I've heard different stories from various sources about what engine they will be using:
The current RS6 engine
The same current 4.2 V8 as used in the S4 but tuned slightly
The New 4.2 FSI engine
I don't know what to believe, but It should have over 400bhp easy [

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Oh yeah and i doubt the finshed car will look like that, It needs to be More aggressive to warrent the RS badge