Dyno Test: 2008 Audi R8 vs. 2007 Audi RS4

Our Four Seasons Audi RS4 has a lot of fans around the office. While it's a great all-around car, it's the 8,250-rpm, 420-hp, 4.2-liter engine that really gets our attention. The V-8's power delivery is so linear that it feels like an electric motor, except of course no electric motor could ever sound this good. I once described its staccato exhaust note as sounding like a thunder cloud. Assistant editor Sam Smith said it sounded "like an angry, drunken bear being shot from a cannon." One of us is obviously a big fan of hallucinogenics.

Anyway, Audi's new R8 shares the RS4's engine, albeit with a couple of small changes. First, it has different intake and exhaust paths due to its mid-engine layout. Then, its dry-sump lubrication system allows the engine to be mounted lower in the chassis, permitting the use of a smaller, and lighter, flywheel.

The lightened flywheel is evident as soon as you turn the key. The R8's V-8 revs much more quickly than the RS4's. So quickly, in fact, that it seems the flywheel must be made of helium. Teensy little prods of the throttle result in huge surges of revs, with sharp barks of anger shooting out of the short exhaust pipes.

In gear under load, the R8 sounds frenetic, evil, and pissed off. By comparison, the RS4 sounds happy, refined, and non-threatening. Never thought you'd hear those words about an RS4, did you? The R8 sounds that good.

On the road, the R8's power delivery felt similar to the RS4's - it has a huge amount of torque available at any speed. Unlike the RS4, though, which hits hard at 3000 rpm and stays there, the R8's torque output builds with revs, gradually coming to its peak.

This isn't surprising - the RS4 uses long intake runners, which help boost low-end torque, while the R8 uses short runners that favor high-rpm breathing. That small difference was enough to convince me to take both cars to a chassis dyno to find out how much of a difference there really is.

We had to massage the final chart because the R8 exhibited a big drop in output between 2000 and 4000 rpm on the fourth gear runs, caused by a glitch in the European-specification, pre-production powertrain computer. The curves shown below are corrected using the fifth gear curves as a guide. The dyno used was a Dynapack 5000, which typically reads a little lower than the more common DynoJet unit.

Since everyone asks about peak numbers first, the RS4 put down 331 hp to the wheels, the R8 put down 338. The difference between those two numbers, which is about two percent, is insignificant. Given the 420-hp rating at the engine, their output corresponds to twenty percent drivetrain loss, which is commendable for an all-wheel drive setup.

The dyno chart confirms what we felt in the seat of our pants - the RS4 has a bigger torque plateau in the mid-range, where the R8's torque curve rises more slowly. Both engines, however, have torque curves so robust that they would have been unimaginable only a few years ago.

And both sound incredible.

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