[urq] Wastegate "Chirp" on Rally, SCCA and IMSA Race Cars

[Martin Pajak]

You are like a dog with a bone... another 5 page essay... you missed your
calling Sir.

 

-----Original Message-----
From: urq-bounces at audifans.com [mailto:urq-bounces at audifans.com] On Behalf
Of Scott Justusson
Sent: 17-Sep-12 08:22
To: lucky_doggg7 at yahoo.com; urq at audifans.com
Subject: Re: [urq] Wastegate "Chirp" on Rally, SCCA and IMSA Race Cars




Archive Search Success!

Roland, who would have guessed that you coming out of the woodwork after a
couple years, would garner so much attention?  Or that the simple question
you asked, managed to find a couple well seasoned urq listers, to opin this
to be some sort of wastegate phenomenon.  The wastegate "Chirp" you refer to
below, is from from a sudden on boost lift throttle, causing a pressure wave
in the intake tract.  This high pressure wave bounces between the closing
throttle plate, and the turbo compressor blades.  When the pressure wave
reaches the turbo, it creates an instability in the turbo compressor blades,
which causes the chirp you refer to.  As I noted, this noise can be quite
loud from the front of the car do to the orientation of the compressor
blades, and in many cases in rally, it could be heard long before the car
could be seen.

A clear example of this 'chirp' phenomenon you refer to can be found in
dozens of instances in this 10min vid of an S1 in rally
https://www.youtube.com/watch?v=ejIhA2UhfNA
In the IMSA 90 GTO, the stall was less, because a closed track has less
unexpected lift throttle moments, so the chirps are much quicker, and
usually heard only between shifts.  You can hear the compressor stall chirps
listening to the sounds of the IMSA 90 on-track in this 4min sound clip
http://www.youtube.com/watch?v=UsMQRRVPmDw

Those sounds you hear are clear instances of lift throttle compressor stall,
plain and simple.  The two responses below claim this to be a wastegate
phenomenon.  These responses represent a very common mistaken opinion,
referred to as the "Wastegate Chatter Myth".  

After several exchanges (available using the Archive Search), it would
appear that at least one of these guys below, changed his tune (pun
intended) regarding his opinion on the "Myth".  I'm hopeful  your question
has been answered completely regarding the 'truth' to the sound and debunks
the 'myth'.

Further, the' Chirp'  has more to do with the size of the turbo vs the
displacement of the engine, and has nothing really to do with the Wastegate
application.  To answer your second question, the wastegates Audisport used
on most of these large turbo cars were usually Porsche 930 units fitted, but
the Pikes Peak and other 'Hillclimb' spec S1 Evo II's used a complicated
arrangement of the 930 and a 944 wastegates The "Umluft", where the 944 WG
was used as an air bypass.  A good explanation of this system can be found
here:
http://www.bufkinengineering.com/Umluft.htm

I hope this answers your questions, and thanks for your post, it resulted in
a good discussion.

Best Regards

Scott J
Chirpin Turbos in
87 4ktq
84 urq
83 urq