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914 fuel pressure

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domino456(at)sbcglobal.ne
Guest

Posted: Sun Jul 01, 2007 5:11 pm Post subject: 914 fuel pressure

Can anyone please tell me what the fuel pressure should be on  a 914; with and with out the second (auxiliary) pump.

  

 Thanks in advance for the help.

  

 Lynn

 Wisconsin

   [quote][b]

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rparigor(at)suffolk.lib.n
Guest

Posted: Mon Jul 02, 2007 8:07 am Post subject: 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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