Are you asking about OE engines or any engine that will fit into a B6 A4? In Europe and RTW, Audi has a lot of different engines engineered to fit into the B6. Anything from a 1.4l, and 2.0l TFSI gas engines, the new 1.8l TFSI I4 and 3.0l NA and the supercharged 3.0l DI gas engine, several 2.0l I4 TDIs, several 2.7l and 3.0l V6 TDIs and the 4.3l V8 NA DI Gasoline engine. There are also other engines I have not mentioned here.
If your are interested in only the two engines available to the NA market, the 1.8T turbocharged I4 and the 3.0l NA V6, both port fuel injection, each engine has it's merits and drawbacks.
The 1.8T is an Audi design originally, and eventually used in VW group vehicles from Audi, VW, SEAT, and Skoda, as well as for industrial and marine applications. The 1.8T is a multiple time 1st place "Wards Best Engines" winner. This engine is amazingly flexible, reliable and fuel efficient while providing unusually good performance. The 1.8T is a very stong engine structurally, and can develope more than 600 HP with enough boost and the correct fuel and ignition timing settings. The 1.8T was developed from the EA113 5 cylinder I5 engines that where produced in the late 70's and 80's, mainly by the development of the ground breaking 5 valve configuration. The newer 1.8T uses continuous intake cam timing adjustment with the exhaust cam timing fixed to the crankshaft position.
The 3.0l V6 engine is a more recently designed engine that has an aluminum block and heads. This engine is designed along the same lines as the 2.7T and 2.8l V6 gasoline engines with a different bore and stroke and the 5 valve heads with intake and exhaust cam timing adjustment. The 3.0l V6 is not known to be a very sturdy engine due to the aluminum block without iron or steel cylinder liners. Performance increases are limited due to natural aspiration and knock combustion onset limits to compression ratio and ignition timing tuning. There is a supercharger kit available for the 3.0l V6 with a poor record of reliability and problematic performance increases.
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