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

[Scott Justusson]

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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  

 

 

-----Original Message-----
From: R Wong <lucky_doggg7 at yahoo.com>
To: urq <urq at audifans.com>
Sent: Fri, Sep 14, 2012 5:58 pm
Subject: [urq] Wastegate "Chirp" on Rally, SCCA and IMSA Race Cars


Greetings everyone in quattro land, I am reaching out to the board for some 
information on what wastegate Audi Sport used in order to get that cool chirping 
sound from their cars in the late 80s and early 90s, meaning the rally Sport 
quattro, SCCA 200 quattro, and IMSA 90 quattro.  I did spend a bit of time 
searching but was not successful so this is the reason for reaching out to the 
board for information.  One last thing, does anyone have a chipped ECU they want 
to sell to me for my 83 ur q?  If so, e-mail!  Thank you very much everyone.  


Kind Regards, 


R Wong


-----Original Message-----

From: Phil Payne <phil at isham-research.co.uk>
To: 'Scott Justusson' <qshipq at aol.com>; ub3ratl4sf00 
<ub3ratl4sf00 at gmail.com>; koenigj <koenigj at comcast.net>; urq
 <urq at audifans.com>
Sent: Sat, Sep 15, 2012 2:56 am
Subject: RE: [urq] Wastegate "Chirp" on Rally, SCCA and IMSA Race Cars

Well, here we go again.  I once had a long history of disagreeing with
Scott.

The chatter that I hear is almost always wastegate.  I've had Snap-off
piezo-electric microphones all over the engine bay and that's where it's
coming from.


>...


-----Original Message-----



From: Martin Pajak <martin at quattro.ca>
To: phil <phil at isham-research.co.uk>; urq <urq at audifans.com>
Sent: Sat, Sep 15, 2012 8:26 am
Subject: Re: [urq] Wastegate "Chirp" on Rally, SCCA and IMSA Race Cars


Hi Phil,

Must be something to it, I have a stock wastegate, a big 50mm Tial dump
valve (setup with proper spring for nice pssssh, it is open at idle).
3071r turbo with mushroom HKS filter.  Yet I still get the chirping sound
from the exhaust side of the engine, in addition to the whoosh from the dump
(so I do not think it is a stall).  You can sort of hear it here, but it is
much better in real life.
http://www.youtube.com/watch?v=qIyj8El8jkA

Cheers
Martin Pajak