(Note, I have no intention of doing this, I was just reading https://www.rotax-owner.com/en/912-914-technical-questions/6136-912ul-fuel-octane-requirements and it got me to thinking about my 914.)

The Rotax 914 is said to be a 912UL with a turbo. If the turbo freezes, or the wastegate is stuck 100% open, the engine is rated to make max 80hp, down from 115.

In this scenario, with 0psi of boost, could 87AKI octane be used (instead of the required 91AKI)? In other words, is the 914 literally a 912UL with a turbo bolted on, or are there still other changes such as spark timing which differentiate the two?

  • Re: 87(AKI) octane in a 914

    by » 9 months ago

    Hello Kenz

    First, the 914 would likely be less than 80 HP without a turbo.  The restrictions of the muffler via the turbo housing, the different camshaft and the airbox breathing plus the jetting for the fuel flow are all wrong.  Second, no I would not even test that idea of how much HP would it make.  In the case of a 914 failing the turbo the most likely scenario is that the turbo fails with the waste gate not closing fully.  This is generally from lead or carbon deposits on the shaft or wastegate seat or butterfly.  If it is a situation of a hunting pressure issue you can turn off the power to the TCU and the servo motor will lock at the last position it was in at the time you switched it off.  There however is no guarantee that it is not partially closed and making partial boost at that time.

    On the question of fuel,  for sure if you have no boost it will react like the 80 hp.  This is primarily due to the 9 to 1 compression ratio.  If for some strange reason you wanted to just remove all the turbo material it would indeed only require 87 AKI fuel at a minimum.  When you run a boosted engine the risk is for detonation of the low grade fuel inside the head and on the power stroke.  Given this I would personally run at least a mid grade in even an 80 hp engine.  The safe bet is to always run a better octane fuel to be sure that detonation is not caused by low grade fuel.  Performance is more predictable also.  


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