Airborne_Again wrote:
I’d rather rely on the manufacturer’s recommendations and not on third-hand statements from a web forum. It is noteworthy that Rotax Service Letter SL-912-016 “Essential information regarding engine behavior, performance and manifold pressure data” which strongly cautions against low rpm/high MP operations with MOGAS 95 does not have any general advice against low cruise rpms. I suppose that Rotaxes in the US run on leaded AVGAS to a much greater extent than in Europe and that may have something to do with the recommendation of running at 5000+ rpm.
If you know who the person on that forum is, then you’d understand that Roger is pretty much the Mike Busch of Rotax Engines. What he does advise against is low RPM with high MAP but he also recommends higher RPM as the engine was built for 5000 – 5500, below this vibrations and harmonics can set in which can damage the engine…..
LeSving wrote:
If everybody (including aircraft manufacturers) actually read all the documents Rotax hands out, they would be all running at 5000 rpm in cruise and fueled by lead free, ethanol free 98 octane
Exactly…..
The Rotax site says this for the 912 ULS/S and the 912 UL:-
FUEL*
min. MON 85 RON 95* min. AKI 91*
Engine performance may vary depending on, among other things, general conditions, ambient temperature and altitude.
gallois wrote:
leaded, unleaded, AVGAS 100LL, Ethanol 10
max. continuous power at 5500 1/min
What Rotax also says is, according to si-912-016, that oil changes need to be completed every 50 hours if running mainly on leaded fuel or every 25 hours if run primarily on leaded fuel, otherwise every 100 hours….
Yes that’s what is in our manual also. It also says do not use aviation oil.
If everybody (including aircraft manufacturers) actually read all the documents Rotax hands out, they would be all running at 5000 rpm in cruise […]
This piqued my interest and I researched it some more and thus found this resource where someone explained the reasons for this and so much more. So much more in fact, that I think it could be of interest to anyone using Rotax 4 strokes. It’s only in German, but deepl.com will surely make short work of it.
Ralf is an expert in all about Rotax 9xx engines indeed. He worked more than ten years for Rotax Franz in Germany and dismantled and investigated thousands of engines, hence has great experience what harmes them. He’s the “real” Mike Bush for Rotax 
Good thing is, that he’s always refering to the official Rotax documentation but translating it for normal pilots
