Good day all,

We're working on a 912ULS equipped with a 914 turbo and are having problems with temperature and power delivery.  The motor is installed on a highwing, with wing fuel tanks + a header tank and 2 auxiliary electrical fuel pumps in addition to the mechanical.  One of the electrical fuel pumps is always on, while the other is on a switch.  

We've installed an Air/Fuel Ratio sensor in the exhaust to ensure we setup the mixture correctly. 

The motor runs pretty well and smooth- with carbs balanced.  We've taken it for a flight and noted the following:

On takeoff, the motor does make a decent amount of power, and reaches a Manifold Pressure of 41inches.  On throttling back to 35 inches however (constant speed prop set at 5400rpm) , the power delivery seems weak, and it is a challenge maintaining altitude even at a low speed.

Immediately evident as well are very high EGTs.  The hottest EGT (left) approaches 900degrees C, with the right being about 850degrees C (a 50C delta).  This is well above the max allowable...

Shortly after takeoff, Water temps also shoot up, and necesitated a quick decend and landing (approaching 105-110C after <3 min; <500ft AGL)

A challenge has been adjusting the mixture to run optimally throught the rev range.  At low and high RPM, A/FR is at about 12:1; and midrange at 14:1; which should be pretty good.  This stays the same while in flight.  To achieve this, we've adjusted needles to pretty rich and tried richening the hot cylinder, without any real effect.  

Starting the 2nd electric pump also did not yield any difference.. 

Pretty large radiator, oil cooler and Intercoolers are installed - about the same used on the 915.. Decent ducting is also in place to allow flow through the coolers (this was where we started trying to solve the problem).

I would be very grateful for some advise on where to look for solutions here.. High EGT's tell me it must be running lean, but AFR says something else, and exhaust burns dark..

This is from an altitude of 4500ft, ambient temp ~ 24degrees C.


  • Re: Rotax 912ULS turbo - Overheating EGT and CHT

    by » 10 months ago

    What you have is a Frankesteined 915 engine.

    The 915 Operators manual shows MAX EGT = 950°C with a 200°C Split (Delta)
    Normal Coolant Temp for both a 912ULS and 915 is <120°C.

    The EGTs may look bad for a 912 ULS, but are reasonable for a 915.
    The High EGT do indicate it is running Lean, They indicate that it is running at a Hgher Power.

    A 912 is Not a 915 and you are severly overstressing the engine.
    A short service life with reliability issues can be expected.

    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Clicking the "Thank You" is Always Appreciated.

  • Re: Rotax 912ULS turbo - Overheating EGT and CHT

    by » 10 months ago

    Fascinating project! To be honest, despite the fact that I don't really need more power I have been caught by the idea, too. 

    However, Bill has more than a point here. Have you done anything to the mechanically stressed parts to make them withstand the extra power? One hears that some tuners weld the crankshaft and use other pistons, for instance. 
    105 to 110 degrees Celcius for water is nothing to worry about. What are oil temps like during full throttle climb and later during cruise?



  • Re: Rotax 912ULS turbo - Overheating EGT and CHT

    by » 10 months ago

    You will definetly shorten that engine's life if not destroy it on a flight one day.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

  • Re: Rotax 912ULS turbo - Overheating EGT and CHT

    by » 10 months ago

    BTT gents! A couple questions: Are you still using the same bings carbs? Which main jet? Have you changed the needle jet and needle? Which fuel pressure regulator? Why are you still using the mechanical pump? Where are you picking up the airbox pressure and manifold pressure. A picture would be nice.


  • Re: Rotax 912ULS turbo - Overheating EGT and CHT

    by » 10 months ago


    By raising the effective compression ratio to above what the 912ULS static compression ratio , will cause detonation because the octane rating of the fuel will be exceeded.

    The 912ULS has a compression ratio of 10.5 to 1 the 914 compression ratio has a 8.0 to 1. So if the turbo raises the effective compression ratio by 2.5 the 912ULS would have an effective ratio of 12.5 to 13 to 1.

    Now your looking at the problems your having. You will need race gas or maybe 100LL.

    Any way you have created a monster.Good luck taming the beast.



You do not have permissions to reply to this topic.