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.

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.

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.

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.

Progress with the Nefmoto community 2.7T Stage 2 tune has spurred my curiosity to find out why the results I’m seeing with a Hitachi MAF equipped car are significantly different from two others car that are participating in the tuning project, but are equipped with Bosch S4 MAF sensors.

I don’t have an S4 Bosch MAF on hand, but I do have a Bosch RS4 MAF, which I decided was close enough to try out in order to see if the car behaved, or rather logged, much different than with the S4 Hitachi MAF.

With that end in mind I replaced the stock airbox with a RS4 airbox, the Hitachi MAF for the RS4 MAF, S4 and RS4 accordion hoses were also swapped, and the stock y-pipe was pulled to allow the RS4 accordion to attach to an RS4 y-pipe.

After the hardware was swapped and I verified that I was getting a good reading from the sensor using a first draft of the revised tune, I began to look more closely at the tables that needed to be tweaked in the ME7 code.

A few that need to be changed are:

• MLHFM
• MLOFS
• KFKHFM
• KFMLDMX

The KFKHFM table is turning out to be one of discovery, I’m needing to adjust this table quite a bit more than I had expected in order to richen up the AFR.

The Nefmoto community tune project is proceeding forward once again.  Here is how it stands compared to a couple “off the shelf” tunes.  The Nef tuning efforts are currently being directed at the initial boost onset to see about getting some more pressure as the actual pressure is falling short of desired.

When that is completed it will be interesting to see if the low end acceleration gap is closed.  At the top end the GIAC tune pushes a few more psi than this tune presently does so the Nef tune may never close the gap up there depending on how we handle the upper rpm boost level.