If anyone knows better, please correct me, but when the FSI engine came out it stood for Fuel Stratified Injection. Then the GTI came out, it was Turbocharged Fuel Stratified Injection. TSI Engines came along and as they were all originaly twin charged they were Turbocharged Supercharged Intercooler, but then VW decided to rebrand all there charged petrol engines to TSI even the ones without twin charged mills. so all VW charged petrol engines are now known as TSI engines, ranging from the 1.2 turbo, the 1.4 turbo, the 1.4 turbo and supercharged, the 1.8 turbo, the 2.0 turbo etc.. when i last spoke to VW headoffice VW UK can not give an explination as to the english translation of the TSI branding, but all charged engines are TSI, and they are referred to as inteligently charged engines. this is of course going forwards, i am not saying your 05 GTI has a tsi, as it is obviously a tfsi.