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At 09:00 PM 6/16/2004 -0500, you wrote:
Now, the Subie guys are
contemplating a re-design of their fuel systems. I'm wondering now
if I should make any changes to my fuel system (2 wing tanks, 6-port
Andair valve, dual EFI pumps mounted on f/w)? Comments would be
welcome, especially from those that have been flying for a
while.
Mark S.
Hi Mark,
We're basically copying the fuel
system that's used on the cars, and you just can't go wrong with the
basic design. Where we get into trouble is when we make changes,
even though we may not realize it.
After reading your message, the
primary thing that sticks in my mind is- keep your pump and lines
relatively cool. I think
a little common sense goes a long ways here. In a car, the pump is
usually in the tank, or very close to the tank, so it's effectively at
OAT. There's a long line between it and the engine compartment, so
even if you boil fuel in the engine compartment, the pump is still
submerged, and will blow the air right through the regulator and back to
the tank. As long as you don't do something silly like run the fuel
lines next to the exhaust, the only time you should have a problem with
fuel boiling is after shutdown.
I have to admit that I'm not
personally comfortable with having the EFI pumps on the hot side of the
firewall, unless you build a box around them, and supply lots of cool
air. I have mine in the cockpit, along with the filters, and
regulator. The only thing in the engine compartment is the lines
that absolutely have to be there.
At first glance, it would seem
dangerous to have high pressure fuel in the cockpit, but not when you
think about it. First, it's extremely unlikely that a line is just
going to blow apart (I don't use automotive rubber lines anywhere).
If there's a leak, it's going to start small, and you'll smell it when
it's still just a tiny leak. If this were on the hot side of the
firewall, it would have to be a big leak before you smell it, and a fire
might actually be your first sign of trouble. Even if you had a big
leak in the cockpit, what's going to start the fire? You can have
all the gas you want, as long as you don't light it with something.
The pumps, filters, and regulator are
all cool, so the heated fuel is minimized. I run both EFI
pumps in flight, so there's a ton of flow as well. In this
configuration, there's absolutely no chance of vapor
lock.
I realize that opinions vary on fuel
systems, so take the above comments for what they're worth.
Cheers,
Rusty
Hi Rusty,
Thanks for your comments. Yes, I have been considering relocating
my pumps back to the fuel tanks. I thought that I would wait to see
what solution the Subie guys come up with before I rip everything out and
start over.
A couple of questions... Your pumps, filters, and regulator is in
the cockpit, not in the tank as in 99.9% of the automobiles today?
How far are your pumps from the tank fitting, and what size line feeds
the pumps. Do you run mogas? If you do run mogas, do you know
the vapor pressure of the fuel you're running? Do you test
it? I have read that mogas has a lower vapor pressure, which means
it can vapor lock easier than 100LL. This may, or may not be a
problem, depending on the altitudes you fly. The point is that if
your pumps aren't in, or very close to the tank, the low pressure created
by the pump's suction could cause a vapor lock at the inlet of the
pump. It may take a hot day at altitude for this to happen (depends
on the fuel). If this happens, your engine will quit due to fuel
starvation. Have you tested your installation to see if it will
re-prime if you inadvertently run a tank dry? That's what happened
to one Subaru flyer resulting in an off-field landing and
roll-over. It would be prudent for all us "hairy chested
aviators" to test for this on the ground. First time this
happened to my setup, the only way I could get the fuel flowing again was
to crack a line open down stream of the fuel pump and let the air bubble
escape. Obviously, that wouldn't work in the air. I now have
a small (.025") orifice that bypasses the fuel pressure
regulator. That appears to have solved that problem. I'm
still concerned about vapor lock on the inlet side of the pumps.
Mark S.
(Could use some high-octane spirits about now)
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