[s-cars] Is 3" enough?

[Trevor Frank]

It would be interesting to see if the size of the tubing was the
culprit, or if a high back pressure muffler was the real issue.  Maybe a
combination of both?  Seems like larger tubing near the turbo would be
beneficial, because the temperatures are so high, so the volume of gas
is high, further back I suspect that tubing size is less important as
the gas cools, You may be able to get away with smaller tubing.  But I
don't really know. 

Good data points though. 

-----Original Message-----
From: s-car-list-bounces at audifans.com
[mailto:s-car-list-bounces at audifans.com] On Behalf Of mlp5
Sent: Wednesday, June 16, 2004 10:10 PM
To: S-CAR-List at audifans.com
Subject: [s-cars] Is 3" enough?

No, Pizzo this is not another trick prom date question.

FWIW I've put a 3.5" exhaust on the Green Garbage can.  Before deciding
to
do the 3.5" I had the down pipe taped for a pressure fitting and spent
several days trying to get a feel for back pressure in an otherwise
stock 3"
Stromrung system.  Two things of note. 

One, and perhaps it is already either already generally known (and
discounted) by the list, or may be it is a "unique" feature to the
Stromrung
taken off the car, but the Stromrung, at least the one on my car, is not
in
fact, a full 3" (& that's OD BTW) diameter system all the way back.  The
rear muffler that came with the Stromrung system seems to be a 2.5" OD
inlet.

Second, while limited to @ 18 - 19 psi by the current AVC-R settings,
and
using a moderately high resolution Dwyer Magnahelic 0 - 80" H2O pressure
gauge, I found the current turbo / intercooler system on the Green car
would
"peg" the exhaust back pressure gauge, i.e. 80"+ H2O (80/27.7276 = 2.88
psi+) beginning @ 6,000 rpm at @18 psi of manifold pressure.  With the
3.5"
resonator & muffler, back pressure at 6,000 to redline today (slightly
colder) @ 20psi, barely reached 40" of H2O on the Dwyer pressure gauge.
I
didn't have a fitting to see how far past 80" (2.88 psi) the motor
would/could push back pressure in the system.  I am aware of some
anecdotal
evidence on another, similar vehicle set to run considerably more intake
pressure, i.e. 25 psi+, that exhaust back pressure on a similar
Stromrung
appeared to have exceeded 5 psi on several dyno pulls. 

So, all other things being equal, stepping up to the 3.5" from the down
pipe
back seems to be of some benefit, maybe as much as a 2 to 3 psi
reduction in
back pressure, once one starts pushing, and holding 18psi+ past 6,000
rpm.

OTOH, at least so far, jumping up the resonator & muffler diameter to
3.5"
has increased the noise level both inside & outside the car.  I'll leave
it
to the list to speculate on, and quantify the imaginary benefits of an
overall 2 to 3 psi pressure drop on the exhaust side of the turbo once
engine speed begins to exceed 5,500 - 6,000 rpm.

mike



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