N75 valve - how does it work

[yorkie]

Just replaced this and car seems quicker but only Vag-Com will really tell.

As above can anyone explain how it works please?

I don't like replacing things unless I understand why they need to be replaced for.

I have read the SSP but it is a bit vague.

To help I do know the following..

It has three hose connections and one electrical.

Its duty cycle (whatever that is) in percentage roughly equates to the percentage of boost given by the turbos which I would expect as it is a boost controller.

What I don't know is..

What duty cycle means.

How it regulates boost - the mechanical process.

Whether it is controlled by the ecu signal from the boost sensor or from the boost output from the turbos.

Sorry if this has been posted before but does anyone really know how it works?

Cheers

[Blue_Thunder]

My basic understanding is that the N75 is controlled by the ECU. The N75 controls how open/closed the wastegates are on the turbos (works the actuators by vacuum?). When the wastegates are open they allow exhaust gas to bypass the turbine blades, therefore resulting in much less boost. I think a dodgy N75 sometimes doesn't allow the wastegates to fully close(or maybe it does allow them to fully close, but not as quickly as they should). This appears on boost logs as actual boost slightly trailing requested boost.

I'm actually pretty interested to know how much of the exhaust gas is diverted from the turbine blades if the wastegates are left fully open (eg: when the N75 is disconnected) if anyone knows.

[saf]

Yes Id like to know also

Ive been thinking of getting a dual map for the car to run around town in. One where I can just run the turbos just off the standard actuator boost levels which should be around 0.7 bar. This would be good for around town driving in traffic etc.

And one for when you want to use it to the max.

this would be good for the Stage 3 S4's around town.

Maybe maybe not.

Well see. But I believe that you can disconnect or do something to the N75 thus allowing a valet mode like effect where the ECU doesnt deliver the boost. Not sure if it will run like stock in limp mode or not.

[S4TAN]

Whether it is controlled by the ecu signal from the boost sensor or from the boost output from the turbos.

I think that's the same thing - i.e. the MAP (boost) sensor lets the ECU know how much charged air (boost) is being produced and the ECU then regulates the duty cycle (the speed at which the N75 solenoid valve opens and closes to control the wastegates - and thus controls boost) ... or something.

[MCB]

I think that's the same thing - i.e. the MAP (boost) sensor lets the ECU know how much charged air (boost) is being produced and the ECU then regulates the duty cycle (the speed at which the N75 solenoid valve opens and closes to control the wastegates - and thus controls boost) ... or something.

One short paragraph - Vorsprung durch Technik

In the old days that would of been 200 words from the much missed Al who is on a fishing Holiday!

[S4TAN]

the much missed Al who is on a fishing Holiday!

2nd that ........... but you've gotta wonder about a man who can prefer a fish over a car ...


I mean c'mon ... it's just a f*ckin fish for christs sakes! Can you drive a fish? NO! Can you spend thousands modifying a fish? NO! (unless you're a cutting-edge geneticist maybe ... ) - i can tell you this much: if i come up against a fish waiting at the traffic lights i know for damn sure who's gonna win!

[yorkie]

From what I understand if the valve is not working mechanical regulation takes over and you are limited to around half boost.

Anyway, more thinking I may have answered my own question - yippee you all shout lol!

It seems to work like a three way valve, it has a solenoid operated plunger controlled by the ecu and is fed with pressure (pneumatic) from the turbos.

Under no boost conditions the wastegate is fully open so there is minimal pneumatic pressure in the feed line to the N75 valve which vents to atmosphere through the fully open plunger in the valve.

As the ecu requests boost the plunger in the N75 starts to oscillate creating a controlled pressure to the wastegates balanced out by vent to atmosphere which causes them to close according to the duty cycle of the N75 valve. If there was no vent to atmosphere to balance out the pressure the wastegates would just fully close straight away.

The more boost the ecu requests the more the plunger in the valve stays open diverting the pressure from the turbos back to the wastegates and closing them even more.

When peak requested boost is attained the opposite happens and more pressure is vented to atmosphere creating a balancing effect and holding the wastegates open at whatever position is required to maintain the requested boost.

As the wastegates are controlled by the boost pressure through the valve if there is a blockage in the system somewhere the turbos will try to over boost as the pressure backs up and this will be picked up by the boost sensor and the N75 vale position modified accordingly to reduce it.

On this engine the maf has no control over the boost.

Now if someone can tell me how mechanical boost regulation works when the electronic system fails you win tonight's star prize.

Cheers

[MCB]

Not sure who would be ars!d to bother unless you tell them what the prize is! Are we talking a million squid?

[Blue_Thunder]

i can tell you this much: if i come up against a fish waiting at the traffic lights i know for damn sure who's gonna win!

Just hope that fish doesn't challenge to an underwater rematch!!!

[S4TAN]

Are we talking a million squid?

Loving your fish-themed work there Paul ...

[yorkie]

Not sure I can offer a million squid within my quota, we are going for Redfish at the mo as they are outside the quota and loved by the continent.

Hows about a packet of the new Foxes Dark sweeties?

[Blue_Thunder]

I've been doing quite a bit of research since having my N75 disconnected due to a major boost leak. I'll spill the beans for Glaciers.

[ChipHazzard]

I mean c'mon ... it's just a f*ckin fish for christs sakes! Can you drive a fish? NO!

are we talkin fish tailing here .................. if so that might explain the amount of rear wheel dive bimmer's on the road

[lemond]

FiSH ...Full indepedant Service History.

For Christs hake

[Ads4]

Is there a porpoise to all this banter?