The S4 is an indirect injection 4.2 V8 with an upper rpm limit of 6800 rpm.
The RS4 is a DIRECT injection (with FSI - Fuel Stratified Injection = lean burn) 4.2 V8 with heavily modified bottom end to allow an upper rpm limit of 8250 rpm.
I swapped from an S4 to an RS4 just over a year ago and, believe me, the engines have totally different characters. The S4 engine doesn't like high revs, it's not worth going over 6000rpm - the RS4 just gets stronger and stronger as the revs rise and it positively throws itself at the 8250rpm redline (I hit the redline almost daily because of that). BTW when the oil is cold - under 60 Celsius - the redline is set down to 7000 rpm and it still surprises me when I can't do 40mph in 1st gear with a cold engine.
When I first got my RS4 is described the S4 engine as "a boulevard cruiser" and the RS4 engine as "a race engine" - after a year I stick by that description.
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