[torsen] Re: [urq] how much boost?

QSHIPQ at aol.com QSHIPQ at aol.com
Sat Mar 2 08:58:03 EST 2002


Guys:
Tried to do a search on Pressure Ratio posts I've put up over the years, no 
success.  The "formula" is 
PR = Atmospheric Pressure + boost Pressure/atmospheric pressure

So

Boost = 17 
PR = 12.2 +17/12.2
PR = 2.4

 Translating that to sea level:

Boost = 20
PR = 14.7 + 20/14.7
PR = 2.4
Turbochargers and their efficiency/output MAPS are always plotted using the 
Pressure Ratio vs output.  Most turbos from KKK peak their maximum 
efficiencies right around the 105,000-110,000 rpms.  Spinning them faster 
than that quickly leads you toward the heat zone.  After 140,000rpm, your 
risk of boom is really high.  KKK doesn't list 150,000rpm on most MAPS, as 
one turbo rebuilder told me, "at that point it's not a turbo anymore, it's 
parts".  The k24 BTW, is a 2.4PR turbo max.  That said, I've seen plenty of 
guys running them at 2.5 and above.  This is not healthy (you are way 
overspinning the turbo), I'd definitely consider 2.4PR as the maximum.   Many 
have argued that slight peaks over max are ok.  I understand that, but you 
best make sure EVERYTHING else in your engine bay is top notch, you are 
playing with fire.

Yes, fronts, humidity etc will affect your baseline, but even the highest 
pressure highs and the lowest pressure lows should effect this less than 1a.  
Most of the information on that site, Bruce, is for figuring/tuning N/A cars. 
 Since audi turbos (and most others as well) are altitude compensating, HP 
equivelancy is commonly figured at 99% for a stock car.  This does change as 
you mess with boost levels, as you are taking out the safety margin that audi 
designed in the turbo/computer for altitude compensation.  You guys at higher 
elevations should be *very* aware of what effects altitude has on your modded 
tq, especially you 1.8 and 2.7tt boys.  Even some of the best tuners from the 
faterlund learned about this the expensive way.

WRT urq's, remember that you have a pressure transducer that only accurately 
reads to 1.95bar.  What this means is you have a PT that can differentiate 
between 1.95bar and 1.94 bar, but can't differentiate between 1.95bar and 
1.96 bar (basically anything over 1.95bar is interpreted as 4.9v -max).  
Pounding 2.0PT at 2.4PR will cause PT failure, or more likely, inaccuracy.  
Something to consider as you are messing wit dem dere Wastegates.

If I find some more my more nerdy posts on the subject, I'll let you know.  
This should be what you need for this discussion.  BTW Brandon, the Blue 
Streak was running 17-18psi peak 15-16 continuous at Steamboat.  

HTH

Scott Justusson
QSHIPQ Performance Tuning
Chicago IL
'87 5ktqw (2.5PR)
'84 RS2URQ (2.8PR)
'83 URQ (2.2PR)


In a message dated 3/1/02 5:27:16 PM Central Standard Time, bbell at surview.com 
writes:


Perhaps the ratio you are referring is the pressure ratio?  There
is a maximum pressure ratio the K24 should be run. 2.8??  Since
atmospheric pressure at 5300' is roughly 12.2 psi then boost at a
2.4 PR would be (12.2*2.4)-12.2=17 psi.  At sea level this would
work out to around 20 psi. Don't forget to turn down the boost
over Vail pass:-)

Here is a handy link for computing absolute atmospheric pressure:
http://home.sprynet.com/~rshelq/calc_hp.htm


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