Mercedes-Benz C63

The Mercedes-Benz C63's compact styling hides a colossal engine

It's the size of a Mondeo – slightly smaller, actually – and it has a 6.2-litre V8 engine able to pull with the power of 457 horses. Even the keenest car nut might wonder if we're into overkill here. What is a compact-ish saloon doing with so much power? And how do you squeeze in so much engine?

Open the Mercedes-Benz C63 AMG's bonnet and it does seem densely packed in there. But the vast engine fits, which it wouldn't were the C63 configured with the transverse engine and front-wheel drive of the C-class range's non-German rivals. True, the BMW M3 and Audi RS4 are also mid-size saloons with unlikely V8 engines. But neither has a thumping great 6.2-litre.

To buy a C63 AMG will cost you around £55,000. That's a lot for a C-class, but then no other comparable car can cannonball itself to 62mph in 4.5 seconds. It's not just the figure, though – it's the way the feat is achieved, which is worryingly easy. There's no messing around with clutch and gear lever here. You just put your foot down and blast off.

Maybe having a seven-speed automatic gearbox takes away the fun. An ESP system keeps the forces under control when needed, but there's more amusement to be had when using the gearbox manually and switching the ESP to its more liberal Sport setting.

The seven-speed Mercedes automatic is usually a frustrating device in manual use, the multiplicity of ratios confusing as they blur together, the transmission taking over when you don't want it to. You might select third but the transmission regards this as "all gears up to and including third", and performs jerky downshifts when you don't want them.

Not so the AMG Speedshift. It stays in the gear you asked for, it locks the torque converter so there's no slurring within gears and the drivetrain stays properly solid, and – best of all – it performs a throttle blip on the downshift like the sequential-manual transmission of, say, a Ferrari. This not only sounds good but also smooths the shift and eliminates surging.

The manual shift is quick enough to be a useful aid to brisk driving rather than a gimmick. You can even switch off the ESP if you want to perform powerslides, but there's little point – it always comes back in under braking, anyway.

So this is one fast car. It's limited to 155mph, but if you buy the optional AMG performance package (as if), the limit is revised upwards to 174mph.

That's allowed because the package includes carbon-ceramic brakes, lower and firmer suspension, a limited-slip differential and a driver training course. So how fast would the C63 go if there was no speed limiter at all? The engineers reply that they do not know, because they haven't measured it on account of the tyres' maximum speed rating.

I see no need for the performance package in the UK. Not just because of the unlikelihood of doing 174mph, but because even the C63's suspension is so firm already as to make it, I suspect, a bit tiresome on our roads.

Lumpy ride apart, the AMG is a delight through fast, twisting roads. The wheels are set more widely apart than in the standard C-class by means, partly, of revised front suspension whose new steering-pivot axes help make the steering particularly precise. BMW's engineers should try a C63 to discover what the M3's anaesthetised steering ought to feel like.

Of course, the C63 looks a lot more aggressive than a regular C-class, with its fattened front wheellarches, and its scoops and vents. The AMG interior treatment is quite restrained, the highlight being the excellent seats with their deep side bolsters.

This is a fine and highly enjoyable car, if a hard one to justify to those who don't understand. And despite the monstrous engine, it's rated at a relatively frugal 21.1mpg. But no one is going to drive a C63 AMG that gently, because then there would be no point in having one. Over to you, conscience.