# N75 Valve Explained



## peter-ss (Sep 3, 2008)

Whilst reading about the N75 Valve I found this post on another forum, which is the best explanation of the operation of the N75 Valve that I've come across, so I thought I'd pass it on. This may have already appeared on the TT Forum, but if it has I haven't found it.

This is the link to where I found the write up and for ease of reading I've copied the contents below.
http://vagfans.com/forum/viewtopic.php?f=4&t=255

N75 valve operation
Before going on I want to explain the duty cycle and how the N75 receives "power".
I'll use a time of one second to explain the duty cycle. If the unit is on for 500ms and off for 500ms it means 50% duty cycle. If the unit is on for 750ms and off for 250ms it means 75% duty cycle. If the unit is on for 1000ms and off for 0ms it means 100% duty cycle. The higher the time that the unit is on for that second higher the duty cycle until 100%. The lower the time that the unit is on for that second the lower the duty cycle until 0%.
To control the boost the N75 receives PWM (Pulse Width Modulation) signal. This is a series of on and off power signals that it receives from the ECU. This goes with in hand to the above. I'm not sure what time frames the actual duty cycle is calculated on for VW, but let us use one second.

This briefly explains the operation of a N75 valve. Also made a quick rough drawing.
The N75 valve function is to control the boost. This is done by using the existing boost by tapping into a boost pipe just after the turbo compressor.










The N75 was designed to let the boost feed to the actuator, follow the red arrows, all the time (from 1 to 2 and 3 is blocked, in above pic) when there's no electrical current flowing or if the electrical plug is not connected. This is a "safety" feature in case the electrical cable has been damage, cut, plug popped off, etc...

By feeding boost into the actuator the actuator will push the actuator arm which in turn will open the waste gate. By opening the waste gate, some of the exhaust gas will bypass the turbine. This lowers the pressure across the turbine which causes the turbine to turn at a slower pace, which in turn will create lower boost.

If the ECU starts to request boost it applies an electrical current to the N75. This will block 1 off. Between 2 and 3 will now open. This will release the pressure against the actuator and the waste gate closes, follow the blue arrows.

If you look at the following picture, it is a waste gate duty cycle from a K03 MKV GTI. Other turbo VW vehicles will work on the same principle.










Right in the beginning of the graph, from 2000rpm and just before 2080rpm, you'll see the duty cycle is at 0%. This means there's no electric current following through the N75 so the 1 and 2 path way is open (red arrows). This happens when no boost is requested. This is usually when you cruising, lifting your foot off the accelerator, idling, etc...
Now towards the end of the graph, from about 7000rpm to the end, you'll see the duty cycle drops from a high percentage to 0%. This means I've lifted off and the boost will continuously feed the actuator to drop the boost levels as there's no electric current flowing through the N75.

Above 2000rpm to about 2300rpm you'll see almost 100% duty cycle, that's when I stepped flat on the accelerator. This means there's electrical current flowing through the N75 and point 1 is blocked and 2 & 3 is open (blue arrows) to release any pressure in the actuator to close the waste gate. In turn the turbo will start to boost.
Once the boost levels have been achieved the N75 starts to receive more off time so the actuator can operate the waste gate to stop the boost from rising. As the rpm climbs the duty cycle percentage climbs. The N75 starts to receive less off time (more on time) and the actuator starts to close the waste gate to try to keep the boost level, as the engine is asking for more air.

Here's another graph with the requested boost and actual boost with the duty cycle. I've scaled the duty cycle by multiple of 10 so you can clearly see it on the graph. Divide by ten to get the actual level.


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## rcall1057 (Feb 12, 2010)

that was really helpful. I've been wondering how it played into the whole equation. thanks for the research


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## KammyTT (Jun 28, 2006)

i only read the first 4 sentences but thanks anyway


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## rcall1057 (Feb 12, 2010)

KammyTT said:


> i only read the first 4 sentences but thanks anyway


lol, you will likley not be able to impress your friends when the conversation suddenly turns to the duty cycles of the N75 valve :roll:


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## KammyTT (Jun 28, 2006)

i doubt my friends will actually know what the n75 valve is :lol:

but hey


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## peter-ss (Sep 3, 2008)

rcall1057 said:


> that was really helpful. I've been wondering how it played into the whole equation. thanks for the research


I'm glad you found it useful. I never really understood why it had three pipes connected to it but after reading the post it all makes sense.


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## TTitan (May 11, 2007)

KammyTT said:


> i only read the first 4 sentences but thanks anyway


 :lol: :lol: :lol: :lol: :lol: -- I feel better now after reading that, I struggled with the first 6 sentences and then skipped to the art work. My engine looks nothing like that artist depiction, must be becuase mine is APX? Maybe I can get my daughter to explain what the words mean -- she is good with computers.

Well I must log off now -- back to work fixing cars in my AA van.

TTitan.


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