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1. 09:07 AM - Re: 914 fuel pressure ()
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| Subject: | Re: 914 fuel pressure |
Hello Lynn
"Can anyone please tell me what the fuel pressure should be on a 914; with
and with out the second (auxiliary) pump."
Very close to between 2 and 5 PSI above airbox pressure. Look up exacts in
Rotax Manual. This is absolute pressure so it is same at sea level and
18K, same 1 or 2 pumps, it is output of fuel pressure regulator to carb
float bowls. The pressure between ambient and what the fuel pump/s output
is moot on a 914.
The fuel pump labor will change, the higher you go the harder they have to
work to maintain flow and pressure to insure the fuel pressure regulator
can maintain 2 to 5 PSI. Who cares what pressure the output side of the
pump/s is, just it is capable of netting 2 to 5PSI to carb float bowls.
You need to watch out using parallel fuel pumps you don't blow above the 5
psi. I chose series with bypass. Remember that the fuel pressure regulator
constant returns fuel to tanks, you need to make sure there is not much
restriction from FPR to tank, read exact check in manual, parallel worst
as far as blowing over 5PSI. You not only need good fuel pressure from
pumps, but good flow as well to supply motor. The Fuel Pressure Regulator
I believe is sized with not much excess pressure ability above what is
needed at 115%. From what I understand if wastegate sticks closed and you
over boost, you will fall below 2 PSI and hence run out of fuel, boost
drops, carbs fill, engine catches, etc. etc. etc. This is one form of
"Surging" that can occur. Also if filter or gasculator is clogged, or
filter in pump intake is clogged or if a pump is malfunctioning.
IMHO all planes with 914 should have a differential pressure gauge
installed. One supplier is UMA.
If for any reason you see needle fall below 2 PSI, you are in danger of
running out of fuel. Turning on second pump, going lower in altitude and
or switching to a second filter can help as well as trying a lower power
setting. Of course the most critical part of flight, right after take off
is the time you most don't want a power out. For any of the above
mentioned reasons, when you dial up full power, and the differential gauge
shows less than 2 PSI, there is a good chance the motor can quit in 20
seconds. It will run perfect till that time. Smart move to glance at
differential needle when rolling, and abort if low. If you get the surging
I mentioned, if you see that the needle is falling below 2 PSI and you
reduce power and the needle goes above, there is a very good chance that
the motor will continue to run.
You may find some useful info here:
http://contrails.free.fr/engine_914_fuel_syst_test_en.php
Ron Parigoris
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