[s-cars] Could it be the GP RS2 turbos are "over" spec?

Robert Myers robert at s-cars.org
Thu Jul 1 18:28:28 EDT 2004

Maybe I'm all screwed up, Gary.  Reason with me.

A more efficient turbo is capable of moving more air - correct?  Boost 
pressure is partially a function of exhaust flow - you need enough exhaust 
to spin the turbo at high speed.  Boost pressure, given  sufficient exhaust 
flow, is controlled by the ECU by use of the WGFV and should, as I 
understand it, reach essentially the same levels regardless of the turbo 
unless the turbo breaks.  (Granted, the more efficient turbo should reach 
that level more quickly and therefore get on-boost more quickly.)  The 
volume of air used in the engine is a function of cylinder volume, rpms, 
and air pressure.  The mass of that air charge is a function of both 
temperature and pressure.  Higher pressure is good lower temperature is 
good (assuming more air charge is better than less air charge).  A more 
efficient turbo simply moves more air at lower rpms.

There are two main sources of heat for the charge air.  1. contact with hot 
metal of the turbo and 2. adiabatic compression.  Perhaps the air may 
remain "inside" the efficient turbo for a shorter duration and therefore 
pick up less heat than it might with a less efficient turbo.  I suspect 
that this is likely a spurious argument.  The amount of adiabatic 
compression should also be essentially the same in each case.  A volume of 
air at one pressure is being compressed to a smaller volume at a higher 
pressure.  The two pressures in each case should be basically the same - 
~one atmosphere outside and say 28 psig inside.  The two volumes should 
also be essentially equal at the same RPM and manifold pressure.  The only 
difference I see between the two turbos is that one may be operating at a 
lower rpm than the other.  They are both putting out air at essentially the 
same pressure and at essentially the same flow rate.  The only possible 
significant difference might be that the more efficient turbo doesn't heat 
the air as much as the less efficient turbo and that difference would have 
to come from heat transfer from the hot side of the turbo to the cold side 
(by conduction, I presume) and the into the air flowing through the cold 
side.  I'm having difficulty seeing why this might be the case.  The two 
turbos are basically similar in structure and should transfer heat 
similarly as well.  So, why does a more efficient turbo produce cooler 
air?  I don't see it.

At 03:01 PM 7/1/2004 -0700, Lewis, Gary M wrote:

>I still don't believe it.  You'd be seeing LESS psi from a more efficient 
>turbo, not more.  Why?  Cooler air = more mass at the same PSI.  Think 
>about it.  Fill up a basketball with 20 psi of 400F air, (hope it doesn't 
>melt, but work with me here), wait a few hours for it to cool, and the 
>thing will be pretty flat.  Push a turbo outside it's efficiency range, 
>and you get hot air, like the RS2 setup with a K24 (btdt, broke turbo 
>nicely, even after that nice man Pastore told me I would, "Drive nice, or 
>yu' gonna break the F'n turbo")...
>If the turbo is more efficient, you should see lower psi numbers, not 
>more.  Something else is wrong Dave.
>Gary Lewis
>1995.5 S6 Avant, Green/Ecru, RS2 MAF, RS2 Exhaust Manifold, RS2 Injectors, 
>Custom Turbo (K26 Turbine, Factory K26 Cold side (Audi Works '999' P/N) 
>with 50mm intake and custom Innovative Turbo compressor wheel), TurboXS 
>Type 25 Bypass Valve, Bilsteins, Eibachs, Big Reds, Spec II Clutch.
>-----Original Message-----
>From: Dave Forgie [mailto:forgied at direct.ca]
>Sent: Thursday, July 01, 2004 11:06 AM
>To: s-cars
>Subject: [s-cars] Could it be the GP RS2 turbos are "over" spec?
>  Hmmm...  One lister responded to my question about the GP RS2 turbos off 
> line and said:
>"Tim McLean tells me that Dahlbach did infact modify the turbos for 
>increased efficiency and flow, so this is not surprising to learn.  I have 
>been seeing 24-25 psi with stock exhaust (and no pinging)....I am using 
>Dahlbach software that Tim programmed specifically for my setup. It could 
>be there's something in their code that 'smoothes' out this issue?"
>Maybe/Probably.   If this is true, how do I get out of this situation?
>The Dahlback software seemed too expensive so I passed and went 
>elsewhere.  Re-tuned chips after some wide band O2 sensor results?  So 
>much for saving money on a GP.  (four letter expletive deleted).
>Dave F.
>S-CAR-List mailing list
>S-CAR-List at audifans.com http://www.audifans.com/mailman/listinfo/s-car-list
>S-CAR-List mailing list
>S-CAR-List at audifans.com


If you're not outraged you're not paying attention.

  Robert L. Myers 304-574-2372
  Rt. 4, Box 57, Fayetteville, WV 25840 USA WV tag Q SHIP
  '95 urS6 Cashmere Grey - der Wunderwagen ICQ 22170244

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