Seeking Advice on MAP and RPM Limits for Rotax 912ULS (Post-SB-912-079)

Hi Andrew,

I'm not an expert, but based on the fact that your engine is turbocharged, you should be limiting the manifold absolute pressure (MAP) according to the RPM using the table provided in the service bulletin. A turbocharged engine will naturally run higher MAP values since the turbo is increasing intake pressure above ambient levels, which is why it's crucial to follow the recommended limits for different RPM ranges.

Regarding the risk of detonation, yes, high MAP can increase cylinder pressure and temperature, leading to detonation. Since the Rotax 912/914 doesn’t have traditional CHT monitoring, signs of detonation would typically include engine knocking, rough running, or even a sudden loss of power. Monitoring exhaust gas temperature (EGT) trends closely could also help—an unexpected drop in EGT could indicate detonation.

Hope that helps!

Great, thanks Claude.  I'll need to explore other avenues on the MAP with the turbo, as I'm not able to run within limits at any normal phase of flight.

Thanks for the detonation tips

Andrew

Andrew Retallack wrote:

Thanks Claude...

A couple of follow-ups to help my understanding, please. 

1. My 912ULS is fitted with a turbo by the manufacturer - my MAP consistently runs above 30" at around 5000-5200 RPM in cruise, and is well within the POH operating limits.  It would be interesting to understand how the turbo affects MAP and the load on the engine

2. My understanding of the SB is that the risk of high MAP is detonation.  How would one detect this on a Rotax which doesn't have CHT - assume engine knocking or rough running?

Thanks

Andrew

While not knowing specifics about "the manufacturer" regarding a turbo added to a stock (?) 100 hp 912ULS, this certainly got my attention. They seem to be "turbo-normalizing" rather than "turbo-charging" which is wise considering doing so requires reducing the engine's compression ratio as is done with the 914. However, notice the Rotax chart from the service bulletin limits MP to 25.3" when carb heat is being used. Compressing air by itself will heat it and the turbo itself is hot. Have you measured the temperature of the inlet air post-turbo? I'd guess that it's a warm or warmer than one would get with carb heat, especially if there's no intercooler installed. It would make me more than a little nervous without knowing exactly how "the manufacturer" has dealt with this relative to the Rotax SB.