I'm going to install a couple of EI CGR-30C Primary Engine readouts in my TWINCO. I have to fill out a Configuration Sheet with redline, green and yellow arcs, min/max values.

One thing I want to measure is Fuel Pressure at the flow divider.

Can anyone tell me what the fuel pressure MAX (redline), normal (green arc), and warning (yellow arc(s)) should be? I've talked to Lycoming, the servo overhaul people, Piper, local mechanics, etc, and NOBODY seems to know. The best I've heard is 10 psi max, and I've read measured values of around 7-8.5 psi.

Thanks.

Frank

I never heard of fuel pressure gauges in a Twin Comanche. As Piper doesn't have that as part of the Type Certificate, then it must be part of the STC to be legal. As such, the STC holder has to be the one to give you those values based on their certification of the unit. I am guessing that no such values exist. The pressure would vary anyway with the engine speed and the mixture setting. I cannot see that it really helps to have a fuel pressure reading taken at the spider. If one was going to have it, the inlet to the servo would seem to make more sense.

Thanks, Kristin.

I know that the PA30 calls them fuel flow gauges, which are nothing more than a pressure gauge calibrated in flow instead of PSI.

And I was going to measure the pressure at the spider because that's where the "fuel flow" gauges are now connected. It made sense to me to put it there, as that's the last place in the road from fuel pump(s), servo, fuel lines and hoses, before the fuel gets to the cylinder. A fault anywhere downstream would be pickup up on the readout.

So maybe I should ask what the MIN/MAX pressures are at the input to the fuel servos (output of the engine and electric pumps)?

Thanks,

Frank

Thanks again, Kristin, for your thought-provoking answer.

Measuring the fuel pressure at the spider was not the best idea. So I took your suggestion, and will use the pressure at the input of the fuel servo (output of the fuel pumps).

Upon further research, I found the MAX pressure of the electric fuel pump to be 26 psi. (From the PA-30 service Manual). And the engine driven fuel pump, according to the Lycoming engine Parts Manual (Lyc P/N 75247), generates between 25-30 psi. So that's how I marked the gauges on the STC'd Primary Replacement Electronics Instruments CGR-30C.

Good-bye analog gauges; hello digital world!

 

The CGR Fuel Pressure display is only an option with the system. Makes no difference if you install or not. As suggested where you place transducer will dictate what reading you have. There is a max fuel pressure listed in the limitations section equating to gph reading. Bob