Fuel Pressure Relative to Manifold pressure

[neville]

I have 260 hrs. on my 912Is installed in an SS7, and using a Advanced 5600 EFIS. Fuel pressure will show +/- 47 psi when start power is applied and drops to 44 psi when engine starts. (only one pump on for start). It stays within specs until I exceed 10,000 ft when it drops to 40psi. The Advanced 5600 uses the formula;
Fuel pressure= analog fuel pressure + ambient pressure- MAP pressure. The off-set shows up on the 5600 admin page for fuel pressure settings.

[aviator79]

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.

[JoeRuscito]

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.

[aviator79]

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.