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You may remember my fuel pressure fluctuation problem I described a couple of weeks ago. With the boost pump off, I would see pressures in the range 19 to 26 psi on my VMS EPI800 gauge. That was with no restrictor fitting (transducer line connected to the output of the engine pump at a tee with the transducer mounted on the firewall).

I made a restrictor with an orifice size of 0.040, #60 drill size. With the restrictor installed, the fluctuations were noticeably smaller, but this time I wrote down pressures for a couple of minutes so I could calculate a mean value and standard deviation. The mean was 23.9 psi with a standard deviation of 1.8 psi, but the fluctuations still seemed a little too large.

I made another restrictor with a orifice size of 0.020, #75 drill size. With this restrictor installed, the pressure is now fairly steady at about 24 most of the time. I wrote down pressures for a couple of minutes again (the display updates about once a second). This time the mean pressure was 24.2 psi with a standard deviation of 0.68.

I seems that if you want to measure fuel pressure at the pump output, and you have a VMS or other system that is very fast, you must install a restrictor to damp out the pump action (mechanically average the pressure a bit).

Mike Reinath
360 Standard Build
N3602M

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