The displacement on the C5 RS6, RS4, and R8 are all the same @ 4.2L. Same stroke and same bore for each. The RS6 makes different power due to forced induction while the RS4 and R8 make the same. My question is this: Why is the RS6 engine designed to rev 6500rpm while the others 8000 rpm? When you take the engine management out of the equation along with the forced induction, you are left with a long block that for some reason is designed to perform at lower RPM's. What factors are behind this? Cam duration? Vale spring rates? Certainly the main and rod bearings/bolts are designed to handle the higher revs so what is it? What I ask?? Just curious as it would certainly be nice to get the engine revving more in line with what it can do in the other high power Audi's.