Turbo chargers  
This is one change to be done if you want ridiculous amounts of power increase and torque. If you have had first hand experience of what a Renault 5 turbo is like and then times it by 10, you should be getting closer to the feeling it gives you in a GTi.
A turbo works off the exhaust gases to drive more air into the engine using a turbine, which is where the name 'forced induction' comes from. The increase in power comes when the turbo kicks in at usually around 3,000 revs and the surge of power is outstanding. Not the most cost effective tuning product but depends on your taste; a recommendation is definitely consult someone before doing it, as reliability of them has always an issue.
A lot of people believe that unless an engine comes direct from the factory with a turbo, its not worth the hassle, others say with the correct setup, you should have trouble free fun. Lets face it though, there's nothing better than the sound of a dump valve spinning WHCCCCSSSSHHHH!

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106 1.6 16v GTi-turbo
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205 1.9 8v GTi-turbo
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206 1.6 8v XS-turbo
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309 1.9 8v GTi-turbo



Article written by Ian Dews. 'Courtesy of Ian from Pug106GTi.com'