The fourth generation Golf GTI, known as the MKIV, is one of the best GTIs that VW has made over the years. After losing the plot a little with the third generation GTI, the MKIV returned to the GTI's roots of being a fun-to-drive hatchback. Part of the appeal is the turbocharged 1.8-litre motor that was available from 1997 to 2006 models. There were two variants of this motor, the 138-horsepower version and a more powerful 178-hp version. Both engines take well to modifications.
Like most turbocharged cars, this engine is perfect for tuning. It's easy to get over 200-hp and there have been MKIVs built with over 500-hp. GTI drivers can take a variety of steps to get more horsepower out of their particular model.
One of the most common and effective routes to getting more power from a 1.8T motor is by having the ECU (Engine Control Unit) reflashed. This changes the ignition and fuel maps to a more aggressive curve as well as raising the rev-limiter and top-speed governor. '99 to 2001 cars will end up with 195 to 200 horsepower, and model years 2002 and later will realise about 220 horsepower. Companies like Neuspeed offer this service (which requires that they have the ECU from your car) for about £260 (all prices as of September 2009).
You can also replace the restrictive stock intake on the 1.8T with a cold air intake like the one that Evolution Motorsport offers for £122. This adds another 8-12 horsepower. The V-Flow intake, for £194, adds 10-15 horsepower. Most companies that make parts for GTIs offer intakes.
Turbocharged cars benefit greatly from a free flow, larger diameter exhaust, and the GTI is no different. An aftermarket exhaust from a company like APR is usually good for another 10-15 horsepower.
Heat is the enemy of horsepower, so a larger intercooler is a good idea as well. By providing more cool air to the turbo, the motor gains more horsepower. Adding an intercooler is also an important step if you decide to get a larger turbo.
By far the biggest gain in horsepower on a 1.8T motor comes when the relatively small factory turbo is replaced with a larger turbo. GReddy makes a turbo kit that uses a TD04H-19T turbo with a bigger turbine and compressor wheel. This is also the most expensive upgrade and runs over £1,950, not including installation. Turbo kits usually come with a software upgrade, larger fuel injectors to provide the proper amount of fuel as well as any associated tubing and oil lines.