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I have a Rotax engine which, when the oil is cold, shows a mild pressure drop when the rpms increase.

Here's a video which shows it happening. The oil pressure is shown in the top right gauge, and the tach in the middle left: https://imgur.com/a/CM5F4vR

I don't have a healthy explanation for why a positive displacement pump would see a pressure drop at the output when it spins faster. So the question is: is there ever a situation where it is okay for oil pressure to drop with increased RPM?


Additional details in case case it matter:
  • The oil is fed from the oil tank to the oil cooler and then to the pump via rubber hoses.
  • The above event only happens when the engine is cold, perhaps indicating suction hoses collapsing under vacuum.
  • There is an oil bypass downstream of the oil pump, it is designed to trigger at 5bar
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  • I think you should be directing your questions in aircraft/Rotax forums or Rotax. Air cooled aircraft engines require specific engine runup and cool down procedures. Non pilots may not be familiar nor appreciate responsibilities of preflight check lists to ensure what goes up comes down in a controlled manner.
    – F Dryer
    Nov 28, 2023 at 0:13
  • @FDryer Alas, Rotax people know less than I would like. Pilots have some very strong opinions which are based much less on science and much more on "my flight instructor taught me this in 1967". The run-up and cool-down are not as specific as you might imagine, the flight manuals just say simple things like "prime as appropriate" or "don't take off below 50C oil temp". FWIW, the Rotax is not an air-cooled engine, but a hybrid water-/air-cooled one. Nov 28, 2023 at 16:16
  • OK, my Rotax info is mostly air cooled aircraft engines and familiarity with preflights, starting, engine runup to ensure needles are in the green before takeoff and landing cooldown before engine shutdown. I'm not aware of hybrid water/air cooling but run-upshould be similar. While there are opinions in the pilot community, if they're not in agreement, I would ask Rotax directly.
    – F Dryer
    Nov 28, 2023 at 16:30

2 Answers 2

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I'm not entirely sure about a Rotax engine, but most engines have a bypass valve. The oil pressure is read from the other side, which means if the bypass opens when the oil is stiffest (most resistant to flow), the oil pressure could drop when you increase RPM. If it is doing this, it could also mean the bypass may have a weak spring on it, so may be worth getting checked out.

I'd suggest if the oil pressure is still within the "normal" range, there shouldn't be any issue. If you are worried about it, I'd also suggest you allow the engine to warm up before you need to rev it, that way you'll get what you consider to be "good" oil pressure when you need the engine running at speed.

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  • Thanks! The Rotax has a bypass as well, it's designed to open at 5bar. A weak spring is an interesting idea to explore. When the oil is good and hot, I see about 4.5psi when running at the same or higher RPM. Nov 27, 2023 at 20:39
  • @KennSebesta - Can't say that's the reason for what you're seeing, and to be honest, it's probably a bit outside as far as reality goes, but it's the best I could come up with. Nov 27, 2023 at 20:41
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According to this website, the Rotax has an oil pressure regulator. An oil system that has a regulator would act just as the video shows, with oil pressure decreasing as rpm sudden increases. Notice the oil pressure also increases as the rpm drops. That's perfectly normal, especially when the oil is cold and more resistant to flow.

In the world of automobile engines, it's never ok for oil pressure to drop with increased rpm and vice versa. Car engines, however, don't ordinarily have a regulator in the loop.

Has this fluctuation happened since the engine was new? If so, it's normal. If you're not sure, contact the manufacturer.

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  • Thanks, that's nice to learn. One thing I don't understand is why a regulator would see a pressure drop when RPM increase. I would have expected a regulator to keep pressure constant. Also, from my understanding of the Rotax, until 5bar the regulator stays shut. Regarding the engine age, it's 12 years old, and I am its second owner so difficult to say what happened when new. Nov 28, 2023 at 16:13
  • When the oil is warm, the regulator keeps the pressure even, as you would expect. When cold, the oil offers greater resistance and thus causes more of a pressure difference on both side of the regulator. Anyone who has ever tried to suck air through a PCV valve would know from first hand experience.
    – Carguy
    Nov 29, 2023 at 3:37

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