I went ahead and did the 1.8 bar WG spring here are some observations.

I have MTM Stage 1 and am near Denver so my atmospheric pressure is right at 12psi. I think it is helpful to express boost pressure in terms of BAR this way folks at sea level or in between can compare apples to apples (In my case - (boost pressure + 12psi) /14.5 = BAR). I also have a pressure gauge plumbed into the pressure line that feeds the ECU since a 95.5 S6 does not have the dash display which isn't very accurate anyway.

Stock spring baseline:
Before buying the 1.8 spring I tried to turn the WG ajustmet screw down. I ended up turning it all the way down and saw the spring only pressure (WGFV disconnected) increase from only 7psi/1.31bar to 8psi/1.37bar. This is the main reason I thought my stock spring may be tired. Does anyone know what the stock spring is supposed to be?

With the WGFV disconnected (conector pulled) and adjust screw turned all the way in I was seeing about 8psi or 1.37bar. With WGFV connected I was seeing peak 20psi/2.22bar and sustained 18psi/2.06bar.

I think of the spring, WGFV and full throttle switch as kind of a three step process. The spring keeps the WG closed unitl X psi then the WGFV takes over and controls boost up to Y psi and if the full throttle switch is closed the WGFV will control up to the maximum z psi.

1.8 Spring:
When I first put the 1.8 spring in I turned the adjustment so three threads were showing. First drive with WGFV disconnected (spring only) showed 16psi/1.93bar and with WGFV I was seeing about 24psi/2.48bar max. with some bucking/surging. I don't think it was fuel cutoff as I have felt that before in my 5KTQ and it is like hitting a brick wall. Probably just the ECU feaking out!!!

I then turned the adjustment all the way out and now am seeing 14psi/1.8bar with spring only and 22psi/2.3bar max 20psi/2.2bar sustained with WGFV. After driving a week it does seem that the ECU has reduced the max. slightly and now I am not getting any bucking or cut out.

The 1.8 spring does seem to provide better throttle response. It seems to spool up a little quicker and I am getting 2psi more max boost. I'm not sure why this is the case unless the WGFV just could not respond with quick enough with the stock spring? Maybe the max boost will come down some more after more time. I think the 2.3bar max 2.2bar sustained is more inline with what the MTM stage 1 is supposed to be.

It is a little more peaky meaning that with the stock spring the WGFV was very smooth in the transition from spring to WGFV to full throttle switch. Probably because the WGFV was having to work harder since it was basically needed above 8psi with stock spring. Now it is not quite as smooth and requires a little more throttle modulation by me at times. I don't mind it a bit and actually prefer it as it feels similar to how I had my 5KTQ setup for years.

Anyway, hope this helps if anyone is thinking about doing this mod.

Russ
95.5 S6
MTM Stage 1
Bilstein/Eibach Suspension

off 1.5 turns and things are much better. Nice kick in the back. No bucking as long as you ease into the throttle and give the ECU some milliseconds to make the corrections. I think the problem comes when the 1.8 bar spring is turned into too much (e.g. three threads showing is too much) and you hit the throttle too fast. This is new territory for the ECU and it can't handle the rapid rise in boost - so it dumps it as a safety measure.

Backing off the spring and using a little restraint on the throttle reaps big rewards.