Thorp Air Command - T18.net

Old Fiesty has 2 fuel pumps in parallel. The fuel line tees off at the fuel strainer/drain outlet. One hose goes to the Facet electric fuel pump, the other hose goes to the Lycoming mechanical fuel pump. Each pumps' discharge hose goes to a tee with a small 1/8" npt tap for the fuel pressure gauge as it enters the carburetor.
Feisty has good fuel pressure when cool during the runup, but when hot, or at altitude, the fuel pressure reads low, sometimes even 0! So I run with the fuel pump on when the pressure indication is low. I can still enrich the mixture and observe the EGTs go down, indicating adequate fuel is entering the carb, but it's a bit creepy.
I'm planning to add a fuel computer, and a Rotec TBI unit, so I don't want to go gravity feed (yet).
I think the heat is causing vapor lock in the stagnant part of the system. Either the fuel flows equally thru the parallel circuits, at half the rate, absorbing heat thru twice as much hose, or one pump discharge has more pressure and the other has stagnant flow, absorbs heat and fuel vaporizes, effecting the output of the other pump?
So I plan to reroute the hoses (eliminate 2 hoses) and run the pumps in series like my son's RV-4.
Any thoughts?

_________________
Scott Emery
EAA Chapter 326
T-18 N18TE

One line . Gascolator to Facet, Facet to FDS (FC-10 , my set up) "Red Cube" , Red Cube to Lycoming pump , fuel pressure tee on outlet from Lycoming pump to Carb . No problems with "zero" fuel pressure , even in SOCAL Summer heat . I indicated about 4-5 psi with the Facet on and about 3-5 psi with the electric off . I know , I know ! Gravity feeds works Use as directed ! May cause baldness , problems sleeping , do not drive or operate heavy machine while taking . BOOM !

RB

Sounds like you have way too much complication under that panel. With two lines you would need a check valve. I run from the tank, to my fuel sending unit, to a Facet pump that is wired to the Master, to the carb.
Cubes

I did re-route the fuel lines as mentioned above. I noticed a check valve is mounted on the discharge side of the mechanical pump.
The results were better, but at full tilt boogie @ 4,500 ft, the fuel pressure is still low, under 2.5 psi with both pumps on, just about 1 psi with the boost pump off.
I wonder if the fuel pressure gauge is effected by altitude or temperature. It's a liquid filled "Holley" brand 1.5" unit from the Summitracing aerospace department...
No fuel transducer yet.

_________________
Scott Emery
EAA Chapter 326
T-18 N18TE

Scott, I have a very similar setup. I come out of the bottom of the tank, back towards the spar for the fuel shut off, forward (all this is in the tunnel), through the firewall with a bulkhead fitting, number eight solid line to the gascolator, then up to the firewall (#8 solid) mounted "aircraft" style fuel pump (not the Facet they put on RV's and cars), from there to the engine mechanical pump with a flex hose, then to the carb with a solid #8. No check valve installed. I have never put a pressure gauge on it, I assumed when the fan in front quit turning I had no FP and somewhere in eye site was my airport. I have been told by the Marvel people that the carb requires at least 1 pound of pressure.

Go to the Summit Racing site and look up SUM-800115 . Fuel pressure gauge . It gives a detailed brief on how temp and altitude affects the pressure reading . Significant difference . An electric gauge with a pressure transducer would be the way to go . A lot more $$$ than the $20.00 Summit gauge , but it works !

RB