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

[Phil Payne]

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.

However, I do very much agree with the problems that turbo stall can cause
on turbos - especially the K24.  What exacerbates this problem (in the UK)
is that the dominant turbo rebuilder (http://www.aet-turbos.co.uk/ - good
guys) won't accept a turbo with a broken shaft as an exchange item.  Having
seen them strip one, I can understand why.

I also agree with the comments about bypass valve capacity.

I've got other reservations.  Over here, Bailey is a dominant supplier of
bypass (and blow-off) valves, but their product has a non-linear opening
characteristic that I believe results in some back pressure before it opens.
The unit Audi fitted to the ur-quattro 20V (035 145 710) has a much smoother
opening profile but - as Scott implies - does have a restricted capacity.

Nevertheless - I do agree that fitting such a valve increases turbo lifetime
and is in general a very good, cheap and easily reversed modification.  For
most untweaked 10Vs I recommend the 20V part (number above) but do make sure
that you don't impede airflow over the turbo and exhaust manifold when you
fit it.

-----Original Message-----
From: urq-bounces at audifans.com [mailto:urq-bounces at audifans.com] On Behalf
Of Scott Justusson
Sent: 15 September 2012 04:22
To: ub3ratl4sf00 at gmail.com; koenigj at comcast.net; urq at audifans.com
Subject: Re: [urq] Wastegate "Chirp" on Rally, SCCA and IMSA Race Cars

Except all WRC and ProRally urq's chattered, from day 1, btwitnessedt.
Read: Only the later cars had the recirc setup.  In the early cars and even
the reference below, it was not 'wastegate' chatter it was always compressor
stall from a turbo spooling up and the throttle slamming shut.  There were
no bypass valves in the early cars, so with big turbos, you heard the
compressor stall.  On Buffum's drives back in the early 80's, you would hear
that chirping 5 miinutes before you even saw the car.

We actually get a lot of it quite noticeable at Steamboat with the 20vt
k24's.  When I discussed it with Ned Ritchie years ago, we figured it was
the turbo working beyond the ability of the bypass valve to dump at altitude
(with tires on ice spinning up under load), that caused turbo stall.  If you
really like that noise, put a cone filter in place of the airbox, you will
hear compressor stall during street driving.  Or put a restrictor in the
bypass valve, or plug the bypass valve all together.

All that said, it's not a healthy noise for turbo longevity, especially on a
k24, btblewthat up at steamboat in my 10vt k24 urq.  That small shaft can't
take the thrust load of the stall.

Cheers, hey Roland, and my .02

Scott J
chirping 20vt 4kq cone filter
84 urq
83 urq

 

 

 

-----Original Message-----
From: Stephen Kraus <ub3ratl4sf00 at gmail.com>
To: John Koenig <koenigj at comcast.net>; urq <urq at audifans.com>
Sent: Fri, Sep 14, 2012 9:03 pm
Subject: Re: [urq] Wastegate "Chirp" on Rally, SCCA and IMSA Race Cars


For more info:
http://en.wikipedia.org/wiki/Audi_Quattro#Sport_Quattro_S1_E2

The boost pressure was recirculated into the turbo when the driver comes off
the throttle, the throttle plate closes, the extra boost pressure
recirculated back into the turbo, keeping it spooling for when the throttle
plate re-opens.

No loss of boost pressure when the driver shifts and engine resume high
speed instantly, but the brief blockage of the pressure and recirculating
air makes the turbo chatter.
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