It takes a serious set of stones to go after the BMW M5 and the Mercedes-Benz E55 AMG. The two have been the undisputed kings of four-door performance for nearly two decades. Challenging them is like picking a fight with Clubber Lang and Drago at the same time, and even Rocky wasn't punchy enough to do that.

Well, it turns out Cadillac has grown a pair worthy of Easter Island. The 2006 Cadillac STS-V is a high-performance version of the rear-wheel-drive STS, created with the sole intention of taking on the legendary German super sedans. And it just might have what it takes.

Big power
Engineered by GM's Performance Division, the same bunch that cooked up the killer Chevy Trailblazer SS, the STS-V is not only the first production Cadillac ever to be supercharged, but with 469 horsepower, it's the most powerful production Cadillac ever.

Under the hood is a 4.4-liter interpretation of Caddy's familiar all-aluminum Northstar V8, which features double-overhead cams, 32 valves and variable valve timing. In STS-V spec, the engine is topped by a Roots-type supercharger making 10-12 pounds of boost and four tubular water-to-air intercoolers that look like foot-long metallic sausages. Cadillac calls it the Northstar V8 SC (supercharged) and considers it to be a clean-sheet engine.

"We didn't just bolt a supercharger to a Northstar," says John M. Barrick, the engineering manager for Cadillac's Performance Series of cars. "It's based on the Northstar, but over half the parts are new."

Those new parts include a stronger engine block, new cylinder heads, a lower compression ratio, and the most powerful engine control module ever used by GM. The combination is good for 469 hp at 6,400 rpm and 439 lb-ft of torque at 3,600 rpm, which means the 4.4-liter Northstar SC makes more than 100 horsepower per liter.

Impressive for sure, but we still can't help but be a little disappointed. We can't help but wonder, "What if?" As in, what if the Northstar SC displaced the same 4.6 liters as its normally aspirated counterpart?

Answer: It would make even more power.

But it isn't, and it doesn't. Instead, it's smaller in an attempt to strike a better balance between fuel consumption and the radical increases in power. And it worked, but not enough to make the STS-V the mileage leader in its class. Meanwhile, that drop in displacement assured it wouldn't wear the power crown, either.

Zero to 60 mph in less than 5 seconds, sort of
It sure sounds good, though, delivering a harmonic blend of blower whine and V8 rumble, which only intensifies as the tach needle climbs. Torquey, too. With a great big wall of the stuff right off idle. If it weren't for the hard-core fuel cutoff at 6,700 rpm, it feels like the big V8 would just keep pulling.

Cadillac claims the STS-V accelerates from zero to 60 mph in less than 5 seconds. And it does, sort of.

The first time we tested the STS-V, we recorded a best run of 5.1 seconds. So we called the guys at GM to tell them their car doesn't accelerate from 0-60 mph in less than 5 seconds. Figuring there must be something wrong with the car, General Motors sent a powertrain engineer from Detroit to check it out. The car was fine. But he did notice we didn't use any roll-out in our acceleration tests, which GM and many car magazines do.

Roll-out essentially gives the car 12 inches of movement before you start the clocks. You know, a head start. So we humored the guy, recalculated our radar gun and made a few more runs. Sure enough, with 12 inches of roll-out, the car's best 0-60-mph run was 4.9 seconds. Too bad we don't test with roll-out. Our 0-60-mph test starts at zero mph. So our official 0-60-mph time for the STS-V is 5.1 seconds.

With that settled, we made a few quarter-mile passes, and clicked off a best of 13.6 seconds at 106.2 mph, which makes it the quickest Caddy we've ever tested, but not the quickest car in its class. The last E55 wagon we tested hit 60 mph in 4.9 seconds and finished the quarter-mile in 13.1 seconds at 112 mph. We haven't run a 2006 M5 through our instrumented testing yet.

Odd thing is the STS-V feels even faster than it is. It rockets off the line and gathers speed like it has fallen off a building. The weak link in its acceleration equation seems to be its six-speed transmission, which shifts slower than we would like. There is a manual mode for self-shifting, which we always appreciate, but it seems to take a year before the transmission grabs the gear you've asked it for.

On the plus side, the six-speed is very attentive around town and on the highway, where it responds to throttle inputs quickly and accurately.

Shockingly smooth, surprisingly agile
And it's the highway where the STS-V is most at home. Despite a retuned suspension, with 15-percent higher spring rates and 12-percent higher damping rates than a standard STS, the STS-V hums down the highway like the luxury sedan that it is. It's the kind of car you pack with three of your rowdiest hell-raiser buddies for a late-night banzai run to Atlantic City. Trump Taj Mahal, here we come.

It also handles like a car half its size. This 4,300-pound sedan is actually flickable. Although the STS-V is softly sprung compared to the M5, body roll is well controlled, and the big Caddy never feels floppy or sloppy. And as it should be, power oversteer is just a throttle stab away.

Feedback is spot-on. Although the speed-sensitive, variable-effort power steering is a little overboosted turning into slow corners, it's perfect in the faster stuff. Cadillac developed the STS-V on racetracks and proving grounds all over the world, including the legendary Nürburgring in Germany, and it shows in the car's high-speed stability. The STS-V is perfectly balanced in quick transitions. In our slalom test, it managed a very impressive 65.2 mph through the cones, which is 5 mph better than the Mercedes.

The "Ring" is also hell on brakes, which is why the STS-V has an awesome set. Brembos front and back, with four-piston calipers and huge 14-inch front and 14.3-inch rear rotors. They haul this big, heavy sedan to a stop from 60 mph in just 122.4 feet. Fade is simply a nonissue, and pedal feel is very good.

Big price
That kind of performance never comes cheap, and the STS-V's $77,090 sticker price makes it the most expensive Cadillac to ever reach the road. For now, that is. The title will transfer to the $100,000 XLR-V when it hits dealers soon enough.

That price, which includes a $2,100 gas-guzzler tax, also makes the STS-V about $5 grand cheaper than an M5 or an E55. Although the Caddy lacks some of the gadgetry and features found in the German sedans — things like the M5's clutchless manual transmission and the E55's adjustable suspension — everything you really need is standard on the STS-V. In fact, the car only comes one way, loaded. Standard equipment includes stability control, a sunroof (which can be deleted), DVD navigation system and XM Satellite Radio.

Caddy only plans to build between 1,000 and 2,000 STS-Vs a year. You can spot them by their larger, stainless-steel wire-mesh front grille, which Cadillac says increases airflow 34 percent to help cool the engine. The model also gets a prominent front splitter, a domed hood to clear the engine's supercharger and a larger rear spoiler, which looks a little glued-on for our tastes. The 10-spoke wheels, which measure 18 inches in the front and 19 inches in the rear, are also unique to the STS-V. It's all very tasteful without being boring, and Caddy says the splitter and spoiler have an aerodynamic benefit.

Inside, Cadillac leather-lined the standard STS interior and added some suede inserts on the standard STS seats. It's a nice touch and goes well with the Olive Ash Burl wood and aluminum trim, but the headliner is still crude mousehair. At this price point, it should also be suede.

World-class Caddy
According to Barrick, the 2006 Cadillac STS-V is about refined performance. He says the goal was to build a smooth, refined and quiet car that was a blast to drive and track-ready. And that's exactly what the STS-V is. It may not be the fastest sedan in the world, but it is the most luxurious, best performing and best built Cadillac ever. It's also further proof that good things happen when a car company grows a pair.