fuel pumps

Fran and Bud quest100 at gte.net
Sun Jan 31 03:57:52 GMT 1999


Marc,
Using a low pressure electric pump to fill a reservoir that then supplies a
high pressure pump that feeds the fuel rails is often used when converting
an old mechanical FI to EFI.  The pressure regulator or pressure relief
valve routes the return fuel from the rails back to the reservoir.
(Pretty much as you outlined) This combination can be made to work very
well. 
   
In such systems the reservoir is configured to act as a liquid-vapor
separator with the vapor coming off the top and being returned to the gas
tank through a low pressure check valve or back pressure regulator.  In a
configuration like this, the electric pump is mounted below and close to the
reservoir which it thinks is the fuel tank, so that it always has a good
head pressure at the pump inlet. The pressure on the top of the reservoir
(back pressure from the check valve) also assists in assuring a good supply
to the pump, and at the same time the vapor returns to the rear fuel tank
which is still vented. One advantage of this approach is that even if the
fuel tank level gets so low that the low pressure pump picks up air, the air
only gets as far as the reservoir where it returns off the top and  back to
the tank - and the high pressure pump never sees air until you are bone dry.

If the reservoir is fabricated with a flat side, a regular Holley float bowl
can be fitted to provide the needle valve and float mechanisms so that you
don't have to make everything.

The conversions of mechanical FI sometimes use a mechanical FI fuel pump as
the high pressure pump to feed the rails. Starting and tuning become quite
complex since mechanical pumps are generally positive displacement and the
faster they are turned the more fuel they deliver and the higher the
pressure until the regulator or relief valve opens. Sometimes a second low
pressure pump is required to provide pressure for starting until the
mechanical pump takes over.

The only down side to all of this is that it is much more complex and costly
to do right.  Spring rates in check valves, regulators, back pressure
regulators, and relief valves have to be played with depending on what
components are used, lots of tinkering time, development, etc.   

But if you use a normal EFI approach, and address only the fuel delivery
system (as you outlined) you should have no more problems than any EFI and
will avoid the problems of vapor, low fuel tank levels, and air pickup
during cornering/slosh. The reservoir design should have 1) a low or bottom
outlet for the high pressure pump feed, 2) baffled areas for the low
pressure supply and the rail return to enter near the bottom but not where
they could interfere with the feed to the high pressure pump 3) a top outlet
for the vapor return to the tank and 4)possibly another top outlet for a
pressure gage.

DONT FORGET to assure that the reservoir supply and the return from the
rails enter the reservoir into/under liquid and not into the air space above
the liquid.  Same would go for a fuel return line that was routed back into
the tank (if there was no reservoir).  OK for the vapor return line to enter
high but the returning gasoline should always return to near the bottom. 
Static electricity/spark reasons)

Most of this stuff is is "old time racer" lore learned the hard way on
mechanical FI systems, not on EFI, but the gasoline doesn't know the
difference - except that it is being delivered more accurately with EFI.
Bud
----------
>From: Marc Piccioni <mpiccioni at attcanada.net>
>To: "'diy_efi at efi332.eng.ohio-state.edu'" <diy_efi at efi332.eng.ohio-state.edu>
>Subject: RE: fuel pumps
>Date: Sun, Jan 31, 1999, 1:15 AM
>

>I will also be converting to EFI shortly and wonder if the following will 
>work:
>
>Use a low pressure electric pump mounted by the tank feeding forward to an 
>insulated fuel resovoir ( say 1 liter in size) mounted in the engine 
>compartment equipped with a float to keep the level constant. Then feed the 
>high pressure pump locally from the bottom of the resovoir and route the 
>regulator bypass outlet back to the top of the resovoir. The only problem 
>that I can see is possibly vapour being trapped at the top of the tank.
>
>----------
>From:  Fran and Bud[SMTP:quest100 at gte.net]
>Sent:  January 30, 1999 2:17 AM
>To:  diy_efi at efi332.eng.ohio-state.edu
>Subject:  Re: fuel pumps
>
>Best performance will be gained by mounting the pump low and as close to 
>the
>gas tank as possible, making the low pressure (inlet) flow circuit short.
>Most pumps don't do well with suction, but are happy pushing all the gas
>they can get.
>----------
>>From: "Mike Pilkenton" <mpilkent at ptw.com>
>>To: "DIY-EFI" <DIY_EFI at efi332.eng.ohio-state.edu>, "4/6 Cyl. Performance"
><DomesticPerformance at onelist.com>
>>Subject: fuel pumps
>>Date: Sat, Jan 30, 1999, 4:34 PM
>>
>
>>Well I'm deep into my engine conversion project and need to install an
>>electric fuel pump for the EFI engine (3.1L V6).  Question is do I have to
>>put the fuel pump back in the tank or can I install the pump up by the
>>engine.  I know I need the high pressure type made for FI engines and not
>>the carb (low pressure).  Mounting anaftermarket in-line pump up by the
>>engine sure would be more convenient.  Any advice would be appreciated.
>>
>>Mike Pilkenton
>>
>
>



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