For 35 years, Hartge GmbH has been mis-matching BMW hardware, squeezing oversized, range-topping engines into small saloons and coupes. Its creations are never anything short of spectacular, yet usually come wrapped in subtle bodies ready to scare unsuspecting, self-assured supercar drivers. But not any more. Few cars have ever oozed menace like the H50. It's almost cartoon-like, bursting at the seams like a steroid-filled body-builder. Just look at that raised, heavily vented bonnet, the slashed wings, the four fat pipes, the oh-so-low suspension and those enormous 21in wheels. Stealth, it seems, is off the menu.

The car you see here was once an unassuming 330i. Now, one major transplant and extreme makeover later, its heart beats with four extra cylinders. Crammed into the engine bay is the extraordinary 5-litre V10 unit that usually powers the M5 and M6. Now this may sound crazy, but fitting a standard engine with 'just' 500bhp was never going to be enough for Hartge, so they reworked the cylinder heads, modified the butterflies, fitted new camshafts and remapped the ECU. Hitting the 'Power' button now unleashes grunt of colossal proportions, 550bhp and 384lb ft of torque, armed and ready to fire this 1670kg machine headlong at the horizon.

Or at least it will soon. You see, it has taken longer than expected for Hartge MD Volker Schu and his team to match the M5's electrical systems to the E90 3-series' loom. Schu had hoped the electrical work would have been one of the more simple tasks within the project, but it has become one of the biggest hurdles they have had to overcome. For our test today - arranged a few weeks ago to allow evo to be first behind the wheel - the big guns have been disarmed. The hyperdrive Power button is out of bounds, so somewhat disappointingly we only have a mere 400bhp to play with.

'It was a surprise to us,' explained Schu, 'that two cars produced at the same time should be so different. It's almost as though we have taken the engine from a different manufacturer. We have had the engine running at full power, but have had to disconnect it again to work on other systems such as the DSC traction control and the onboard computer. It's very nearly there, though, so we will be ready to build the first customer cars in April.'

Thus far the project has taken around 12 weeks, beginning with two complete cars - the 3-series and an M5. In future, Hartge will simply buy-in the V10 engine and other required M5 parts, but to understand how to mate these two machines, two whole cars was the logical starting point. The engineers have had to fathom out the conversion in stages, the first of which was simply to get the massive motor and the SMG gearbox to fit in a space never designed to swallow something so large. The steering rack required a little modification, and new homes had to be found for all the ancillaries, but as it turned out this was all kids' stuff compared with sorting out the massively complicated electrical system BMW employs for the M5.

At the rear, the M5 propshaft and M differential both had to be shortened to fit 3-series dimensions. The axle ratio was altered to match, with stronger link-arm bearings and (fully adjustable) multi-link arms to prevent it all ripping itself apart.

To stop the H50 spitting you off the road, new springs and dampers were developed in conjunction with Bilstein, with increased front spring rates to cope with the extra weight of the V10. The H50 currently sits a full 30mm lower than the 330i, although the final set-up won't be perfected until the Nürburgring is no longer snowbound, and in any case customer cars will be fitted with 20in tall wheels - 9in wide at the front, 10in at the back - not these whopping 21in ones. Filling those wheels will be cross-drilled floating discs a monumental 380mm in diameter at the front (328mm at the rear), with eight-pot callipers ready to grab them hard and lose enormous speed in a serious hurry.

Sitting behind the carbonfibre steering wheel, your eyes are drawn to the bonnet bulge that fills the bottom couple of inches of the windscreen. This again will be changed and smoothed off for customer cars, as the extra height is not needed at the top edge of the hood. The interior is now a cool mix of perfectly finished carbon and soft hide, set off with Hartge's own shimmering aluminium pedals. The bespoke speedo is rated to a heady 330kmh (205mph).

The key and immobiliser weren't quite ready, so to wake the mighty V10 we had to twist the barrel internals, making it feel a little bit like we were stealing the H50. Still, it barked into life instantly, with the custom exhaust adding a harder edge and more volume to the soundtrack you get from either the M5 or the M6. In fact it's exactly the noise you want to hear from a car that looks like this. When revved, your ears are treated to a full-on wall of sound, overlaid by an intense metallic rasp and unique bassy backbeat. This is definitely not a car for those wishing to remain anonymous.

We head out of the factory gates and up into the snow-covered hills above Hartge's Beckingen base, just below the southern tip of Luxembourg. In the first few days of March it's almost impossible to find long stretches of road that are neither icy nor coated with drifting snow, but whenever even the smallest opportunity arises to unshackle the V10's might, I flick the left-hand steering-wheel paddle and bury the throttle. The savagery takes you by surprise every single time, pinning you hard into the seat and firing this 400bhp Three down the road as hard as an M5 with all five hundred horses in play. Be in no doubt, with 37.5 per cent more power, the full-cry effect will be devastating.

I expected a rough ride and unruly handling, particularly with those giant wheels and the unfinished suspension, but the H50 was never wrong-footed or thrown off track. There's plenty of detail to the steering too - it turns in decisively and, as you'd expect with so much rubber on the deck, grip levels are high.

And so to the big question of price. With all that work, combined with the fact that you're effectively buying one pricey car plus another car's expensive engine to make one über-saloon, it ain't gonna be cheap. Indeed, 145,000 euros is a big pile of money (to which you'd still have to add VAT), making the H50 probably the most expensive 3-series the world is ever likely to see. But if money presents no barrier and you crave something a little on the wild side, your search ends here.

Hartge currently has two orders on its books and expects to sell between ten and 15 examples, with Coupe and Touring versions also available to order. We've been promised another go once the serious ballistics have been hooked up, at which point we'll see if the finished H50 can earn its final half-star. I, for one, can't wait.