[s-cars] Is 3" enough?

[mlp5]

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