Steve,
Examining the Walbro pump specifications at:
https://walbrofuelpumps.com/walbro-gsl-series-universal-inline-fuel-pumps
and looking at the data for the individual pumps enables the following speculation. It is only speculation.
If you have GSL392 pumps, as are almost all of the pumps listed now on Ebay, they have a flow rating of 255 L/hr (70 gal/hr) at 40 psi and 8 amps. If your engine could use 20 gal/hr, that would still leave 50 gal/hr returning to the tank. With two of those
pumps running at 65 psi
they should be moving 60 gal/hr each with each drawing 10 amps. If your engine was using 20 gal/hr, 100 gal/hr would be returning to the tank. This would seem to be moving much more fuel than necessary. Depending on where you are measuring the fuel
pressure and the details of the fuel flow path, the pressure at the pump outlet (and the current draw) could be much higher with flow rates this high.
At these flow rates, your whole fuel system could be a constriction and any fittings (tee's etc) may have significant effects.
That is why it would be good to establish which pumps you are using either by identifying them or measuring their flow rates. GSL393 pumps
(45 gal/hr at 40 psi and 5 amps) might be more appropriate for your installation if your present pumps have high flow ratings.
Steve Boese
From: Rotary motors in aircraft <flyrotary@lancaironline.net> on behalf of Charlie England <flyrotary@lancaironline.net>
Sent: Friday, March 24, 2017 12:24 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: Fuel Pump current and pressures
Hey Steve,
Just re-read your post with the measurements, & I'm not sure which had higher current draw. It was the bottom pump, right?
Thanks,
Charlie
On 3/24/2017 8:06 AM, Stephen Izett wrote:
Good idea Charlie.
That would clear it up as far as the manifold flow characteristics.
Just a pain in the but to get to.
Thanks
Steve
Have you tried swapping the position of the pumps & making the same measurements? It's hard to imagine it making that much difference, but the bottom pump does have a tight right turn and then a sharp edged 'T' turn to the left. I couldn't guess
how much, but that would account for at least some pressure increase.
Charlie