1) A 4ooHP TFSI 3.0 V6 out of the new S4 . That's unlikely , because Audi isn't dum enough to put a smaller engine in RS5 than in the S5 .
2) A 590HP TFSI 5.0 V10 out the RS6 . That's nearly an impossibility . Because the RS5 will be alot lighter than the RS6 . So with this kind of power , it'll take out the R8's supercarish shadow . And they simply wouldn't do that .
3) A 450HP FSI 4.2L V8 out of the S5 . This is the most sensible . the same engine that gives out more power . It's the same trick they did with the old S4 and RS4 . And UK's CAR magazine claim they got an early look at the RS5 , and say this is the engine they went for .
But what ever engine it'll have , this will be the BMW M3's biggest rival .