Differenzial fuel pressure sensor on position p2

Back to  the item 8 screw. Referencing IPC 912i series ED4/Rev0, Feb01/2019, in the index section there are two entries for intake manifold/airbox. The first entry is for the original 912is. It shows item 8 as one screw. It also shows item 21 as being a "snap-in" wiring harness support. The second index entry is for the 912iS Sport. It shows that item 8 is now 4 screws which attatch item 21. 

Another issue is; going on line with Fly Rotax, selections for either 912iS or 912iS Sport IPC are available. However both show the original 912iS information. It appears the sport illustrations are in error as they do not conform with ED4/Rev0,Feb01/2019.

About the absolute pressure sender; The issue is that the engine sensors mounted on the 912is engine are an input to the engine computers (CPS"s). There is no fuel pressure sensor on the engine. Fuel pressure is regulated by the mechanical regulator which is referenced to airbox (MAP), therefore while the CPU's have MAP and Atmospheric pressure they have no fuel pressure reference to put fuel pressure data on the CAN buss. The pilot would like to monitor fuel pressure so an absolute fuel pressure sensor is added to the aircraft fuel sytem as outlined in the installation manual (just before the final fuel filter) and its output is routed the an electronic display system (EFIS). The EFIS has all the data now to use the formula Fp= Analog (absolute) fuel pressure+ambient (atmospheric) pressure-MAP  and displays the true engine fuel pressure. No differential guage is required. If the aircraft had no electronic display system (really bad if using a 912iS) a differential pressure guage could be mounted in the instrument panel and connected as per the diagrams shown in the install manual with the MAP going to the low side and fuel, from just before the final filter, to the high side. The guage must be a diaphragm type NOT A PISTON TYPE.  Big problem, most differential guages are to big to be used.

The intrest in locating an airbox source for a differential guage is to confirm that the EFIS and  the differential guage agree on what the fuel pressure is. In other words a mechanical system is put against an electronic system.

Interesting thing. The CAN buss has fuel flow available. It it in grams per second and is derived from injector open time. My EFIS accepts the data and I have it setup to display gal per hr. No separate fuel flow sensor required.

The problem with using an absolute fuel pressure gauge sender is that many EFIS Displays assume that the pressure measured is the pressure displayed and there are no differential calculations available, just a linear calibration to correct for a 0-7 psi sender as opposed to a 0-70 psi sender. 
The EFIS manufacturers may not have anticipated Turbo-Fuel-Injected engines in the original design.
Your EFIS will vary!

Marco, the original poster, is asking about a Garmin G3X EFIS installation.
The Garmin G3X installation manual states, on page 24-10, that for the Rotax 192is and 915is engines you should  ... 
" Use UMA 1EU70D differential sensor for fuel pressure. "   (70psi)

And for the Rotax 914 engine, you should  ... 
" Use UMA 1EU07D differential fuel pressure sensor. "   (7psi)

The Garmin G3X is a "What you measure is what it displays device." 
A differential sender is what he needs.
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An ABSOLUTE sender requires taking the Map and Ambient Senders into consideration, both of which will have their own inherent small but not negligible errors that will add to the total measurement error.

A single sender's error will be preferred to the summed errors of three senders.

You would be better off using a GAUGE Sender and then adjusting only for MAP.  Two errors.
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Why all the aversion to Differential senders???
Every sender is Differential at its core.
The "Differential" Senders just have an added thread on the "Ambient"  Port.
Leave the Differential Port disconnected, and you have a GAUGE Sender.

Thank Bill,

I've order differencial one

MArco

The blank screw next to the hose nipple feeding the pressure regulator can be removed and does provide access to MAP.  The Garmin G3X installation manual, page 32-3, notes using Rotax hose nipple 840-672 with sealing washer 230-415 to attach the differential fuel pressure sender hose tho the air box.  This specified hose nipple does have 6mm threads that fit into the port, but the problem I had is that the threads in this port are recessed, and the threads on the hose nipple are very short and would not reach.  I could not find a 6mm thread hose nipple with longer threads, so I ended up buying a stainless steel 6mm male/female stand-off with longer threads (13mm) and drilling a hole down the center of it.  This was then installed first with a sealing washer, followed by the hose nipple with a second sealing washer.  It works great and looks super clean.  I liked this method better than putting a T in the fuel pressure regulator MAP hose, though either will work.