Power Upgrades, Opinions? (fwd)

["Barton P. Chambers" <bartonc@annap.infi.net>]

Hi, fellow q-heads,

This is a couple of days or more old, but given the recent discussions on turbos
and turbo-building, I thought some of you might find this of interest.  The 
delay was due to getting permission to repost this.  

Greg Brown, owner of BRD (Brown Racing Development), not the publisher of 
European Car - which used to be VW & Porsche, has had many of his 911 and 930 
projects profiled in EC.  He knows his stuff.

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From: Greg Brown
Subject: Re: Power Upgrades, Opinions?

Farzaan:

Sorry, my host wouldn't deliver my mail yesterday.  Looks like I'm a little
behind.

O.K.  Now regarding the bigger turbo.  I said that 1 bar of boost is 1 bar of
boost.  

>>we can see that 1 bar boost does not mean the same thing on different
turbos.
>>The stock turbo has to work very hard to compress 1 bar of pressure and
creates a lot of heat.  The larger turbo, with it's greater capacity to
compress air (because of it's larger compressor section) does not create as
much heat, thus producing a denser air.  

Actually, just the opposite occurs in this application.  The stock turbo is
not doing a lot of work to produce pressure.  It is simply rotating from
exhaust passing it and producing pressure.  Lets say that it takes 300 cfm to
make 1 bar of boost at 5400 rpm.  If you install a bigger turbo, it still
takes 300 cfm to make 1 bar of boost at 5400.  However, the turbo is making
more cfm than the engine needs, so the excess is dumped.  The problem is that
the waste gate only can flow so many cfm's too.  The K-27's seem to actually
stall the compressor wheels because they are too large for the needed
application.  The exhaust pressure goes up like crazy because the compressor
wheel is stalling.  The temperature then goes up like mad.  The exhaust can't
get by the exhaust turbine blades and this acts like a potato in the exhaust
system.  This heats up the intake side becasue the turbo is getting very hot
and intercooler temperature goes really high.  Further, the exhaust is
restricted from getting out of the cylinder head and volumetric efficiency
goes out the window.  The engine has automatic EGR.  This makes the cylinder
head temp rise and the whole engine is now fighting itself.

There is definately a need for a slightly larger turbo on the 951.  The
biggest turbos Porsche offered have a tough time maintaining boost to 6,000.
 Haven't seen the correct one yet however.  Turbo mapping is very critical.
 Taking a late 951 Turbo hot housing and slapping on a K-27 cold housing just
doesn't get it and this basically what the aftermarket is offering.

Greg Brown

 

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