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Thread: Fuel Pressure Relative to Manifold pressure

  1. #11
    Senior Member aviator79's Avatar
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    Default Re: Fuel Pressure Relative to Manifold pressure

    I'm pretty sure Neville's setup is correct. The Pressure sender is probably gauge pressure, i.e. Absolute fuel pressure - Ambient pressure. and the regulator regulates relative to manifold pressure. So to read the pressure that your regulator is actually regulating, you need to add the ambient pressure back in, and then subtract off the manifold pressure to get a differential pressure between absolute fuel pressure and manifold pressure.

    It looks to me like your display is showing the gauge pressure.
    -Before start, your MAP is as high as it can be (ambient). Since your regulator maintains some specific setpoint above that, it too is at its highest value.
    -After start and at idle, your MAP is about as low as it can be, so you see a huge drop as the regulator does its job.
    -Fuel pressure increases again as you increase power because the regulator is maintaining pressure relative to MAP.

    If you can make your G3X do what neville's AFS is doing, you should see pretty constant fuel pressure at any power setting and altitude. An "offset" to me sounds like a constant bias, and not what you want, but I'm completely unfamiliar with the G3X.
    --Brian
    Flying - S7SS

  2. #12
    Senior Member JoeRuscito's Avatar
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    Default Re: Fuel Pressure Relative to Manifold pressure

    Yes totally agree with everything said.. The g3x only allows you to set an offset, everything else is done in the background and I do believe it is doing what you both describe. The interesting part is if I set a 0 offset I see about 15 psi low readouts throughout the range... of course coincidentally 15 psi is about ambient pressure at sea level (Im at sea level). So it makes me think I need to set the offset to ambient pressure when the engine is not running. In that way the fuel pressure reads 0 when engine stopped. ~44 before start and then ~44 through the whole range. This seems great to me... but then begs the question what happens at altitude? When the ambient pressure is not ~15 psi. Im really interested to hear what Eddies offset settings are.

    Thanks for thinking this through with me guys.

  3. #13
    Senior Member efwd's Avatar
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    Default Re: Fuel Pressure Relative to Manifold pressure

    Well.... I need to do some study. This is all new to me. I flew yesterday and found 44psi at 7500ft. I have done nothing to the G3X. I mentioned this long ago to the Rotax Mechanic. He told me my readings were normal. I aim to have this discussion again as well as looking into the G3X instructions since you have posted this thread. I removed, tore open and then replaced the fine filter after about 4 or 5 hours of flight because of these readings. I was looking for the reason for the low readings. Filter had nothing on it whatsoever. I now have 102.5 hours without trouble. Wish I had more for ya Joe.
    Eddie Forward
    Flying
    SS7, 912iS, Garmin G3X

  4. #14
    Senior Member JoeRuscito's Avatar
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    Default Re: Fuel Pressure Relative to Manifold pressure

    I wonder what yours would show if you went into configuration > engine > fuel pressure > and switched to manifold compensation. I would guess if anything low values would indicate a restriction on the low pressure side. Whereas a clogged fine filter would increase the pressure I think.

  5. #15
    Senior Member efwd's Avatar
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    Default Re: Fuel Pressure Relative to Manifold pressure

    I agree Joe. At the time I was looking for anything because I was finding fiberglass fragments in my fuel samples. Still am. I will check out the G3X. Thanks for the guidance.
    Eddie Forward
    Flying
    SS7, 912iS, Garmin G3X

  6. #16
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    Default Re: Fuel Pressure Relative to Manifold pressure

    HI Joe,

    I no longer have my plane but I consistently saw 40-41psi at idle and 48psi at TO rev with both pumps. turn one pump off and 5000rpm and I had 46-47psi. As this cruise fuel pressure was on the high side it worried me at first but after talking to people smarter than me I decided to monitor fuel pressure for changes rather than absolute readings - an increase of fuel pressure above the norm indicating time to change out the fine filter.

    By the way, in your earlier post you said the you were concerned about your idle pressure being below the min fuel pressure of 46.5psi. That is the max. The min fuel pressure for the 912is is 40.5psi

    cheers

    ross
    Ross
    Mt Beauty, Vic
    OZ
    Sold to Richard and Scott Taubman in OZ, 2019. Kitfox SS7,Rotax 912is Sport, Airmaster CSP 75" blades.
    Landcruiser and Cub off road camper (doesn't get any kudos on this forum!)

  7. #17
    Senior Member aviator79's Avatar
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    Default Re: Fuel Pressure Relative to Manifold pressure

    Quote Originally Posted by JoeRuscito View Post
    if I set a 0 offset I see about 15 psi low readouts throughout the range..
    I think you've got it Joe. This would indicate that you're seeing the difference between the raw sender measurement and the MAP. Neville's AFS adds back in the ambient pressure, which would make your readings perfect at sea level. I would guess that if you offset by your home field elevation pressure, you'd see artificially high readings as you climb, but they'd be more in line with the limitations in the Operating Manual. If you can confirm that is the case, that's probably what I'd do.

    If you were really serious and you can't configure the G3X to compensate for both MAP and ambient pressure, you could tee into the line from the regulator to the airbox and install a differential pressure sensor. That's basically the recommended way to measure fuel pressure on the 914. If it were mine though, I probably wouldn't go to that much trouble. I think the G3X can get you a good enough value to make smart maintenance and flight decisions even if it's not absolutely what Rotax wants you to monitor.
    --Brian
    Flying - S7SS

  8. #18
    Senior Member efwd's Avatar
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    Default Re: Fuel Pressure Relative to Manifold pressure

    The information Ross has is precisely what I was told by Brian at California Power Systems. I have been monitoring for changes to date.
    Eddie Forward
    Flying
    SS7, 912iS, Garmin G3X

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