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I have the Vision Micro Systems EPI800 system in my 360, which recently had its first flight. The fuel pressure fluctuates between about 19 psi to about 26 psi in cruise with the boost pump off. With the boost pump on, I still see the fluctuation, but the pressure is about 2 psi higher. I have an IO360-B1B, and I am measuring the fuel pressure at the engine pump output (Tee at the pump output with the transducer mounted on the firewall). I also have a cooling shroud around the engine pump. I have a temperature sensor to monitor the temp. inside the shroud, so I know that it is working (22 deg. C at 8500 ft, 22.0 in. and 2300 RPM, with an OAT of 49 deg. F).

Is this fluctuation reasonable and expected? If the engine pump is a diaphragm pump,  it seems reasonable to me to expect fluctuations in pressure at the pump output, depending on when the pressure signal from the transducer is sampled relative to the position of the pump diaphragm. If the pressure is sampled just as the pulse is exiting the pump, a higher indicated pressure will probably result. If the pressure is sampled directly out of phase with the pump pulses, a lower indicated pressure will probably result. Anyone disagree?

Anyone else see similar fluctuations?

Test flying is going well. I now have about 19.5 hours on the Hobbs.

Mike Reinath
N3602M

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