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I'm currently running an unchipped S4 with a Milltek Catback, a BMC CDA Filter AND a modified N75 Boost Valve. I'm told roughly the way it functions is to somehow trick the wastegate into overboosting and result of which could range from 1-3psi extra boost according to ECS Tuning. Contrary to other discussion topic, I do feel a noticeable difference when boost is called upon, spool up is apparently quicker.

The time has come for me to upgrade to an MTM ECU mod, and I'm stuck with the following question: Should I stick with my modded N75 and continue to trick out the ECU for that extra 1-3 psi, or should I revert to stock N75 under the assumption that any reconfigured boost will have been optimized by the MTM programme, hence no further tricking needed?

Many thx for feedback.

Modding the N75 is a risky business, so i have been told.

If you are going to get teh MTM conversion done then i would revert back to a standard N75. Who will you get to do the conversion? They will advise you better regarding modding the N75, but I guess that they woudl say steer well clear.

whats the N75? I'm new to S4s so I need help.
cheers

I considered an uprated N75 last year, but after researching the subject on Audiworld, it can cause problems with overboosting on chipped cars, which can lead to surging or even cause the ECU to go into limp mode - works fine on un-chipped cars.

I suggest giving it a try with the MTM software - if it gives you any problems, revert back to the original N75.

Nige.

Great, reinforces the shop's recommendation and my prevailing inclination. In fact, I'm not even totally trouble free with the N75 on unchipped ECU...keeps throwing up a code saying 'Maximum Charge Pressure Exceeded'... Well, I'm reverting to the original for sure!! Thanks guys.

what is a N75?

Nutta, do a search on this site for "N75"

N75 is boost pressure control valve. Generally on Turbo'd cars these can be discarded for two/three way bleed valves. However Audi has a sophisicated control system of which N75 is an integral part and the ECU monitors requested against achieved. I believe it is used to restrict/allow requested boost against achieved boost. On non chipped cars these can be "tweaked" to the upper limits of what is "acceptable" often giving a HP gain. I believe the likes of APR/AWE in the US often turn them down to stop throttle surging etc...when chipping....a common problem. Think that about covers it.......

KayGee wrote:N75 is boost pressure control valve. Generally on Turbo'd cars these can be discarded for two/three way bleed valves. However Audi has a sophisicated control system of which N75 is an integral part and the ECU monitors requested against achieved. I believe it is used to restrict/allow requested boost against achieved boost. On non chipped cars these can be "tweaked" to the upper limits of what is "acceptable" often giving a HP gain. I believe the likes of APR/AWE in the US often turn them down to stop throttle surging etc...when chipping....a common problem. Think that about covers it.......

How does one modify it? If at all possible?

There is a small screw under the plastic cap - it is very fragile, so be careful. Sorry, this assumes you know where the N75 is - here's a pic ... modules/pnCPG/Coppermine/albums/userpics/10231/rs246_norm_n75.jpg

thank you kaygee. at last someone who helps.

Nige_S4 wrote:There is a small screw under the plastic cap - it is very fragile, so be careful. Sorry, this assumes you know where the N75 is - here's a pic ... modules/pnCPG/Coppermine/albums/userpics/10231/rs246_norm_n75.jpg

Nige - sorry to be a complete numpty but the small screw you refer too, is it visible from the metal housing on the top of the valve or does the whole valve have to come out and the black plastic casing come off? There is little movement in my screw that is visible from the metal housing (perhaps 1/20 of a turn).

Appreciate your help?
Thanks,
Rob

Rob,

It's on the top of the valve, and easily adjustable in-situ. My guess is it's sealed at the factory, as it take some effort to make it want to turn the first time - you should get about 1/4 turn of movement out of it from the central position (1/8th each way that is - be careful, they are fragile). Turn clockwise it you want to decrease the boost limit, anti-clockwise to increase the boost limit.

Nige.

Thanks.

Has anyone done any VAG-COM logs to prove that adjusting the N75 actually increases boost.

I am thinking that if boost pressure is monitored by the ecu and cotrolled via the N75 to open the wastegates, the ecu will simply put more demand on the N75 to reduce the actual boost back to meet same level as the demand boost.