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Engine options, speed, acceleration and refinement

Despite being two cylinders down compared with its 63-badged brethren, the CLS 53 still conjures some of the aural thrills (albeit slightly synthetic ones) you’d expect from one of Affalterbach’s more menacing creations.

While not as earth-shatteringly raucous as those V8-powered models, the 53’s inline-six is a characterful unit, being silky-smooth in timbre at low speeds, while an extended prod of the throttle will be met by a crisp, mellifluous growl that gets more dramatic as you approach the 6500rpm limiter.

The V8, 549bhp CLS 63 S we tested in 2011 was just 0.1sec quicker from 30-70mph - a metric that perhaps gauges usable accelerative potency better than any other

Volume-wise, the CLS was measured at 78dB at max revs in fourth gear – 2dB quieter than the V8-engined GLC 63 S Coupé we tested in June. Off the line, the CLS doesn’t suffer from any noticeable shortage of traction or hesitancy from the nine-speed ’box. It recorded a two-way average 0-60mph time of 4.3sec, which is well in line with Mercedes’ 0-62mph claim of 4.5sec.

The manner in which acceleration is delivered is impressively linear, and there’s little turbo lag evident at any point in the rev range. Lock the CLS in its most aggressive Sport+ driving mode and upshifts are executed with just enough ferocity to remind you that, despite its lack of V8 engine, the 53 is a potent straight-line machine.

That potency is evident when it comes to the CLS’s overtaking ability, too, with the 30-70mph sprint being dispatched in 3.7sec. The last-generation CLS 63 S we tested back in 2011 (5.5-litre V8, 549bhp, 590lb ft) bettered that time by only one-tenth of a second. The earlier car was superior in terms of flexibility; locked in fourth gear, the 63 S would accelerate from 30-70mph in 5.6sec, while the new 53 takes 6.1sec.

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Still, this is evidence that, while it might not have the laid-back, any-condition pace of an AMG V8, the CLS 53 will hit a comparable outright performance level if you’re prepared to really put its six-pot to work.

Of course, without that whopping V8 under the bonnet, the CLS 53 doesn’t quite have the propensity to drink fuel at the rate its predecessor did. It returned a brim-to-brim touring test economy result of 39.2mpg, while the 63 S could only manage 25.8mpg.