Hi Frans
"With both pumps on (I have them in series) the flow increases to over 140
liters per hour, and the pressure goes slightly over the red line as
indicated on the "fuel over airbox pressure" gauge. Do I have a problem
here?"
First off understand that Airbox pressure is in absolute terms. The
pressure coming out of a fuel pump if you measured it would be over
ambient pressure.
That said, I hope did not add confusion, but you really should follow
Rotax manual procedure to test if you have between ~2 and 5PSI over airbox
under worst condition. Worst condition of course would be when you are
flying in thin air where the fuel pump/s need to work overtime.
Rotax simulation of thin air is to fool fuel pressure regulator into
thinking you are at a high altitude and you then need to be able to meet
minimum differential pressure.
OK this is the most critical meeting worst case for chance of not being
able to deliver fuel thus you will run out of fuel.
Next scenario is in thicker air. The fuel pressure regulator works based
on flow. It is calibrated based on a maximum amount of resistance return
flow of fuel will have going back to tank. If you increase resistance of
return flow, your regulator will be fooled into delivering a higher fuel
pressure over airbox pressure. Just a note, I was supplied with my Kit and
FWF 90 degree fittings that some were not too restrictive, some were poor
at best! A simple blow through test will tell the tale by the sound. Same
goes for straight nipple fittings, try not to use restricting fittings.
To answer your question as direct as I can---------perhaps.
Is your differential fuel pressure gauge really telling the proper tale?
Gauges are notorious for having error.
If you had parallel pumps, turning both on would in most cases blow well
above 5 PSI, Rotax does not recommend that, but plenty of folk seem to
have good results. I would be not tremendous concerned if next test I
mention proves the same under both circumstances.
Check your return line, better yet let fuel pressure regulator pee into a
pail with a non restrictive hose, if pressure within limits, then if you
reconnect your return line and pressure goes up, go after a less
restricting design.
Do test as per Rotax under worst case by cheating fuel pressure regulator
into thinking you are at altitude.
Good Luck.
Ron P.
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