What is the N75 Valve?
The N75 valve is a fundamental part of controlling boost on a 1.8T engine. Its a small electronic solenoid valve that the ECU regulates to control how much boost the turbo has to create. The value that many logging tools show is the ‘duty cycle’ which is what the ECU is requesting the valve controls the turbo to make more or less boost depending upon the maps inside the ECU. (more on that later). A HIGHER duty cycle, will mean that the ECU wants the valve to close the turbo wastegate, thus forcing exhaust gases past the exhaust turbine, creating more boost. A LOWER duty cycle will mean that the ECU doesnt want to see more boost as per the mapping instructions.
If you look at a N75 valve you will see that it has 3 ports and a 2 pin electrical connection. The electrical connection plugs into the cars engine loom, which in turn is powered by a +12v connection and a ECU controlled ground. Also on the valve are 3 ports. You will notice that one of these pipes is about 10mm longer than the others. This is the atmosphere port which returns any boost pressure bled off back into the cars intake system to be re-used. (especially since this air has been accounted for initially by the MAF sensor, therefore fuelling accounted for).
The port directly opposite the long one goes to the turbo’s actuator. This controls the wastegate in the turbo to bleed exhaust gases off depending how much boost the ECU is requesting.
The pipe that’s remaining, and opposite the electrical connection, goes to a BOOST source. This cannot be a boost/vacuum source as you would find under the inlet manifold post throttle body. The best place for this is the charge pipe straight off the turbo, as per many OEM applications on K03 and K04 setups.
Above is a small schematic as to how the N75should be connected.
Why is the N75 the best option?
VW/Audi/Skoda/Seat spent millions of ££’s developing the valve to ensure that the engines boost was controlled as smoothly as it could. The N75 isnt a on/off valve like you find with many aftermarket manual boost controllers. The valve can regulate exactly how much boost at given RPM’s and engine load situations without any ill feeling to the driver. I’m sure a lot of readers have heard about stories where people have fitted a manual boost controller (MBC) and their turbo has felt like a on/off switch or had jerky, and often dangerous, periods at partial throttle such as pulling out of a junction where only small amounts of boost may be requested by the ECU from the throttle input from the driver. The reason for this is because a MBC is set by the user to 20psi for example, so this then because 100% duty cycle or nothing. There is no inbetween where a N75 would be able to request 50% duty from the turbo to ensure the driver could complete the maneuver safely. More on what the ECU sends to the N75 in a bit.
How reliable is the N75 and what are it’s limits?
Having spoke to many of the UKs best 1.8T tuners, who in themselves have tuned tens of thousands 1.8T engines over the years, the N75 valves VERY rarely fail. Maybe a failure rate of 1 in 1000 cars through their doors would have a genuine N75 valve issue.
The N75 will also run no problem on turbos that are bigger than the factory K03/K04 and we have seen cars running GTX35 Turbos with the stock N75 well in excess of 550-600bhp and still controlling boost and part throttle boost demands like they would as they had just drove out the factory.
The ECU and the N75 relationship
Inside the ECU, is a small number of maps that control the boost from the turbo. There are maybe 8-10 maps inside which control the turbo and N75 valve. These work together in a PID setup to ensure that the turbo is controlled smoothly. But that’s a bit more advanced than our discussion today. We will however, look at the final map the ECU controls for the actual valve itself. This is where the duty cycle is set against 2 axis; Boost pressure in hPa (hectopascals) and RPM (engine speed)
Below is the ECU map which determines the duty cycle that is sent to the N75 valve. (Click image to view full-size)
As you can see from the image, the ECU determines the boost duty by a map based on RPM and boost pressure. For example: 1100hPa at 5000rpm, the ECU will request 95% duty from the valve, closing the turbo wastegate and creating more boost until the demand is met. Of course, these valves aren’t fixed as the Motronic structure will interpolate these values.
As you can see from lower boost pressures, and lower RPM, the boost duty request is much lower; around the 25-40% region. This is your partial throttle boost to ensure you as a driver get a nice smooth response. If it was a MBC in the same position, the value would be 100% as they are fixed duty cycle.
To be continued….