[s-cars] Fuel Pressure Test Results

[Dave Forgie]

Ned:  Thanks so much for your input on this problem. At least I know that I
am NOT wrong in thinking it is a fuel pump issue.  Sean Douglas took your
info and tested the fuel pump pressure at a bunch of simulated manifold
pressures (but no actual flow) from 0 to 28 psig.  The actual pressures
ranged from 98% of theory at 0 psig to 92 to 94% at the upper pressures.  I
will have to test mine but it looks like we have a couple of off spec fuel
pumps.

Now the trick is how do we score some "on" or "over" spec fuel pumps or do
we just bite the bullet at go for the Audi Sport fuel pump?  (Which several
people told me was overkill/un-necessary for less than 400 hp).

Dave F

Ned Ritchie wrote:

> Just so that all comments are on the same page:
>
> Fuel pressure regulators are set to maintain pressure ABOVE manifold
> pressure.
>
> If engine is off at standard temp and pressure then fuel pressure
> controlled by the regulator should be your set pressure above the
> manifold pressure.  14.7 psig/1.01 BAR
> Therefore a 4 BAR regulator with engine off should give 5.01 BAR/(5.01 x
> 14.504)-14.7 = 57.96 psig fuel pressure
>
> 24 psig boost should give the following pressures:
>
>              (24 + 14.7) x 0.06895 = 2.668365 BAR manifold pressure
>                        (2.668 + 4) = 6.7 BAR fuel pressure
>              (6.668 x 14.504)-14.7 = 82 psig fuel pressure
>
> Then new Audi fuel pumps have been known to be weak right out of the
> box.
> Weak as in not enough flow and pressure under load.  In fact 4 in a row
> bad.  They went back to be sold to someone else.
>
> Ned
>
> -----Original Message-----
> From: s-car-list-bounces at audifans.com
> [mailto:s-car-list-bounces at audifans.com] On Behalf Of Dave Forgie
> Sent: Sunday, July 25, 2004 10:19 PM
> To: Sean Douglas; s-cars
> Subject: [s-cars] Fuel Pressure Test Results
>
> S-gruppees:  We (Sean Douglas, Dave L. and me) need your collective
> wisdom.
> We are still (I know, its boring but) trying to solve are RS2
> install-related pinging at higher boost pressures (Sean actually had
> some
> pinging before with an MTM+1 chip set).  I have contended
> (unsuccessfully)
> that the new batch of turbos is either over spec OR the OEM pumps aren't
> up
> to the task (the List said NO to both of these).  Last weekend Sean and
> I
> tested my fuel pumps fuel flow (700 cc in 15 sec at 11.75 V - within
> Spec
> and identical to Sean's).  This weekend Sean tested his fuel pump
> pressure
> with these results:
>
> With just the fuel pump on = 57 psi  (3.93 bar at 1 bar = 14.5 psi) Spec
> =
> 58 to 61 psi
> Residual pump pressure = 52 psi (spec 49-54 psi) (after turning the pump
> off)
> With engine at idle = 50 psi (3.45 bar)  (this would be with the
> manifold
> under vacuum)
> (No tests yet under load)
>
> I am confused.  In my calculations with a 4 bar fuel pressure regulator
> and
> an engine making 24 psi boost (for example), I thought the fuel pump
> should
> have been able to make 4 Bar x 14.5 psi/Bar = 58 psi plus 24 psi boost =
> 82
> psi (5.66 Bar) on a static pressure test.  If I am right, then we have
> the
> reason for the pinging - not enough fuel pressure at the injectors under
> load.  However, then I have to wonder why the OEM pump (and fairly new
> one
> at that) isn't up to snuff (back to the Audi Sport pump or another OEM
> S4
> pump?).  IF I am wrong (I have been known to be wrong) in interpreting
> these
> results and how the FPR is supposed to work, I (We) need to know why we
> are
> wrong on this.
>
> Your wisdom is required. (Please).
>
> Dave F. (on behalf of the Sean D. and Dave L.)
>
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