N75F valve

**GT3** · 05-02-2008, 11:52 AM

Curious to know if anyone has changed or investigated the option of changing their Polo's N75F valve with another one, eg N75J ? Some explanatory info below:

The N75 is an electronic air pressure regulating valve. It sees ambient air pressure (1.0Bar, our atmosphere is pressurized) and the pressure of your charge air after it leaves the turbo. Then based upon the duty cycle signal that the valve receives from the ECU, it will allow more or less of that charge air pressure to flow to the wastegate actuator on your turbo. This in turn will allow the turbo to either make higher boost pressure or drop pressure by letting the exahust gasses bypass the turbine wheel on the turbo.

The main change between these various N75 valves is the way that they react to the duty cycle signal. Some valves have a small threshold for the need to lower or level out boost pressure. Some valves are intended to allow much higher boost levels and hold them even throughout the entire RPM range. In the end, the ECU still controls the valve's open/closed state. The valve just reacts faster or slower to these signals.

**ryan_GTI** · 05-02-2008, 01:25 PM

Changing to the 'J' valve seems to be a very popular mod for the 1.8t over in the states, so i guess it's a very worthy mod.

Also read in a UK forum that boost picks up a little quicker... so better response overall perhaps?

EDIT: Just did a little bit more reading about the 'J' valve... apparently it's abit of a hit or miss - some people are experiencing unhealthy boost spikes and surging, whereas others are gaining extra pounds of boost with no issues and loving it!

Also read that APR recommends leaving the 'F' valve - Guy maybe you could provide some insight?

If it does prove to be a success story, then this would be a fairly effective cheap mod!

**GT3** · 05-02-2008, 01:51 PM

That's the issue - by pairing an upgraded N75 with a high boost ECU program you can hit "limp mode".

The N75 version "J" is popular, and in some 1.8T applications, reacts in a very linear way throughout the RPM range and may not trigger "limp mode" as encountered with many 1.0 bar chip programs.

This is why I'm curious to know whether it's been attempted on a Polo GTI.

**Guy_H** · 05-02-2008, 02:31 PM

Polo runs an 058 906 283F valve - the only AUS delivered car that ran the "J" valve was the Audi 200T (circa 1982) - you can buy them new from your Audi dealer for about $120.

I have only seen them tried on MKIV GTi's & A3's (DBC models) and although it picks up boost a little quicker, it ran a little flatter through the mid range.

So it "felt" a little quicker, but was not actually, and maybe a touch slower through the mid range.

Some one needs to spend the cash & data log it - I'll wage a carton of beer it will see no quantifiable gains on a dyno!

**Guy_H** · 05-02-2008, 02:32 PM

P.S. Most software tuners frown apon it as they spend hours & hours tuning it for the valve supplied on the car! On a stage 3+ car (where we are hitting 28 - 30PSI) - the difference is so great, the wrong valve will send you into limp mode pretty quickly!

**Timbo** · 05-02-2008, 02:59 PM

Originally Posted by Guy_H

Some one needs to spend the cash & data log it - I'll wage a carton of beer it will see no quantifiable gains on a dyno!

Struth! That's upping the ante. I thought I was the resident cynic, but I've never waged beer!

**fuzion** · 05-02-2008, 03:06 PM

thats interesting...

from what ive been told those valves will start to leak or fail, if you have to change it, a j valve will generally give a better result, at worse the same as a new stock one.

as for extra boost, variying results have been found, depending on other factors, on hard ware in the car and ecu tune itself

theres a lot of this in the vortex in FAQ under the 1.8t forum and golf iv thread as well that says basically the same thing as i had a search and read as i was a little confused about 'why the hell do we change these valves for?'

Originally Posted by N75 info elsewhere

H, higher spike than J, but holds less.
K, I haven't really heard much, and what I've heard is conflicting, but it does hold more than the others.
C, older stock valve, holds a bit more than F
F, newer, smoother valve found in 1.8Ts

Originally Posted by vortex user

Yeah stock turbo with APR software. I talked to the APR rep on here and he said to get an "H". Where Can I get one.

Theres endless threads on VW Vortex from every ecu tuner and modifier around saying changing the valve is the go and of course varied results, some working better then other cars, or no difference at all on others dependant on mods, and tune.

So im confused

gpk_gti · 05-02-2008, 03:16 PM

I ran a dyno without the n75 race valve and then with it...gain was 1psi across the rev range.this was before my chip change.

**GT3** · 05-02-2008, 03:43 PM

Originally Posted by gpk_gti

I ran a dyno without the n75 race valve and then with it...gain was 1psi across the rev range.this was before my chip change.

when you say 'race' valve, you mean "j" valve right? ..was there a power difference on the dyno? (there could be a carton of grog coming your way)

anyway, here's another perspective to perplex us even further:

http://www.ozaudi.com/forums/showthread.php?t=1308