[torsen] Re: [s-cars] RE: RS2 turbocharger

[JShadzi at aol.com]

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<<Greetings turbo fans,

I have watched this thread develop with some interest and will chime in with
my impressions. Big turbos are not a cure all for big performance. If you do
go beyond the RS2/Garrett hybrid sized turbos there are some very important
and very very expensive requirements to take advantage of the flow that can
be obtained from such a turbo. >>

Agreed, but I think there is a lot of acceptable application as long as the
turbo is not falling to the left of the surge line in a given application.
If one's preference is huge lag response that is fine, as long as the turbo
is matched wrt the compressor map.


<<> First would be intercooling. The bigger the turbo the quicker it can pump
> the
> charge air through your I/C and the sooner your stock I/C becomes heat
> soaked. Brian Billotti's trip to the land of the s-car mafia this spring
> proved to him anyway the need for an adequate(as in BIG) I/C. As he drove
> Mike P's and my car he just couldn't believe how efficient our big Garrett
> I/Cs were. He was continually amazed to see I/C out temps at or just above
> ambient. The last I heard he had begged off the very last short radiator
> core
> from Mike to allow a big front mount I/C. Speaking of charge air temps,
> another benefit from a larger turbo that is operated short of its
> efficiency
> peak is lower charge air temps to the I/C to begin with. This is the
> primary
> reason I went with the size turbo that I chose. Oh yeah, that and the mid
> and
> high end power.>>

Definitely, I think practically in any Audi application (due to space
constrains) you can't really install a big enough IC, especially when boost
*and* flow #'s are up considerably.

> <<As Rod so elloquently put it, no I haven't given up on drivability, its
> just
> located a little farther up the tachometer. One thing I have noticed is
> that
> my turbo is very sensitive to back pressure. We had removed the kitty and
> tried a test pipe and I lowered my full boost rpm to about 3800. The
> exhaust(Stromung SS) was real loud when I did this so I substituted a Borla
>
> racing muffler for the test pipe. That threshold jumped up to 4100 rpm. I
> think this is somewhat related to my RS2 exhaust manifold/Garrett big
> turbo.
> My next step will be either the Sport Q or Spaghetti manifold and a big MAF
>
> intake to go with the RAIS(ram-air intake system).>>

Yes, the RS2 is hardly the best design, I've been very happy with the 10v
header I built, long smooth runners (see 80tq.com)

> <<One last thing about drivability. With a big turbo you can't drive the car
>
> looking at the boost guage. The flow off my turbo at 5 psi is significanly
> greater than a K-24 or RS2 or T3/T4. The other given is that I'm getting a
> lot more flow for an equivalent boost level and my IN I/C temp and
> resultant
> OUT I/C temp are all much lower. The seat of the pants feel to my car is
> rather invisible. You just don't realize the big turbo is there until your
> foot is in it. Big turbos are not an easy answer to all the challenges of
> life with a urS4/6 but, they aren't hard to live with either if you do
> everything else that is required. >>


Sure, but the reality is that a big turbo won't be flowing as much MAS
airflow at say 2500 than a K24, but the advantage quickly diminishes 500rpm
later, and by 3500-4k rpm, the K24 is running out of steam and a properly
sized large turbo is boiling all the way to redline.

Small turbos like the K24 are a severe compromise (on part of the factory) to
satisfy the broader auto market, which has been proven over and over, likes
down low torque, not high rpm hp.  In fact there are many who are convinced
that revving a motor over 5krpm will surely cause it to self destruct.  Many
of us know differently, however =)

Any insight into what turbo you're running?

Javad