Re: Low boost in 91 200q? PT swap ramifications...

["Ned Ritchie" <NED@IntendedAcceleration.com>]

Scott responded to Sargent Schutt,

And I basically agree. . .

Except I'd like to add that in addition to using the stock spring in the
wastegate Sargent could turn it down no more than 6 turns, but only if you
have a non stock turbo.  The stronger spring however, allows the boost to
overshoot the programmed limits.

Also when the computer is struggling to reduce the boost the stronger spring
prohibits the computer from potentially protecting the engine.

Ned Ritchie
http://www.intendedacceleration.com/
1991 200 TQ
1989 90 TQ   "Yellow Bird"  480+ hp
1987 5000CS TQ
1986 5000CS TQ  "The Beast"  430 hp
1986 5000CS T
1986 4000CS Q
1985 4000CS Q
1984 4000S Q   "Marni's 4000"  150 hp
1981 4000 5+5 Rally (Old JB car)
1985 VW Quantum Syncro
Dodge Van

Date:	Thu, 18 Jun 1998 17:11:46 -0700
From:	"Scott Mo." <scottmo@teleport.com>
Subject:	Re: Low boost in 91 200q? PT swap ramifications...

Sargent Schutt wrote:
> I do have a boost problem, though. As follows: I had my TAP chip updated
to the 2.5 bar pressure transducer (PT) and, in conjunction with the stiffer
TAP spring (there from the start - not new), I get between 1.8 bar and fuel
cuttoff, depending on how far down I screw the spring (mit allen-key
adjustment under WG cap).

I don't think you have a boost problem, I think you have a calculation
problem with the stock boost gauge. With the 2.5 bar PT installed,
multiply the readings on the dash gauge by 1.25, I would recommend using
the stock WG spring adjusted as it came from the factory. Your 1.7 gauge
reading is actually 2.13 bar but seeing as the stock gauge is not all
that accurate, and can be a little slow to respond to rapid boost
changes, you are more likely seeing the 2.25 bar (18 psi) that this chip
set produces with the 2.5 bar PT installed (depending on air temp/fuel
etc).

The 1.9 bar reading you were getting with the stiffer WG spring and the
cranked down spring perch corresponds to ~2.38 bar (~20psi) but again if the
stock gauge is off at all you may be getting even more than this.
You really should have a good analog boost gauge installed to get a
better idea what your boost is running at.
>
> So, I decided to run the stock spring, since this is really what the
> code was designed for, along with the 2.5 PT. After the swap, I get
>only 1.7 bar max, even though I have screwed the spring down as hard as
> the allen key will let me (I did this to compensate for 'tired spring'
> disease, what with the 175k I  have on the car).
>
> Is this gauge a relative instrument? EG is it deriving boost values from
the ECU, or from the vac line direct?

The dash computer which drives the boost gauge receives a voltage from the
ECU from ~0-5 volts and with the 2.5 bar PT installed, this boost gauge is
not reading correctly.
HTH
- --
Scott Mo.
1989 200TQ
1988 5000TQ
1966 VW Beetle
http://www.teleport.com/~scottmo