Volkswagen 1.8T turbocharged four-cylinder engine has been called the German equivalent to Mitsubishi's legendary 4G63--and for good reason. The 1.8T has a tremendously strong block and rotating assembly, is capable of making 1000+ horsepower on race gas and its ease of modification has few rivals on today's market. One way to increase the 1.8T's boost is to replace the "N75," which is an adjustable valve that sit inline between the turbocharger wastegate and boost sensor line, and uses computer input to restrict pressure in the boost pressure line. The upgrade comes from VW itself in the form of a stock replacement "N75J" valve designed for the same-year Audi TT225.
Remove the stock turbo-to-intercooler pressure tube by loosening the clamps that secure it to the silicone rubber connector on top of the engine. Slide the tube out of the connector and lay it off to one side so that you have easier access to the stock N75.
Identify the N75. The valve has a three-way fitting on the bottom, which has a 90-degree rubber elbow and corrugated vacuum line connected to one side and an angled, moulded-rubber line secured with a hose clamp on the other. The corrugated line is clamped to the plastic intake tube just before the flex portion. Turn the N75 to expose the round top and the wiring harness attached there. Unplug the harness.
Grasp and remove the hose clamp on the bottom of the N75 where the short section of tube connects to the intake. Pull on the valve to release it. Cut the non-reusable hose clamps off the other two rubber lines using either a pair of wire cutters or a hobby-style rotary tool. Be careful not to nick the hoses.
Install the new N75J J valve in the exact orientation as the old N75, with the longer of the two side-facing fittings facing toward the intake tube. Use new hose clamps to replace the non-reusable units on the two previous hoses. Re-attach those two hoses, grasp the first hose clamp with pliers to release the pressure and slip the last fitting into it.
Slide the intercooler tube back into the rubber connector and tighten the hose clamp. Reconnect the wiring harness plug to your new N75J valve.
Only one thing keeps this upgrade from being a true stock-for-stock replacement; the wiring harness plugs into one side on the N75 and the other side on the N75J. The stock wiring harness will reach, but make sure the longer side fitting on the N75J faces the intake tube. The N75J is not only worth about 0.907kg. of boost (about 25 horsepower on the 1.8T) over the original N75, but its more sophisticated design will help to reduce compressor surge and generally help the engine to run smoother under boost.