC

We don't have an RPM/torque number for the O.S. Max .91 FX engine. It hasn't been released by O.S., if they have it.

We normally prop the .91 FX with an APC 15 x 8 and turn it in the high 8,000's. This is in the range that O.S. has recommended. This works mainly to keep noise down. You can go to a 13 x 8 prop, or even a 12 x 9 or so. The engine will turn up much higher, and also make a lot more noise. Propping the engine to run in the 11,000's won't hurt it, but you'll need to play with different propellers and see what kind of performance your engine/airframe combination gives you.

Thanks Bill
Hard to believe a company like OS doesn't have figures on that...
Noise is not an issue... Just trying to keep from buying an assortment
of expensive big props to play with...
Thanks
IC

Any one that comment about this, I ran my <font color="#000000">OS</font> 91 FX with 14X6 <font color="#000000">prop</font> to get 11 krpm +, instead of 16x8 that I was using before (8 krpm). Now I heard a metal knock, too metalic, at the beginning was somekinf less lowder and less frquent, at the end I decided to land after 3 or 4 mins of flying to chech id muffler or something was loose. Nothing all in the right place.

Have you heard something simiar in your engines? colud be some called "Knocking" due to the fuel?? Thanks in advise for uour comments and advise. <span class="info"></span>

With many engines over .90-size or so, there is a "knocking" sound.  We've experienced it on a number of engines in a number of brands.  This affects 2-stroke engines of larger displacement.  Usually it's heard in low- to mid-range running.  We have performed repeated tear-downs of engines and have never seen evidence of metal-to-metal contact that could make such a noise.  Our only conclusion is that the sound is part of the exhaust note and is made by unburned fuel continuing to burn after it leaves the exhaust port.