As the title says, whats the difference in these two engines? Why does the s4 put out the few extra horses, and does the slight weight reduction vs the A6 really add up to that much faster acceleration?

As you may have guessed I have a 4.2 A6 right now, but I'm really getting interested in a S4. If I could only find one for a decent price within the state.

 

S-models need to be always a little bit more powerful that similar stock engines I would guess that only software is different?

 

Well, I've noticed the engines look somewhat different even for the same year so I thought maybe they had some different parts on there. The s4 seems to be much faster though, but only slightly lighter and more powerful.

 

You are correct. These might be differences in S4 engine: all-aluminium alloy 4.2 litre, 40 valve (five valves per cylinder) V8 engine (parts code prefix: 079.C, identification code: BBK), with chain-driven double overhead camshafts, roller-finger rocker arm cam followers, and included variable inlet camshaft timing. It also utilised a strengthened alloy crankshaft bedplate for the five main bearings, and a two-part baffled oil sump, and a light-alloy variable intake manifold (VIM), containing the sequentially actuated fuel injectors.

 

I show the same as above, but the US S4 had the BHF 344 hp cam chain (from the rear of engine) driven 4.2 liter. The A6 had the AWN 300 hp cam belt driven 4.2 liter.