Modifiers have no morals. They are amoral. You’d think among tweakers there was some method of ethical assessment to ascertain what motors actually need modification these days. Step forward small diesel hatches and oil-burning SUVs, both of which would work better with a stronger dobbin count. But not the Mercedes SL55. Merc has already had a slap on the wrist from the German authorities for rating its 5439cc V8 at 479bhp when every car submitted for TUV approval was knocking out 500bhp. The company was forced to republish its claimed figures. You see, on dry asphalt, a standard SL55 often uses its traction control in third gear. On the same road surface, that same car fitted with the Carlsson CK55 RS 580bhp performance kit takes the game one gear further up the mayhem scale. Even wearing 305-section rear tyres, its hips give a belligerent you’re-lucky-it’s-not-wet-sonny shake as the torque arrives in fourth. How much torque? A fulsome 575lb ft at 2700rpm to bolster the alarming power figure that appears in its name. This is an absurdly powerful car and one that confirms a state of desperation among Gemany’s well-established tuning industry. With Mercedes currently turning the dial to 11 on all its in-house quick stuff, the tuners are left looking for 12. For this exercise that means modifying the supercharger and drive pulley, altering the intake and induction systems, completely replacing the enormous Mercedes exhaust and then splicing in a supplementary engine management unit to persuade the V8 to use its new breathing apparatus. The gearbox software is also subject to a few changes and the speed limiter has been removed. Of course, the thing’s quite silly in a straight line. In fact, it’s frustratingly fast because, by the time your brain’s registered the fact that you’ve coaxed the RS into life, the speed limit’s flown by and you have to back off. It’s certainly faster than the standard car, but not as brutally fast as the 625bhp AMG CL65 available from your Mercedes dealer, and the gearbox software isn’t the greatest success. Kickdown is harsh and shifts seems to be quite random. Wheels, preferably of the massive variety, are Carlsson’s other line of business. I challenge anyone not to be impressed by a black SL55 on 20in rims: it just looks superb. But the ride is abysmal and the steering even more cumbersome than standard. Unfortunately, Birds UK (the sole importer of Carlsson gear) didn’t have a set of the ultra-light (and ultra-pricey at £6700) 20-inchers that are claimed to offer far better comfort and steering response. So why would you spend £13,161 on the engine conversion? Not because you wanted a new SL55 with extra poke. That would be a waste of time and money: spend a little more on an SL65 – due later this year – and get a better, faster car. But if you already own an SL55, have been offered a shocking trade-in deal (as most people have, because there are so many about at the moment) and fancy sprucing it up for summer, Birds are the people to see. It’s an expensive conversion, but it makes the SL55 one of the fastest – and best-sounding – cars on the road. And once you’ve seen those 20-inch rims, you’ll be hooked. Chris Harris

Our Verdict

The Mercedes SL is perhaps the most splendid, single-minded luxury convertible in the world

Driven this week

  • Volkswagen Golf GTE first drive review

    Volkswagen Golf GTE first drive review

    First drive
    2 September 2014 2:53pm

    New GTE offers the best blend of economy and performance in the Golf range, but could do with more boot space and a lower price

  • BMW X4

    BMW X4

    Car review
    2 September 2014 2:45pm

    The X6 is a showroom hit. Does its smaller brother deserve to be?

  • Porsche Boxster GTS UK first drive review

    Porsche Boxster GTS UK first drive review

    First drive
    1 September 2014

    Go-faster Boxster renders the Boxster S entirely redundant, but not the slower but still sweet basic model

  • BMW M4 convertible first drive review

    BMW M4 convertible first drive review

    First drive
    30 August 2014

    More visceral than the M4 coupé thanks to more audible exhaust and well-controlled swirl of air in the cabin, but dynamics suffer slightly

  • Radical RXC

    Radical RXC

    Car review
    29 August 2014

    The Radical RXC offers one of the most extreme driving experiences available on public roads