* Cast your mind back to the 2005 Frankfurt motor show and you'll remember one of the highlights, the AC Schnitzer Tension. Based on the then-recently launched M6, the Tension was a starburst orange-and-black be-winged beast, with lashings of carbon fiber and plenty of neat design touches. While Schnitzer claimed it would cost in the region of $345,000 if it were produced, it was fairly obvious this was a show car-a demonstration of what the company could do and a showcase for its new forged-alloy Type V wheels.

Fast forward to the end of last year and Schnitzer had another announcement to make at the Essen motor show-the Tension had just been timed at 331.78 kph at the Nardo test facility in southern Italy-that's 207 mph-making it officially the fastest road-legal BMW on the planet. And no, it's still not for sale. However, if you're dead set on the Tension look for your 6 Series, there's now the Street Tension: a limited run of 50 cars with 95 percent of the show car's looks and a fair chunk of its performance too.

We first catch sight of the (slightly) sanitized 6 in AC Schnitzer's Aachen workshop, and in this car's white-and-anthracite color scheme, it looks no less dramatic than the orange showstopper. Just about all the original's styling cues have made it to the street version, although the detachable rear spoiler isn't one of them. You still get the aggressive front spoiler, widened wheel arches, restyled hood and the subtly reshaped trunk lid and carbon fiber rear diffuser. There are plenty of trick details to take in, but it's only when we get to the business end and open the hood that a shiver goes down the spine-the color scheme even extends to the (admittedly plastic) engine cover, but the 5.1 logo and power and torque curves on top of the painted white airbox cover hint at the available performance.

It's a long time since we've driven a car from a major European tuner with a capacity increase, such is the current vogue for supercharger installations. Upping capacity for a hike in horsepower doesn't normally make sense these days, especially considering three-quarters of all BMWs sold in Germany are diesels. The cost involved for tuners in developing big-capacity conversions on petrol engines just doesn't stack up.

However, with BMW moving the game on with its Valvetronic cylinder head, it's once again made things tricky, as it's harder to make forced induction work with this system. Though not impossible: Alpina's current crop of V8 monsters proves that. But there's an awful lot of R&D to be invested-hence the 5.1-liter lump in the Street Tension.

The engine's had a thorough reworking and, because this first Street Tension is based on a 645Ci, it benefits from some serious power gains. The bores have been enlarged by 2mm, stroke is increased by nearly 10mm, leading to a cubic capacity of 5106cc. Power is 411 hp at 6100 rpm (up 78) while torque's been boosted from 332 lb-ft to 405 lb-ft at 3900 rpm. Naturally enough, there have been a number of major changes to the internals of the N62 V8 to achieve this: a modified block and head, new pistons, connecting rods and crank, as well as bespoke manifolds and exhaust. The ECU has been remapped and the compression ratio raised from 10.0:1 to 10.5:1. Schnitzer's claim of a zero-to-62 mph time of 4.9 seconds is entirely believable, as is the top speed of 180 mph.

However, the Street Tension has more to it than styling and an engine upgrade. Under the skin, there's an AC Schnitzer suspension set-up lowering the car by 30mm, as well as a set of ACS uprated anti-roll bars. When combined with ACS Type V lightweight alloy wheels (9x20 up front and 10x20 at the rear) running 245/35 and 285/30 rubber, you might expect a less than perfect ride. But on smooth German tarmac, it seems to be just as compliant as a standard 6 running 19-inch wheels and run-flat rubber.

It feels good from the steering wheel too, with a quick response to driver inputs; it feels a lot more darty and pointy than you'd expect from a machine of this weight and size.

Inside, there's the usual trick carbon trim adorning the dash, center console and doors, and the neat Schnitzer pedal set, along with the iDrive controller in the shape of an ACS Type IV wheel. But the handbrake handle looks all-new and its black anodized finish looks mighty fine.

There's a plaque on the dash signifying this as Street Tension number one (of 50) and each car will be built to the customer's specification. It can be based on any 6 Series coupe from the 630i to the M6, but obviously only the V8s (both the 4.4-litre 645Ci and 4.8-litre 650i) can have the 5.1-liter engine job. The instruments have also come in for the Schnitzer treatment and the power and performance hike has necessitated a recalibrated speedo, which stretches to a rather optimistic 199 mph.

While the car looks fine sitting in the workshop, to discover how it performs, we slip into the carbon-clad cockpit and, with a certain amount of trepidation, twist the key. There's a brief and typical BMW whine from the starter motor as it wrestles the big V8 into action, then a delicious cacophony from the polished exhausts, like you've just kicked the world's most badass lion in its most delicate of places. It's a brief moment before it settles down into typical V8 waffle overlaid with menace.

As the Aachen morning rush-hour traffic begins to thin, we nose the Tension's aggressive nose onto the street and head for this car's natural playground: the derestricted autobahn. Flooring the throttle as we join the fast-flowing traffic elicits a delicious roar of approval from the twin exhausts, but it's the way the Street Tension piles on the speed that's most impressive. In typical autobahn fashion, a big Audi closes fast and its manic headlight flashing almost prompts us to move over, but given that we've got horsepower aplenty, we let the big V8 do the talking. Knocking the Steptronic gearlever into sport mode and burying the accelerator does the job, as the gearbox drops two cogs, the nose rises and the scenery does that strange thing the stars do when the Starship Enterprise engages warp drive. This thing is seriously quick.

While the V8 engine conversion is the icing on the cake, remember you can carry out a Street Tension conversion on any 6. While you might not have the fastest street-legal BMW in the world, it'll have the original's attitude, and the limited run of 50 will ensure you're unlikely to encounter another on the road.

AC Schnitzer Street Tension
*LayoutLongitidinal front engine, rear-wheel drive

5.1-liter V8, dohc, 32-valve, ACS block and cylinder heads, exhaust manifolds, catalysts, rear mufflers, ACS software

Six-speed automatic

ACS sport suspension, anti-roll bars

ACS front bumper, widened front and rear wheel arches, side skirts, hood with integrated air scoops, trunk lid, sport mirrors, rear skirt with carbon fiber diffuser

*Wheels and Tires
ACS Type V, 9x20 (f)
10x20 (r)
Michelin Pilot Sport
245/35 (f), 285/30 (r)

@ 6100 rpm
Peak Torque: 406 lb-ft
@ 3900 rpm
0-62 mph: 4.9 sec.
Top Speed: 180 mph

AC Schnitzer CEC
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By Max Earey
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