# [s-cars] Fuel Pressure Test Results

Ned Ritchie Q at IntendedAcceleration.com
Mon Jul 26 19:21:10 EDT 2004

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)

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.