It depends upon the version of the car.
For first generation cars there were originally two options, both using a 1.8T engine.
These were either 180 or 225 BHP.
The 3.2 V6 engine was introduced later in the range which had 250BHP.
Audi also produce a stripped down and lightened TT using the 1.8 T engine to celebrate 25 years of quattro, this produce 240 BHP.
A lower output version of the 1.8T was introduced for the roadster version with 150 BHP.
Both the 150 BHP and 180 BHP were revised slightly and later produced giving approximately 163 and 193 BHP.
The second generation cars were initially introduced with with the 2.0T (200BHP) and the 3.2 V6 (250BHP).
The TTS was introduced later with a revised 2.0T engine which gives 268 BHP.
A 2.0T diesel was brought out at the same time giving 170 BHP.
Finally, orders are currently being taken for the TTRS which contains a 5 cylinder 2.5 T producing 340 PS, about 335 BHP, which should get it from 0-60 in about 4.6 seconds.
The 2004 Audi TT has an inline 4 engine.
The 2006 Audi TT has an inline 4 engine.
The 2008 Audi TT has an inline 4 engine.
The 2003 Audi TT has an inline 4 engine.
The 2002 Audi TT has an inline 4 engine.
The 2005 Audi TT has an inline 4 engine.
The 2001 Audi TT has an inline 4 engine.
The 2012 Audi TTS has an inline 4 engine.
The 2010 Audi TTS has an inline 4 engine.
The 2014 Audi TTS has an inline 4 engine.
The 2011 Audi TT has an inline 4 engine.
The 2009 Audi TTS has an inline 4 engine.