It's a very good motor, probably best in displacement range along with the M3, but mainly it's of interest to the less potent brethren in case there are easy upgrades.

I almost want to find out that I'm wrong about component compatibility, because just imagine how irritated you'd be if the 4.2L motor in the S5 started its life out as a 420hp RS4 motor and then Audi intentionally went through the trouble of developing less potent internals for the sole purpose of marketing; satisfying an evenly spaced product lineup so that their cheaper GT coupe doesn't kill their $70k super sedan.

It wouldn't be the first time. Look at Mercedes 6.2L V8 and how the engine output just happens to coincide with the price point: 451 in the C, 481 in the pre-black CLK, 507 in the CL/CLS/E, 518 in the S/SL... and then when CLK black series came out they can magically get 507 out of it, lol.

-Ray

 

I always thought the S5 engine was an S4 engine with direct injection. This is interesting.

I wonder how different they feel.

I'm gonna have to go test drive an S5!

 

Sorry to bump this thread, but the RS4 motor is the same as the R8 correct?

 

Essentially. The biggest difference is that the R8 is dry sump, whereas the RS4 has an oil pan. At a glance it looks like the intake track might be different but that's probably more due to the MR vs FR layout and doesn't seem to have any impact on performance.

 

Just adding some new perspective onto this thread -- I had just the write combination of curiousity and time to kill to create the attached horsepower chart using data scavenged and averaged from the internet (in total looked at maybe 12 different dyno charts).

Since there's only finite resolution of dyno sheets posted on the internet, I didn't bother plotting the actual hp numbers, since they're not perfectly accurate. What the chart does show is that in factory form, the S5 and the RS4 have really similar torque quantity of torque until about 5000 rpm (the S5's 4.2 has actually shown slightly more torque around 3000 rpm). That means all else being equal, the S5 actually moves around town just as easily as the RS4 (assuming your average driver doesn't usually exceed 5000rpm during normal driving), despite a 70hp peak hp differece. The S5 is ~150 lbs heavier, and I'm not sure about the gearing between the two cars, so it's hard to say exactly, but you get the idea.

Now take that concept and add in some typical mods people are doing on their S5's, namely exhaust, and even an ECU remap for some. Per the dyno sheet, by the time you have an exhaust and remap, one might be disappointed to find that their peak HP is still lower than the stock RS4, but looking at the graph you realize that the modified S5 actually has much more power until ~6000 rpm. Even up until 7000rpm, the gains of the RS4 are only marginal, its only advantage being that its powerband enjoys a final hurrah until 8250 rpm.

The point is:

1) comparing the peak hp, even on a dyno, is kind of pointless since by the time you get an S5 4.2 to the same peak hp as the RS4, it will be much more powerful overall

2) other than in situations where the RS4 will be revving to its limit, the engines are essentially similar, if not giving a slight advantage to the S5.

3) compression and cams more than likely explain these differences -- these are classic horsepower curve differences you see when changing to more aggressive cams (slight decrease in low to mid torque in favor of high end torque and higher redline), so doing a 'conversion' should be pretty straightforward and not exhorbitant as someone suggested on page 1.

 

Would anyone happen to know what the gear ratios are for the various 4.2L 6-speed transmissions and if the 6th gears or final drives are swappable?
Changing the gearing of the 6th and the final drive can make the S5 much faster while retaining the highway MPG. Simple and effective way to 'increase power' through torque multiplication while retaining OEM reliability.