Hi Vince,
 Can your "returnless" PWM system have a return line.....of sorts.
 Since
 the purpose of the return is only to "burp" the system, you need to
 pass
 air or gas bubbles. Copper tubing for refrigeration purposes is
 available
 in very small inside diameters. On the order of .020 to .040, and in
 lengths
 to 20 feet. The idea in refrigeration is a combination of dia and
 length
 yields a known restriction the freon must pass thru. If a size and
 length
 could be chosen to pass "air" and gasoline at an extreemly small
 rate--say
 a pint an hour at max system pressure-- the return line could be
 routed to
 any tank. A four hour flight would then put an additional HALF
 gallon into
 a tank, likewise on that four hour flight your "in use" tank would
 be short
 a half gallon. Flying to envelop limits that are going to be
 hindered by a
 half gallon over a four hour flight is just not prudent anyway.

 Dave M

Dave M -

I like your terminology: Burping.  We wouldn't just be "burping"
though, we'd also be "stirring".  We'd BURP for hot start issues
(vapors).  We'd STIR for contamination (water and oil).

To address all three of Dave Leonard's issues, the "return-less" PWM
EFI Fuel Pump system would need to purge 1) vapors, 2) slugs of water,
and 3) slugs of 2-cycle oil.  The builder's choice would purge back to
the fuel tank(s), a waste tank, or overboard.  Personally, I'd go with
purging back to the "main" tank, AKA stirring.

I mentioned your "leaky" option as an alternative before, but proposed
returning through a small orifice instead.  The motorcycle carb jets
others are discussing sounds like a place to start.

My concern with a FIXED orifice size is that the three things we want
to purge will pass at different rates.  Vapors will whisk right
through, where oil may clog small orifices.  Or refrigerant lines.

A ball valve will QUICKLY purge a fuel rail under pressure.  A quick
open-close should move contaminants right along.  In fact, it could be
part of the start-up procedure:
...
- Open Fuel Purge Valve
- Turn ON right fuel pump; Stir for 10 seconds; Turn OFF right fuel pump
- Turn ON left fuel pump; Stir for 10 seconds
- Close Fuel Purge Valve
- Check Fuel Pressure (should be 45-60 PSI)
...
(takes off on LEFT tank)

This is my leading design at the moment.  For a "returnless" PWM EFI
system it lowers power consumption and extends pump life, but still
provides a purge option for Dave Leonard's nightmares.  After all, he
DID say to trust him on this one.  :)

If other ideas are out there, let's hear them.  By the way, I'm still
looking for a fuel pressure sensor for EFI fuel systems - any
suggestions?

-Vince Orton


Vince,
the production Ford part is easy to adapt. just needs 2 screws and has it's
own oring for sealing. used on million of cars since 2001.
I haven't looked at the output of the sensor, but I'm sure the ford computer
doesn't have any goofy exciters for it, so it is probably a voltage ouput
relative to fuel pressure.
cost around $105 new at your local Ford dealer, but I am sure you can fine
one at a salvage yard. used on all 2001 and up mustangs, focus and others.
There are other sensors out there, but most require to be "excited" and
would need an interpretor board to make work. Of course if you are building
your own board......
If you need more data on the ford sensor, let me know and I can dig it up.
Larry