Setting fuel pressure on Garmin G3X

You Guys are going to make me even crazier that I already am!!!  

The Problem is Not, "...  that the Ambient Pressure is being included in the calculation where it should not."

The Problem is that the Ambient Pressure IS NOT being included in the calculation where it should!

The MAP Sender is an ABSOLUTE Pressure Sender and the Fuel Pressure Sender is a Relative GAUGE Pressure sender.

They are calibrated to different reference pressures.

Refer to the ATTACHED Drawing.

Assuming... The fuel pressure is fixed at 45psi above the MAP and the MAP is adjusted to ~10"hg (5psi Absolute) for all conditions.

At sea level the Fuel Sender will report 35psi Gauge and the MAP will report 5psi Absolute.

This would give an Assumed MAP Ambient Vacuum Pressure of  15 Ambient - 5 Absoulute = 10psi Vacuum.

35 + 10 = 45 psi correctly Calcutaled Display Pressure.

At 10,000 feet the fuel sender reports 40psi Gauge, not because the pressure increased but because its ambient reference Gauge pressure decreased with alititude.

The MAP still reports the same 5psi Absoulute pressure.

The MAP VACUUM pressure is no longer 10psi, it is only 5psi because the ambient Vacuum reference pressure decreased with alititude.

If you INCORRECTLY continue to assume 5psi Absolute MAP = 10psi Ambient VACUUM ...  40psi Fuel Sender + 10 PSI VACUUM = 50psi Displayed.

It should CORRECTLY have been 5psi Absolute MAP = 5psi Ambient VACUUM at 10,000ft  ...  40psi Fuel Sender + 5 PSI VACUUM = 45psi Correctly Displayed.

The Formula is NOT ... P = Fuel Pressure + (Sea Level Pressure - MAP)

The Foumula IS ... P = Fuel Pressure + (Pressure Altitude - MAP) 

I wonder how many manufactures may have overlooked this fine point.

To be fair, the error amounts to ~1/2 psi per 1000 ft.  An easy error to overlook if you seldom climb over 5000.

If your fuel pressure appears to slowly increase with altitude it is nothing that you did wrong, it's just what it does.

- - -

As Jeff found out, he could correct his fuel pressure for a particular altitude with Garmin's "Calibration Factor".

4500 ft = 25.4" hg divided by Sea level = 29.92" hg  = 0.85  ... times 14.7 psi  = 12.5 

Which is just a convoluted way to tell the the display to compute the Manifold VACUUM Pressure based on 12.5 psi instead of the 14.7 psi sea level standard.

That will keep him within 2 psi from ~1000 to 8000 MSL.

- - -

Now, if you just install an actual Differential Fuel Pressure Sender, All the the corrections take place invisibly within the sender and the accumulated inaccuracies of three different sensors (Fuel, MAP, Altitude) do not need to be accounted for. 

V V V   Attached Drawing   V V V

Bill,

We are on the same page, though you were more precise in the explanation. I agree with the formula as you state it, and it’s unfortunate Garmin does not use it. Moreover, its difficult to understand why they don’t when they have data inputs for all three required variables. Instead they give you a rather odd user entered “offset” precisely where “pressure altitude” should be in the formula.

The UMA differential sender is surely the most accurate, but I decided to see how well I could do with the Kavlico sensor that came in my aircraft. At this point I’m used to the numbers and I know what is normal for my plane. And who knows, maybe some future G3X software update will get this right.  

Bill, Jeff,

Thank you both for the replies and info!

Bill, I think we are on the same page, like Jeff wrote, strange that Garmin is not taking into consideration the ambient pressure, but this is the formula they claim to have received from Rotax...

Jeff, I have one more question for you (little bit diverting from the main topic)

- What did you configure for Volts1, Volts2?

Garmin installation manual says: Rotax FADEC ECU Bus A Volts & Rotax FADEC ECU Bus B Volts 

  But how do you measure the battery voltage?

Uri,

My aircraft is factory built (SLSA) and those parameters were entered by the factory, and I have never had the need to investigate or change them.  Let’s hope someone else here has gone through this process and can help - you might need to start another thread  with a new title.  The Rotax installation manual lists minimum and maximum output voltages as well as maximum current for alternators A and B.  These might be good starting points for developing your parameters. 

Hi Jeff,

If possible, it will be of great help if you can send me a snapshot of:

1. The Engine and Airframe Configuration Page 

2. If you can go inside the Volts 1+2 and Shunt 1+2 just to see the Gauge display range and marking

attaching my cfg snapshot

Thx!!