[s-cars] Is 3" enough?

[Joe Pizzimenti]

On the big mean green machine, you might also want to try dumping the
wastegate free instead of back into the exhaust.  Yes, it's loud, but
you eliminate some turbulence at that juncture and backpressure will
decrease as a result.

There is a law of severely diminshing returns when you're talking
about a "bigger is better" approach to exhaust pipe diameter.

What effect did this exhaust mod have on boost threshold?

Joe

On Wed, 16 Jun 2004 23:10:29 -0600, mlp5 <mlped at qwest.net> wrote:
> 
> 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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