RE: RE: Toasting an O2 sensor, hesitation at boost, and more

[QSHIPQ@aol.com]

Jouko writes:
>My '88 MB Urquattro MTM ECU has a modified fuel-ign map chip and a
>resistance on the output of the pressure sensor, along with a significantly
>stiffer spring.  I run around 1.8 to 1.85 bar with no detonation, will try a
>little more boost in the spring.

I'm not familiar with the MB ECU specifically, however, when I hear of 
resistors modifications on a 2.0bar PT with a car under 2.0 maximum boost, 
chances are the overboost cutoff hasn't been changed in the chip, nor have 
the WGFV values, both values are just being 'fooled' (computer thinks it has 
a percentage of actual boost voltage value).  The above would be the 
equivelent of an early Stage II Mac-11 modification with revised 
fuel/ignition tables only (or really more similar to a MAC-02 without WGFV 
operation).  My own thinking is that if your spring is really stiff, the WGFV 
probably is operating less than you think (if at all).  A chip that has 
properly addressed the WGFV tables, should be able to go higher than stock 
boost levels with a totally stock spring.  For an easy test of this, unplug 
the WGFV from the top of the WG, and run the car, you will find little if any 
difference in boost vs rpm values (I posted this a couple years ago, and 
found a huge percentage of Mac-11 owners found this 2B true).  Most of the 
resistor modded Mac-02>mac-14 cars can be tested the same way (and my way of 
road testing an unknown customer tweeked car without tearing out the ECU).  

>I think tomorrow I'll hook up the Bosch PMS100 oscilloscope to see how the
>WGFV works on the MTM ECU.  I'll be interested to see how Peter Link
>programmed the WGFV into the equation.  My spring preload is so low the the
>WGFV should have a noticeable effect.  I've got to figure out how to record
>boost/rpm on the oscilloscope - I guess I'll need to install a boost gauge
>with an output to the scope.

A more telling test might be to get PT values vs WGFV DC - Actual vs 
Resisted.  I'd bet they are the same (specifically, no WGFV values have been 
changed).  Only the more recent chips really change the WGFV values 
specifically, and most use a less stiff baseline spring because of it too.  
You could compare the stock chip WGFV values to the MTM chip you have, IME, 
the earlier chips with resistor mods sometimes mess with WOT DC for fuel, and 
some of the timing tables, but usually not the WGFV tables or the overboost 
cutoff.

Bottom Line:  A properly programmed 2.0bar PT chip modded ECU shouldn't need 
a resistor modification at all to achieve </= .95bar boost.  Proceed with 
care messing with it.  

HTH

Scott Justusson