N75 Valve Tuning Investigation

During the course of the S4 community tune efforts a couple other cars that tried using the tune I am helping to develop experienced over-boost on the initial ramp up.  The immediate action was to lower DIMX on their tunes to try and bring the boost curve closer to what I was experiencing.  This left unresolved the question of why we were seeing different boost profiles when using the same tune.

I recalled a similar occurrence when I had APR’s Stage 3 kit and I was comparing my results to a couple other cars that were equipped with the same kit.  As in the case of the S4 community tune the APR tune had been built on my car, and other cars would occasionally see boost overshoot on the initial boost ramp up.  It was discovered at that time that the setting of the N75 valve on my H-box car was quite a bit less aggressive than the A/M/T box cars.  This was determined to be a logical cause for the over-boost seen with the other cars.

After contacting the owners of the other cars involved with the S4 community tune we took photographs of our respective N75 valves and compared the setting.  Again, as in the past, the setting on my car’s N75 valve was a good bit less aggressive than the other cars.

Audi B5 S4 Boost Frequency Duty Cycle Valve
N75 Valve

While this investigation is presently incomplete, it appears to be a good lead into the cause for the disparate boost ramp-up curves between my car and the other two.

I dug up a spare N75 valve that I have and worked on changing the setting to try and increase the rate at which the boost ramps up.  I vaguely recall that the adjustment screw may be fixed in place with loctite, because getting it to budge took some effort.

After making the adjustment I am now more closely set to the two other cars.

Audi B5 S4 N75 valve
New setting for N75 valve

After installing the N75 valve with the updated setting I took a drive and logged the boost onset.  Shown below is the difference between the previous setting boost ramp up, the solid line, and the new boost ramp up with the more aggressive setting that mirrors the other two cars, dashed line.

N75 adjustment affect on boost onset.
N75 adjustment affect on boost onset.

My car is now over boosting initially, unlike prior to adjusting the N75 valve.  This is a great result because it will enable me to make tune changes that will hopefully produce similar outcomes amongst the three cars involved with the tuning project.