# Stock n75 225 amu?



## jacobm (Aug 5, 2011)

What is the stock version of the n75 valve for an AMU engine? Mine came with a race but I need to go to stock


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## racin2redline (Sep 25, 2008)

I believe it's a n75 "C". I'm stock also running the j valve. Boost curve seems to be more consistent less jumpy compared to stock.


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## Marcus_Aurelius (Mar 1, 2012)

You guys do know that the stock N75 are adjustable right! They won't perform magic tricks but will allow some adjustments.


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## Currancchs (Feb 24, 2011)

I believe stock is a "C" (that's what my car had on it when I bought it), but the new equivalent is a "F" (according to the local dealer), which is what I am running currently. I had a "K" on there for a while, but decided to go a bit milder, was pulling a concerning amount of timing.


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## jacobm (Aug 5, 2011)

Thanks guys. Having problems holding boost so FIgured Id go back to a brand new stock valve to start emlinating possible problems (the people that did my tune suggest the stock valve)


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## racin2redline (Sep 25, 2008)

Marcus_Aurelius said:


> You guys do know that the stock N75 are adjustable right! They won't perform magic tricks but will allow some adjustments.


The screw? I'm scared of that lol. Did that once on my GTi and never the same again..unless you have some tips in adjustments


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## speed51133! (Aug 5, 2002)

Marcus_Aurelius said:


> You guys do know that the stock N75 are adjustable right! They won't perform magic tricks but will allow some adjustments.


please elaborate...


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## racin2redline (Sep 25, 2008)

There's a tiny screw inset on the bottom of some n75 valves. Some of them like the j valve I have have a cap over the screw. Basically it adjusts how much air the n75 bleeds into the Turbo inlet pipe. The tighter you make the screw the less air it bleeds therefore increasing the pressure in the wastegate line. Its definitely not a way to increase boost more or less the onset of boost and the The adjustment are extremely fine. IMO I wouldn't play with it unless your having surging issues( or make them worse). Just my .2


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## Marcus_Aurelius (Mar 1, 2012)

speed51133! said:


> please elaborate...





racin2redline said:


> There's a tiny screw inset on the bottom of some n75 valves. Some of them like the j valve I have have a cap over the screw. *Basically it adjusts how much air the n75 bleeds into the Turbo inlet pipe*. The tighter you make the screw the less air it bleeds therefore increasing the pressure in the wastegate line. *Its definitely not a way to increase boost more or less the onset of boost *and the The adjustment are extremely fine. IMO I wouldn't play with it unless your having surging issues( or make them worse). Just my .2


First let's look at the N75 valve! It's a very simple device, but somehow in the VAG community it's turned into some mythical voodoo item that has a mind of it's own. It can control pressure sent to the wastegate actuator or bypassed/bleed to the TIP, but has its limits. The limits are in it's reaction speed and the amount of pressure it's designed to control. Go outside of the factory parameters and the N75 will struggle (regardless of what duty cycle you preset in the ECU). There is only so much the valve can do when it's limited by its restrictor size.

Now to come back to our adjuster screw discussion, there is screw that allows the *static* preload on the plunger to be adjusted (some variation of the valve has the screw blocked, so people wouldn't mess with it). What does that mean? The plunger is what allows 'bypass' or air to be bleed back to the TIP when the valve is electrically cycled. By preloading the plunger more than factory set, you can allow some bypass even in static state (no electrical cycling), and to some extent how early (in pressure) the initial bleeding process starts. Clockwise increases the preload and counter clockwise reduces the preload. Simple as that! It is to be noted that the adjustment range is very fine (super soft spring), so a full turn could mean always bleeding significant pressure and allowing overboosting and surging. So make small adjustments with logging as your reference. 

With that said, is it for the noob to fu*# with? The answer is NO! However, if you understand what you're doing and do it properly, you have some adjustment room that allows you calibrate the valve's operation to your specific conditions. This IMO is better than blindly swapping N75 versions and making a prayer that it'll do whatever you're looking for. I don't run an N75 anymore because I'm way out of the range of pressure that it could accurately and consistantly control. When I used it, I ran two MBCs with different spring rates. One controlled the boost signal with soft spring (acting as an adjustable restrictor orifice); and the second, with stiffer spring, controlled the pressure going to the wastegate actuator. This allowed me to have independent control over onset and boost ceiling. So it's something to consider if anyone wants super accuracy/control over their N75 and wastegate duty.


N75 taken apart so we can visualize what preloading the plunger would achieve.




























Valve cutaway (H valve). Here you can see how turning the screw clockwise preloads the spring and forces the plunger down effectively blocking/restricting the wastegate port. The more adjustment rotation the more blockage is achieved, up to the point where completely plug the wastegate port and causing overboosting and surging.













Exploded diagram to illustrate the screw and plunger action in relation to the wastegate port. You can see how the plunger is able to restrict or block the wastegate port, but not the bypass port. By moving the screw, you can basically manually adjust some wastegate port blockage without electrical cycling.


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## 20v master (May 7, 2009)

Max, can you describe the orientation of the MBC's in relation to the N75? Soft before, stiff after, both before, both after, parallel, series, etc? I used to mess with this all the time, and usually ran an MBC in parallel with the N75, but didn't really understand why it gave the results it did. I have some surging at boost onset (only spiking to 21) and slightly after, then it smooths out, but I'd like to dial it out as it's quite noticeable. I had planned on swapping to another N75 as I have one laying around, but I never heard of anyone actually having any good resuls with attempting to adjust preload on the N75 spring. LIke you said, they're a crap shoot if you're just blindly trying versions to find one that works.


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## Marcus_Aurelius (Mar 1, 2012)

20v master said:


> Max, can you describe the orientation of the MBC's in relation to the N75? Soft before, stiff after, both before, both after, parallel, series, etc? I used to mess with this all the time, and usually ran an MBC in parallel with the N75, but didn't really understand why it gave the results it did. I have some surging at boost onset (only spiking to 21) and slightly after, then it smooths out, but I'd like to dial it out as it's quite noticeable. I had planned on swapping to another N75 as I have one laying around, but I never heard of anyone actually having any good resuls with attempting to adjust preload on the N75 spring. LIke you said, they're a crap shoot if you're just blindly trying versions to find one that works.


Let me break it down! 











Port on the left of this diagram is the wastegate port
Port on the right is the bypass/bleed port
Port at the bottom is the boost pressure reference 

Now, with the dual MBC setup this is what you want to do:

Make an inline type ball and spring MBC and don't include a bleeder hole in your construction (not needed at that location). This MBC goes to the boost reference port (bottom port in the diagram). You want to use a relatively soft but long spring in this, as it will give you more control and travel. This is will trim your boost ceiling, as it is acting as an adjustable restrictor (like the one built in the N75). The more pressure you allow/restrict to the N75 the more or less pressure it can send to the wastegate actuator to mechanically control the overall boost generated.

The second MBC can be a regular "L " type or inline with vent hole (it can even even be a bleeder type because it's going after the N75). This MBC can use the standard "stiff" springs and will adjust your boost onset. It goes in the wastegate actuator line and controls the pressure at which a signal is allowed to the actuator. Make it soft and the onset will be gradual and tamed. Make it hard and the onset will be untamed and brutal (the way God intended it to be, but Satan had to introduce weak cast connecting rods to the world ). I like to set mine so that the wastegate don't see pressure until pass peak torque, you probably want the opposite. 

I'll see if I can find pictures of the old setup for you. :beer:


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## 20v master (May 7, 2009)

Makes sense. :thumbup:



Marcus_Aurelius said:


> Make it hard and the onset will be untamed and brutal (the way God intended it to be, but Satan had to introduce weak cast connecting rods to the world ). I like to set mine so that the wastegate don't see pressure until pass peak torque, you probably want the opposite.


I chuckled. :laugh: I'm not so concerned with the violent onset as smooth delivery. I hate feeling and seeing in logs the yo yo roller coaster on the MAF/boost readings. :thumbdown:


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## Marcus_Aurelius (Mar 1, 2012)

20v master said:


> Makes sense. :thumbup:
> 
> 
> 
> I'm not so concerned with the violent onset as smooth delivery. I hate feeling and seeing in logs the yo yo roller coaster on the MAF/boost readings. :thumbdown:


 That's PID boost logic for you! Always re-acting. That's why I finally said the hell with it and went completely mechanical (why fight with the yo yo of ECU-based boost control when it can all be controlled with simple mechanical devices that you set and forget about..). In your case, you need to fine tune both N75 signals (to and from), in order to achieve perfect linearity.


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## 20v master (May 7, 2009)

Marcus_Aurelius said:


> That's PID boost logic for you! Always re-acting. That's why I finally said the hell with it and went completely mechanical (why fight with the yo yo of ECU-based boost control when it can all be controlled with simple mechanical devices that you set and forget about..). In your case, you need to fine tune both N75 signals (to and from), in order to achieve perfect linearity.


 Nobody knows what a PID controller is. I used to be pretty good at PID equations, but that was a while back. Anyways, I think the bigger issue is the variance in the preload in N75's and stock WG arm positions. The only reason I'm inclined to avoid full mechanical is the part throttle full boost that I'm used to with the K03. I'm assuming it's about the same on the K04-02x. I have two ball and springs I'll play with on Monday since I'm off work. I also have a Greddy Type S EBC, so that might be the better solution, but then means I have to come up with a mounting solution for the control box.


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## jacobm (Aug 5, 2011)

Wow these are the most informative posts about the n75 I've seen. Thanks for all the info guys


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