found this in the 1.8t forum......interesting.....
-----------------------------------------------------------------------------------------------

Here's the actual thread since the links didnt copy over...
http://forums.vwvortex.com/zerothread?id=1811131
-----------------------------------------------------------------------------------------------


Barometric Pressure Mod yields 10% HP gain

Remember this?: Adjust Timing using Barometric Pressure Circuit

I did some more experimenting, and went from 200WTQ/170WHP to 220WTQ/190WHP by setting the barometric pressure signal equivalent to near sea-level (I live at 5300 ft.). Here's the before and after dynos:

Before Dyno
After Dyno

The "before" dyno is one that I've been against for a long time - I thought I was going to be stuck there until I got some fuel management. I ran four dynos "after" and they all agreed. Seat-of-the-pants it's been a tremendous punch, even better at lower altitude. After doing this mod I drove to Tennessee and back for Christmas. The car ran flawlessly at all altitudes in-between, and pulled like a mule at 1500 ft. with five people in the car.

I do not know why the mod does what it does. I have suspicions, but that's all - I suspect it primarily modifies lamda regulation, making it run richer closed-loop. Secondarily, maybe alters load tables, and may or may not affect timing directly. The reason I suspect lamba regulation is that, as I adjusted the device "downward" elevation-wise, the car started gaining boost while running richer (O2 sensor voltages increasing during open-loop). I compensated this behavior by turning down the peak fuel pressure on my Cartech, from 90 to 75 psi. And my fuel trims actually went richer, from -6% to -5% or so. If the ECU knew it was running richer, it would have trimmed out more fuel, i.e. gone more negative. So I got almost 20 extra HP with 15 lbs less peak fuel pressure than when I started.

I missed the O2 behavior the last time I tried the lower altitude (parallel-resistance) settings, because I wasn't logging O2 voltage. I only logged O2 for the higher altitude, series-resistance settings.

Note that the boost for both graphs is the same. The mod gave me more boost (from 15 to 17) but I turned it back down to 15.

The other reason I think lambda reg (or maybe load table adjustment) is that it pulls so much stronger during closed-loop. Around town, I have so much power now, it takes very little boost to keep abreast of traffic; as a result, around-town mileage has gone up from 15 to 20mpg. Highway mileage is about the same, 28-32 depending on how fast I go. It'll be interesting to see my next emissions inpection report.

And then there's Speedy G's theory that it affects requested boost...

I used the "retard timing" a.k.a. "lower the altitude" setup, where the added resistance is in parallel with the signal, thus lowering the effective resistance, corresponding with lower altitude. The final value was 2-1/2 turns on my 50K Ohm 10-turn pot, which is 12.5K Ohms. The total effective resistance calculates to be 2.58K, which is 72 feet based on the altitude data I logged (see first baro pressure post).

The funny thing is that the VAG-COM data block for barometric pressure compensation doesn't agree with this. At my altitude of 5300 ft, the value is typically -17%. I would expect it to go to zero or near zero, but it's currently reading -21%. Whatever - it works!

I'd like to see someone else try this... at your own risk, of course. Log a bunch of data and pay attention to it. I can't guarantee anything other than it works in my car. It would be interesting to see the effects on other engines, especially other engine codes - mine's a cable-throttle AEB. Maybe this interacts in a fortuitous way with one of my other mods, like the Holey MAF? With that in mind, here's the mod list:

'98 AEB cable-throttle engine
Stock ECU (Motronic 5.9.2 IIRC)
Stock K03 turbo
Stock 2.25" exhaust (with resonator removed)
Dawes MBC 15lbs boost
440cc injectors
2 bar base fuel pressure
5 bar fuel pressure at max boost via Cartech FMU
Holey MAF
Port-matched intake
Port-matched exhaust
FMIC
Baro Mod