[s-cars] More dyno results

[Trevor Frank]

I think there is an optimum back pressure for torque and power band, but
I don't think that you need any restriction for max hp.  Look at any
high hp race turbo motor, they have a very large very short exhaust,
typically long enough to get it out of the car.  In Rally cars I have
seen a continuous taper exhaust system.  I suspect that there is some
conservation of linear velocity that they want to maintain to tune the
car for a specific torque band.  The theory goes that you need to get
the motor to work a bit to make heat to spool the turbo sooner.  Any
pulses that where there have been chopped and cut into a million pieces
by a turbo charger, just listening to a turbo motor gives you the idea
that there is only a continuous wave left.

It would be interesting to remove then retune and see how much you can
gain back by increasing timing, or the boost map..  Going from a stomung
back to stock I lost 4psi of boost.

Of course I could be completely wrong.

-----Original Message-----
From: s-car-list-bounces at audifans.com
[mailto:s-car-list-bounces at audifans.com] On Behalf Of Dave Forgie
Sent: Tuesday, September 14, 2004 7:58 AM
To: s-car-list at audifans.com
Subject: [s-cars] More dyno results

Dave:  This is not another "vote".  I am just going to modify my
previous post.

I still think that there was a loss when you removed the exhaust.
However, as I thought about it more (while shaving - ouch!), I think it
has more to do with the fact that with an exhaust system, the individual
exhaust pulses (pressure waves) leave a "vacuum" (low pressure) in their
wake.  This low pressure helps to draw out/along the next pressure
wave/exhaust pulse, and so on. However, it you remove the confining tube
(the exhaust system) and dump the exhaust pulse directly into the
atmosphere, this low pressure assistance phenomenum is lost.  As a
result, you make less hp/less torque than with an exhaust system.
Having said that, I  think there is an optimum exhaust size for every
level of horsepower and, at some point, the stock exhaust is indeed too
restricting for these higher hp exhaust pulses (pressure waves) to get
down the pipe efficiently and the end result is less power than possible
with a somewhat larger diameter system.

Dave F.

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