N75 valve - anyone ever tweaked the screw...?

[CliveH]

Yes, good logic, S4TAN.

I've seen MRC tune a few cars now (including mine) and I've not noticed any tinkering with the N75 - we should have a chat with them about this next week.

[S4TAN]

Jules, MRC didn't adjust the N75 when they did my custom map - don't know about anyone else.

From what I've read on AW tweaking the N75 screw anti-clock reduces turbo lag + gives spikes, tweaking the other way increases turbo lag but gives smoother response - I prefer the aggressive setting that my car now has by tweaking slightly anti-clock....

BTW - how odd! I'm often called Jules (my name is Julian) and my surname begins with an M.....!! Spooky!

[CliveH]

BTW - how odd! I'm often called Jules (my name is Julian) and my surname begins with an M.....!! Spooky!

Devilish!

S4TAN - when you say 11 o'clock, where are you standing in relation to the front of the engine when you measure this?

[S4TAN]

Clive - just standing directly in front of the engine (not at the side of the car) shins touching number plate - see the small screw in the recess on top of the N75? Superimpose clock over this so that 12 o'clock is directly ahead (towards windscreen); the top of the slot in my screw was at 11 o'clock......

[CliveH]

Now, that's what I assumed Jules - and out of pure curiosity, I had a look at mine this afternoon (haven't tinkered with it, honest) and the screw is sitting at about 2 o'clock - quite different to yours. Now you've got me thinking...but I'm not going to mess with it until I speak with Doug or Mihnea (is he back, by the way?)

[dummi]

it is a vac valve AFAIK, it will be effected by opening and closing the screw, it is activated by solenoid control which is electronically controlled by the ECU, AFAIK its used to request or control boost by the wastegate. MRC will have mapped it electronically i would have thought no need to physically touch it. you are essentially changed the settings your self manually, my thoughts are it may feel better, and you may feel you have done something good but it would be worth checking vag-com on what is actually happening, likely its in the range of the adapting ability of the ECU. reading other posts MRC prefer to use timing rather than boost so your likely a little less boost but more timing, you've now upped the boost delivery and it would probably be wise to consult MRC. though i also agree with your theory on them not coming with the exact calibration from the factory so maybe no harm done.

[S4TAN]

Clive - I haven't popped into MRC for a week or two so don't know if Mihnea's back - guess it's best to raise this on Saturday (as you suggested earlier in this post) amongst the myriad of other topics we will undoubtedly bombard them with!!

@dummi - it's always possible that I have upped the boost (although i think it's more likely that i've set the valve such that it controls the wastegate so that boost is produced earlier - hence the reduction in turbo lag) but it's also equally possible (and probable IMO) that my new valve was set "softer" than my original valve and all I've really done is tweak the new valve up to the state that the original valve was in when my re-map was done.....(albeit with a better vacuum seal/solenoid action because it's a nice shiny new valve as opposed to a 6 year old one that has surely deteriorated over the 6 years it's been in-situ!)!

I've read quite a bit on AW that these valves, although ostensibly calibrated in the factory, are all slightly different (hence the different screw "clock" positions that people observe in their cars) - no 2 valves can be exactly the same so I think that a VERY SLIGHT tweak (experiment both clock and anti-clock) is valid........

[Dippy]

Guys, if you look into the biturbo engine design, you'll see that the ECU controls boost using a closed-loop with the MAP sensor and the N75. Remember that VAG-COM can log demanded boost and actual boost? That's where it is doing the control.

If you change the N75 setting, you won't increase the SUSTAINED boost because the ECU will simply adjust the solenoid rate so that actual equals demanded. However what you do change is the characteristics of the control loop.

My control theory is very rusty now, but the ECU probably uses PID control or at least a derivative of it. This is far from the simple "on-off" control that you get with say a central heating system. With PID, the ECU takes account of the overally magnitude of demanded boost as well as the rate of rise and the amount of time elapsed. With good tuning this allows the ECU to deliver the required boost as accurately as possible within any given environment.

By changing the N75 you will change the tuning to a different OPTIMISATION. This means that something might be better, but AT THE EXPENSE of something else.

So if the N75 is changed such that response seems better, this means that the ECU will be raising the boost quicker than before, but you'll know what the expense is? Overboost. The worst thing about this is that it won't be controlled overboost. So the ECU will not be adding more fuel and you won't know how much excess pressure there is.

Adjusting the N75 is a crude method of tuning which will reduce long-term reliability. ECU mapping is the best way to tune an engine.

[S4TAN]

Hey Dippy - unlike yourself I'm certainly no expert (as I'm sure is obvious) and everything you say makes good sense - however, this then prompts the question: are you saying that if/when your N75 dies/deteriorates (and this DOES happen) then you have to go and have your re-map re-done after installing your new one??

I repeat: no 2 valves can be exactly the same (of that I'm sure) - i've read several threads over on AW on this and the US guys seem to tweak their N75s (even after a re-map!!) - some of them even turn the screw quite a large amount (I've seen threads where people have turned it 1/4 turn which equates to 3 hours using the "clock" method!!! - the amount i turned mine equates to about half an hour or 1/24th turn) - I also suspect that maybe I turned my new valve to a setting that roughly matched the old one i took out (I think that it's unlikely that the new one was set at the factory EXACTLY IDENTICALLY THE SAME as the old one that was in my car when i had my re-map done)

The reason I posted this question was to find out if anyone on this forum had done this to their N75, and just to gauge what their results were..........

[Julesm]

Guys, if you look into the biturbo engine design, you'll see that the ECU controls boost using a closed-loop with the MAP sensor and the N75. Remember that VAG-COM can log demanded boost and actual boost? That's where it is doing the control.

If you change the N75 setting, you won't increase the SUSTAINED boost because the ECU will simply adjust the solenoid rate so that actual equals demanded. However what you do change is the characteristics of the control loop.

My control theory is very rusty now, but the ECU probably uses PID control or at least a derivative of it. This is far from the simple "on-off" control that you get with say a central heating system. With PID, the ECU takes account of the overally magnitude of demanded boost as well as the rate of rise and the amount of time elapsed. With good tuning this allows the ECU to deliver the required boost as accurately as possible within any given environment.

By changing the N75 you will change the tuning to a different OPTIMISATION. This means that something might be better, but AT THE EXPENSE of something else.

So if the N75 is changed such that response seems better, this means that the ECU will be raising the boost quicker than before, but you'll know what the expense is? Overboost. The worst thing about this is that it won't be controlled overboost. So the ECU will not be adding more fuel and you won't know how much excess pressure there is.

Adjusting the N75 is a crude method of tuning which will reduce long-term reliability. ECU mapping is the best way to tune an engine.

Cheers Dippy,

As I thought then.

I reckon that Satan is ok with what he has done with his replacement valve, he has already asked MRC to check it out and they are fine with it, as I mentioned earlier most tuners will allow an element of safety in the mapping from a fueling perspective so unless the valve was adjusted radically then I suspect all will be good.

Jules.

[Dippy]

Ha! I'm certainly no expert!

I think the remap is generally independent of the N75 within a certain range. The whole point of the map is that it is just a n-dimensional look-up table. The ECU reads its sensors and then finds the corresponding values in the relevant tables to use to control its outputs (such as the fuel injectors). For a given instant, the setting of the N75 should have no impact.

The only impact should be in the dynamic state, or rather if the response time of the N75 affects the rate at which the ECU is reading its sensors. I gave an example - if the N75 allows the boost to spike above requested then the ECU will not provide any extra fuel. However I really don't know what the cycle time of the ECU is - as I noted, I am not an expert.

Therefore, as MRC have shown, a minor change in the N75 setting won't affect their map, and similarly a replacement N75 with a slight variation won't affect it either.

[AdamS4]

Would this "tweak" work on non re-mapped cars ??

[Ads4]

Ask Julian, Adam, he's tweaked everything.