[s-cars] RE: tubular exhaust manifold and turbo options

[Trevor Frank]

It would really be interesting to see it's effects, larger exhaust
tubing, on em pressures, I am sure that more flow at a given
backpressure will reduce egt's and allow you better optimize timing and
fuel, same with better cam timing. 

Another take on this might be the higher the power band that you want,
the larger the turbo, and the shorter/larger diameter the exhaust to
balance things out.  You might be able to maintain the same pressure
ratio, exhaust manifold to intake pressure, with a shorter/larger
diameter exhaust and better cam timing.  This might explain my WRC
quandary.

I have a bunch of 3.5" tubing to do my exhaust system with.  Any race
car that can live with a limited power band and is turbo charged really
doesn't have an exhaust, more a deflector to get the heat out of the
body work.

Although WRC cars seem to have a constant velocity exhaust system,
tapered tube from large to small, last time I looked. I wonder if making
the engine work a bit doesn't yield better low end torque numbers due to
the increased egt's and the ability to run more advance. 



-----Original Message-----
From: MLP [mailto:mlped at qwest.net]
Sent: Wednesday, March 17, 2004 10:13 AM
To: Trevor Frank; Audi20V_Kruemmer at yahoogroups.com
Cc: s-car-list at audifans.com
Subject: RE: tubular exhaust manifold and turbo options

>-----Original Message-----
>From: Trevor Frank
... snip 
>
>Oh ya, I too work with a guy who used to work in the Garrett
>research group for turbo's.  He talked about spending 100k+ 15
>years ago on a test bench, just one to test large turbo
>chargers, with all kinds of crazy valves and fittings and
>continuous high temperature gas supply including it's own
>building. 

The other item that caught my eye in the Kavanaugh post were his
references to, and description of Pressure Ratio's (PR) as applied to
the turbine's side of the turbocharger equation. 

It seems like most of the various list's focus is usually on the
compressor side of the turbocharger.  Kavanaugh's comments re, "Here's a
worked example (simplified) of how larger exhausts help turbo cars:  Say
you have a turbo operating at a turbine pressure ratio (aka expansion
ratio) of 1.8:1. You have a small turboback exhaust that contributes,
say, 10 psig backpressure at the turbine discharge at redline. The total
backpressure seen by the engine (upstream of the turbine) in this case
is:  (14.5 +10)*1.8 = 44.1 psia = 29.6 psig total backpressure

"So here, the turbine contributed 19.6 psig of backpressure to the
total.

"Now you slap on a proper low-backpressure, big turboback exhaust. Same
turbo, same boost, etc. You measure 3 psig backpressure at the turbine
discharge. In this case the engine sees just 17 {sic. a typo maybe,
shouldn't this be 17.5?} psig total backpressure! And the turbine's
contribution to the total backpressure is reduced to 14 psig (note: this
is 5.6 psig lower than its contribution in the "small turboback" case).

"So in the end, the engine saw a reduction in backpressure of 12.6 psig
when you swapped turbobacks in this example. This reduction in
backpressure is where all the engine's VE gains come from.

"This is why larger exhausts make such big gains on nearly all stock
turbo cars-- the turbine compounds the downstream backpressure via its
expansion ratio. This is also why bigger turbos make more power at a
given boost level-- they improve engine VE by operating at lower turbine
expansion ratios....."

I'd like to find more information on turbine "sizing" and its relation
to the compressor cold side's operation.  FWIW, I'm looking at what, if
anything can be done to make mounting a Garrett BB series to the
stock/RS2 exhaust manifold.  It is not easily doable, if doable at all,
with the Garrett GT30r turbine scroll. 

Mike





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