[s-cars] Partial Throttle Surging

[Robert Pastore]

I don't know what compressor wheel MLP is running, but I thought I saw in
another post that it was a T04E/46 (but this sounds small in the land of
HAP)
Plugging numbers into the turbocalc software from Ray Hall Turbocharging,
none of the data (in the limited scenario of part throttle acceleration up
to 18 psi boost) suggests flow requirements to the left of the surge line.
(But at WOT, the most AAN boost Maps WILL bring many of the popular
compressors to the left of the surge line because of the low flow/high PR we
demand, while selecting the wheel based on max flow/High PR).

My bet is the problem is with the Dave Jones secret software.  At part
throttle, you are in closed loop, and the combination of MAF
signal/Software/fuel pressure/injector size is giving you an A/F ratio the
O2 sniffer doesn't like.   What you are feeling could be the Motronic
cycling against Lambda control...a little rich, then a little lean, then
back. I've had my car feel like I was towing a trailer, then surge ahead,
over and over in cycles of a few seconds.   It turned out to be a stuck
thermostat and an old coolant temp sensor.   For some reason, a tired 02
sensor( > tired=longer, slower cycle)  or coolant temp sensor seems to
amplify this cycling.  When you go to WOT, you go open loop, and the surge
goes away.

I can't explain the boost pressure fluctuation.  I don't like the pressure
wave theory, because I think if it existed it'd be mostly absorbed in the
IC.
Bruce Mendel experienced something very similar, and it turned out the
woodruff key on his crank pulley sheared (twice!) and the slight out-of-time
between the crank and cam was picked up by Motronic resulting in this
behavior.  Ruling out problems with the WG, WGFV, and bypass valve come to
mind too.

Finally, I think the stock WG spring only pressure is somewhat south of 12
PSI --  about 7 PSI is what I recall, but someone else can chime in and
confirm and/or correct this.

Bob



-----Original Message-----
From: PAUL GAILUS [mailto:gailus at mindspring.com]
Sent: Monday, August 26, 2002 9:30 PM
To: mlp; 's-car-list'
Subject: Re: [s-cars] Partial Throttle Surging


Sounds like these symptoms could be caused by
compressor surging as the pressure ratio rises above
what the compressor can support without stalling at
the relatively lower flow rates you get with a small throttle
opening. In other words, this would be operation to the
"left" of the surge or stall line on the compressor map.
This is more likely when running a big turbo.
You could run a bigger turbine nozzle A/R but
then your boost threshold and lag go up.

If this surging only happens under the very limited
conditions you describe, it seems that it could
be handled best by an ECU software / boost controller
change that could activate the wastegate at these times.
How best to do this with the Motronic system I'm
not sure. Maybe Bob Pastore might chime in?

Paul

----- Original Message -----
From: mlp <mlped at qwest.net>
To: 's-car-list' <s-car-list at audifans.com>
Sent: Monday, August 26, 2002 12:48 PM
Subject: [s-cars] Partial Throttle Surging


> Has anyone on the list experienced, as best I can describe it, "partial
> throttle surges or surging" under certain boost conditions?
>
> The condition seems to arise & can be duplicated or created with a long
> steady pull under load, like you can get on a long steady climb, by
allowing
> the car to accelerate in an upper gear, say 5th or 6th, while only
slightly
> opening the throttle just enough, say less than an 1/8th or so, that the
> engine will make, and continue to slowly build boost without cracking the
> throttle open any further.
>
> In my car I when boost gets to @ 12 psi (a number suspiciously close many
> waste gates  reputed "default" base spring pressure), then will begin an
> increasingly aggressive swing, pulse between 12 / 18+psi as seen on,
> reputably reliable, digital boost gauge.  By "aggressive" I mean one will
> see almost instantaneous pressure swings from the base 11 to 12 up to
18~20,
> 13, 14, 15 & 16 etc. don't get to play much if any part in the natural
> progression of events.
>
> One theory that's been tossed out for kicking about, is that there is a
> pressure difference across the throttle body plate, creating a pulse
between
> the intake side (i.e. downstream) of the plate and the turbo compressor?
> Perhaps exacerbated by the ECU taking its pressure reading from the intake
> manifold, which is upstream (on the other side) of the throttle plate.
>
> Any thoughts, theories etc.?
>
> Please Paul, no "choking the chicken" jokes.  I mean I think I may finally
> have at least one car back!
>
> mlp
>
>
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