technicaldata

More on N75 Valves, what are they?

A: The N75 valve controls the wastegate of your turbo. The N75 valve is a simple solenoid, that works on the same concept of a latch relay, but it flows air instead of electrical current.

The N75 has 3 nipples, NO, NC, and a COM. The NO is "Normally Open", the NC is "Normally Closes", and the COM is the common input/output.

When there is no load applied to the N75, air simply flows from the COM nipple to the NO nipple, while the NC is blocked off. There is also a air nipple coming out of the turbo itself, that feeds pressure. The N75 pretty much REDIRECTS this pressure either into the wastegate actuator (lowers/controls boost by opening the wastegate), or REDIRECTS and seals it off, where the wastegate actuator will remain closed to raise boost. When a 12vdc load is applied to the N75, the NO (which normally flows to the COM) is not SHUT, and air will be directed from the NC instead (which is now OPEN to flow to COM).

Based on this, a series of "on and off's" of the N75 (aka "Duty Cycle") will control and regulate boost, controlling how much air is actually bled off into the atmosphere, or how much air will be going into the wastegate actuator to lower boost pressure.

Here is a better description of how it works:

Normal Position (Used to lower boost):

Wastegate <-airflow- [N75] <-airflow- Turbo Pressure

(Pressure from turbo is passed directly into the wastegate actuator, thus opening it and lowering boost)

Under Load (Used to raise boost):

Intake/Atmosphere <-airflow- [N75] <-airflow- Turbo Pressure

(Air from wastegate actuator is bled off into the intake tract, thus closing the wastegate and building boost)

There are 3 most commonly used versions.

N75C also known as stock N75 works best with stock or performance software.

N75J also known as TT225 N75 is used go give slightly more boost (more torque) then stock N75, in some instances limp mode might occur.

N75H Also Known as N75 Race valve will give up to 3 PSI more boost then the stock version. Although giving huge jump in torque it's almost guaranteed to cause the limp mode.

N75J and N75H work on a principle of delaying the speed, at which the ECU signals are interpreted, more delay equals higher peek boost and elevated boost levels throughout the rev range. (From AudiGeeks http://www.audigeeks.com/forums/index.php?topic=1047.0)

Wastegate Functions

Wastegates all have preset tensions on them, as well as a pressure rate. Most standard internal wastegate actuators are set anywhere from 14-18psi, and it depends on actual boost to control the actuator. Now this doesn't mean that a 14lbs actuator will only hold 14 lbs of boost, but more so of how it is tensioned on that rod, as well as how much pressure it sees (which is regulated by a MBC/EBC).

If a wastegate were connected directly into the turbo nipple, the minute it sees a low as 1psi of boost, it will open the wastegate immediately venting out all boost. A wastegate flap only needs as little as a few thousandths to expel most of its boost. Even with a fully opened flap, a turbo can still make boost (depending on its size). A typical 28RS turbine can still make as much as 12-14psi on a fully opened wastegate.

So in comes the boost controller...which has 2 flavors. The first type is your manual boost controller, which works simply by restricting the amount of pressure going into that 14-18psi actuator at a FIXED PERCENTAGE. You can dial in that turbo by adjusting the actuator rod to make the load stiffer, which means it will take MORE boost to open it, or vice versa. It it normally takes 14psi to fully open a wastegate, putting a in-line restrictor like a MBC will either keep that the same, or raise that threshold. By adding a restriction, it means that it will take MORE boost to open the wastegate, and thus regulate boost. So a 14psi wastegate can take as much as 20-25psi just to open it fully, because the actuator sees less. You can also accomplish the same by tightening that actuator rod. Whereas it would normally take 14psi to fully open a wastegate, putting higher tension on the rod will also raise that threshold, while keeping the boost controller baseline the same. This is the reason why the article indicated "more boost" as the rod is tightened, and "less boost" as the rod is loosened--you're simply regulating the pressure needed to control the actuator.

The other type of boost controller then of course is the EBC, or Electronic Boost Controller. An EBC does not work on a fixed-level resistance, but rather in pulses (measured in duty cycles) or pressure being either A) going into the actuator to regulate boost, or B) bled out via the intake or atmosphere. The number of pulses (flaps) on a EBC will determine how much actual pressure is going to the wastegate actuator. EBC's all have it's own dedicated pressure sensor, identical to a MAP sensor, that detects how much boost is being made. So when you dial in your EBC to make 20psi, it will minimize the pulse lengths of pressure going into the actuator, and bleed it into the atmosphere UNTIL the achieved and requested boost is met. Once the actual boost is met, the EBC will then make the solenoid start to vent just enough pressure, to maintain that boost threshold. EBC's are less dependent on how the wastegate actuator rod is tightened, but having a tighter rod will still give you the faster spoolups, but more spikes and less control by the boost controller. (Jeff, AudiGeeks

http://www.audigeeks.com/forums/index.php?topic=2337.0)

Adjusting Actuators

Should be able to adjust the actuator and therefore the boost by turning it out 3 - 6 turns counter-clockwise. Will have to remove the rod linkage at the turbo in order to make the adjustment.

Lengthen Rod for Less Boost

Shorten the Rod for More Boost

(Nicholas, Performance Turbo)

Internal Wastegate, showing "flapper" valve

(Source, Wikipedia)

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