N75 Question

[Nige_RS4]

I adjusted the N75 on my S4 as I was experiencing surging i.e. ECU over-boost, followed by N75 control, followed by ECU over-boost etc.... this was at WOT between 3-4k rpm.

By adjusting the N75 (I did an awful lot of research on AW before I touched it BTW) I managed to reduce the peak in the APR code so that the surging was eliminated. The screw on the N75 is sealed using glue at the factory and when people say they break easily, I can imagine that it is breaking the seal that is the reason that they break easily, as the screw on the top only has a range of about 1/4 turn in both directions.

Unless you suffer from surging (which you will only get on a modified car) there should be no reason to play with it.

If you do adjust it, turn the screw 1/8 turn at a time - CW will reduce the peak boost, CCW will increase the peak boost.

As stated above, boost is controlled by the ECU - the only reason a tuner should touch the N75 is if they've loaded an off-the-shelf map and your car is suffering from surging (which is not a good advert for the tuner!)

HTH

[MarkB]

Very well put Nige

[Dr-Al]

Both valid answers...........but, is the N75 adjusted during a remap? I think that it is as I don't see any other way of increasing the boost, is there another way of increasing boost without changing the setting of the N75? If so can somebody shed some light please?

Jules.

In the absence of a signal from the ECU, the N75 feeds the charge pressure back to the wastegate actuator. The wastegate is held closed by a spring, causing all of the exhaust to go via the turbine. As the control pressure on the actuator increases, the wastegate gradually opens, allowing the exhaust gas to bypass the turbine and slowing the build-up of charge pressure.

Eventually, the system reaches a steady state equilibrium where the charge pressure and the force of the spring balance each other and the boost remains steady. The spring on the wastgate actuator is set so that this mechanical feedback loop will limit the boost to ~0.4 Bar. IIRC, some actuators have adjustable springs so that this pressure can be dialed up or down according to taste.

It is designed this way so that, if the N75 fails, the engine will be mechanically limited to a 'safe' boost until the N75 is replaced. IE, the feedback loop acts as a mechanical failsafe if the electronics fail.

Under normal operating conditions, the ECU will open and close the N75 a number of times per second, allowing some of the charge pressure to bleed off to atmosphere. This results in a lower pressure being fed into the wastgate actuator, allowing it to stay closed.

The percentage of the time that the valve opens for is called the duty cycle. At 50% duty cycle, the valve is being opened and closed so that it spends 50% of the time open and 50% closed. At 100% duty cycle, the N75 is fully open, so the feedback loop is broken, the wastegate remains closed and the boost builds until the turbos reach their mechanical limit.

However, before the boost builds to a level that would damage the turbos, the ECU dials down the duty cycle, feeding some more of the charge pressure into the wastegate actuators to open the wastegate slightly.

The ECU monitors the charge pressure using the G31 sensor just in front of the throttle. The tuning map determines what pressure is required and, by comparing the actual vesus requested pressure, the ECU continuously works out if the current pressure is enough, too little or too high. It then adjusts the duty cycle of the N75 to maintain the charge pressure requested from the map. If there is over-boost it will dial down the duty cycle. If there is under-boost, it will dial up the duty cycle.

This is where the screw comes in - turning this will affect how much bleed-off there is at a given duty cycle. Therefore, if you turn the screw to 'open' the valve, the valve will bleed off more charge pressure at a given duty cycle.

OK, so now you may be thinking "great, from what he said above: bleed off more pressure = more boost". Fortunately for the engine, this is not the case. All that will happen is that the ECU will see the boost building quicker than it expected and it will dial down the duty cycle. This happens in a fraction of a second.

As Nige_RS4 mentions in his post, his generic map caused the boost to 'overshoot' the requested level. The ECU would then dial it down using the N75, but it would then 'undershoot' and dial it back up. This resulted in the charge pressure oscillating aroung the set point. This behaviour is often caused by the control mechanism reacting too slowly to rapid changes in the thing it is supposed to be controlling. His cure was to effectively handicap the N75 by setting it so that the bleed-off was reduced at a given duty cycle, causing the boost to build in a more controlled way. Doing this effectively re-calibrated the N75 to the new operating environment and (assuming there was nothing wrong with the N75), the same adjustment would need to be made if the N75 were ever replaced. In this case, the recalibration of the N75 was necessary to correct a problem introduced by the new map.

Although I don't know the specifics and I could be wrong, but I suspect that surging could have been better controlled by tweaking the map rather than the valve. As Nige intimated in his post, 'bodging' the N75 is not an elegant way to correct a problem caused by a re-map, but it can work.

[S4TAN]

Yep - my (original) N75 wasn't touched during my custom re-map (MRC) - i got a new N75 some months after the re-map (along with new N249)......

Being an engineer and having a logical and inquisitive mind it struck me that, although ostensibly calibrated at the manufacturing plant to be the same as other N75s of the same type/version (albeit to within a certain +/-deviation tolerance) installing a new valve in a car that had previously been re-mapped with a different (old, possibly degraded) N75 may affect the performance of the car in relation to the way the ecu map interacted with it.

So......after some research on AW (where quite a few S4 owners had tweaked their valves) i decided that the installation of a new valve was possibly the best time to have a tweak with the screw as the new valve may be different to the old one anyway even before any tweaking had been performed (I certainly did NOT tweak the screw on the original valve when it was in the car)

I marked the factory setting of the screw on the new valve and had a little tweak (anti-clock) about 1/20th of a turn max - results seemed good, no blatantly apparent detrimental effects.

As it stands now though I've wound the screw clock-wise a little way so it's just a tiny bit further anti-clock than the original factory setting - car goes great.

It's up to the individual to try these things, after all it's their car and they pay the bill if they fvck it up...I reasoned that installing a new valve was the most appropriate time to try a little tweak because of the reasons stated above.

Be a devil, try it if you dare!

[Julesm]

Dr.Al,

That's a great reply and very informative.

So that I've got it right in my head, are you saying that the map within the ECU controls the boost via adjusting the duty cycle of the N75 valve?

and......

In order to generate more boost the screw on the N75 is not adjusted at all.

Thanks
Jules.

[Dr-Al]

Dr.Al,

That's a great reply and very informative.

So that I've got it right in my head, are you saying that the map within the ECU controls the boost via adjusting the duty cycle of the N75 valve?

and......

In order to generate more boost the screw on the N75 is not adjusted at all.

Thanks
Jules.

You got it. The screw may alter the shape of the boost curve slightly, but is should not affect the peak boost level.

[S4TAN]

When I tinkered with mine (anti-clock a little way) it seemed to produce boost earlier (reduced turbo lag) but didn't affect peak boost level. Depenedent upon how you like your car to respond, doing this can have a quite pleasing effect..

Best done when the engine and valve are warm (the screw is easier to turn).....not that anyone here would do this of course....

[Nige_RS4]

@ Admin: can we rename this post to "N75 Question" or something similar, as apposed to "Paging Minhea".

@ Littleblue: if you're wondering about my old S4, then I replaced the N75 after I have the REVO code put on

[S4TAN]

Good idea Nige - I only named it that to try and get Mihnea's attention as i wanted the answer from the expert about N75 in specific relation to the MRC custom map (he wrote the map!)!! But for all other interested forum readers "N75 question" is a better description...

[DavidT]

@ Admin: can we rename this post to "N75 Question" or something similar, as apposed to "Paging Minhea".

Good idea, done.

[Dippy]

Perhaps we could have renamed it to "The experts say don't touch the N75, but hey, come and do it anyway!"

[MarkB]

Perhaps we could have renamed it to "The experts say don't touch the N75, but hey, come and do it anyway!"

Doesn't have the same 'ring' to it