[s-cars] Overboost and Wastegate Springs

[Scott Justusson]

Overboost is not usually a WG spring problem on a 20vt.  Best test given the OP is to bypass the N75 valve from the boost hardware (disconnect the turbo outlet from the N75 valve and the WG feed from the N75 valve and join them together).  This should allow exactly 1.9bar (absolute pressure) boost or ~14psi.  If this happens, then the WG is operating correctly, it's the N75 valve that's likely the problem.  The OP sounds like an N75 valve that is stuck in a partially 'closed' (add boost) position,or a leak in the line that feeds the ECU pressure transducer.  With either of the above the car will 'overboost' beyond the normal software boost profile design parameters.

A fix best done sooner than later, or expect a sudden loss of power due to some more basic loose-metals problem.

Cheers and HTH

Scott J


 

 

-----Original Message-----
From: David Forgie <forgied at shaw.ca>
To: s-car-list <s-car-list at audifans.com>
Sent: Sun, Sep 2, 2012 1:02 pm
Subject: [s-cars] Overboost and Wastegate Springs


One of the things that waste gate springs provide is the minimum boost levels.  
You can see this if
 you disconnect the N75 WGFV.  On a stock spring and perch height this is about 
6 psi.

As far as I know, when people talk about the number of "Bar" a particular 
wastegate spring is 
rated at, e.g. 1.4 Bar or 1.9 Bar, they *should* be talking about absolute 
pressure, not relative
 pressure.  A stock 1.4 Bar spring is about 20.3 psi absolute or about 6 psi 
relative.  Likewise,
at 1.9 Bar spring is 27.55 psi absolute or about 12.8 psi relative. These are 
the maximum "minimum"
boost levels.

My experience with a 1.9 Bar spring was at the stock spring perch height, it 
resulted in too rapid
initial boost rise, which freaked out the ECU, causing it to dial back the boost 
via the WGFV, then
it would try again, and the rise rate would be too high, again a dial back, all 
in fractions of a second.
This resulted in a very jerky stacatto effect.  Not nice.  Solution: Turn the 
spring perch up until the 
issue is removed. Still provides a good initial tip in boost level. (IME). 

Some N75 info: http://forums.quattroworld.com/s4s6/msgs/20987.phtml

But you knew that. ;>)

Dave F.
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