[urq] More Weird UrQ Behavior - fuel starvation

[Cody Forbes]

When it comes to fuel pump output flow is inversely proportional to  
pressure. When you decrease the pressure that the pump has to work  
againt you increase the volume of fuel that the pump moves. Since the  
engine is using basically the same amount of fuel you end up with more  
volume being returned to the tank.

This is why the fuel pump that is stock on a 5000tq, designed to work  
at almost 100psi, is the fuel pump of choice for racers and high  
performance street cars everywhere. When used in EFI systems the flow  
rate of this Bosch 040 (or 044 in out of tank version) pump is one of  
the highest flow pumps on the market.

http://www.boschfuelpumps.com/ for specs.

-Cody (mobile)

On Mar 28, 2010, at 11:16 AM, Grant Lenahan <glenahan at vfemail.net>  
wrote:

> OPK, so it sounds like ( for mysterious reasons) that the EFI  
> systems have a higher proportion of fuel returned, thus a higher  
> gross flow for any given net fuel usage. Strange, IMO.
>
> Grant
> On Mar 28, 2010, at 11:09 AM, Ben Swann wrote:
>
>> Yes, but a CIS system maintains a high head pressure 70-85 PSI  
>> system pressure with very
>> little return flow to the tank.
>>
>> EFI typically runs at much less pressure - typically around 45 PSI,  
>> and much higher
>> return flow into the tank or simply higher flow demand into the  
>> pump than the tank
>> outlet can supply and that results in cavitation.  I observed this  
>> quite clearly when I
>> used a high flow transparent fuel-filter before the pump.
>>
>> I found the pump was cavitating even at idle, but because of  
>> buffering in the lines,
>> damper, etc. the cutout was most likely to occur at higher load  
>> than when not under
>> load.
>>
>> I also think there may have been some problem even with the CIS  
>> system before I did the
>> conversion, but was not as noticable.  The later cars went from a  
>> 3/8" diameter tank
>> oulet to nearly 5/8" feed and I'm sure that helped.  Ultimately all  
>> the cars went to an
>> intank pump - no inlet restriction to the pump because it is  
>> submerged.  The in-tank
>> pump also stays cool - the external pump tends to get hot,  
>> especially when being fuel
>> starved.
>>
>> I could go on about reasons why I believe this problem exists.   
>> There was a lot of
>> people telling me I was F.O.S., etc.  In the end, for whatever  
>> reasons irregardless of
>> explanation, fluid dynamics, etc. the solution I came up with fixed  
>> the problem.  I
>> tried many other things prior to doing this.  The only other  
>> alternatives involved
>> cutting the tank and installing larger feed or in tank pump, or  
>> installing a fuel cell,
>> and I did not want to do any of this unless absolutely necessary.
>>
>> The "fuel capacitor" surge pipe/tank is an inexpensive, simple  
>> solution and works.  It
>> is especially applicable to the Ur Quattro because of the limited  
>> space to install a
>> solution like a surge tank, tandem pumps, etc.
>>
>> I'll create a web-page on this when I can and let folks know.  In  
>> the mean time I found
>> 4 pictures I took that help explain it.
>>
>> Ben
>>