The McLaren 720S Spider is the world's fastest convertible

Ripping the roof off a perfectly good coupe doesn't often end well.

As well as reducing the structural stiffness of the body, it typically adds weight due to additional bracing required to account for its absence.

Suddenly that fun-to-drive two-door turns into something slower, less responsive and all-round less smile- inducing.

Hard and fast

But the McLaren 720S Spider is not your average convertible thanks to the carbon fibre body integral to its character.

The Monocage II-S structure that forms the passenger compartment and is the foundation of the car is so stiff that the removal of the roof didn't require any additional bracing.

Other than the lack of a roof panel, the main changes are the addition of carbon fibre rollover hoops and a revised windscreen frame to accommodate the latching points for the roof.

The window frames have also been removed from the dihedral doors, which still open in a theatrical scissor fashion courtesy of clever hinges.

And the electronic roof mechanism means the switch from coupe to Spider adds just 49kg to the lightweight supercar.

Good timing

The whole roof-down process can be done at up to 50km/h and takes just 11 seconds, among the quickest of the auto folding convertibles.

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Once the roof has been lowered there's only mild ruffling of the hair – even at freeway speeds – although it can be amplified big time by lowering the rear and side windows.

At city speeds it pays to lower those windows to get more of the open-top feeling.

Past the sound barrier

Otherwise, the most noticeable difference between coupe and Spider is the noises emanating into the cabin.

The twin-turbo V8 doesn't emit a particularly seductive sound, but it's hugely purposeful, a raucous growl backed up with whistles and whooshes from the turbo, something more prominent with less between your ears and the bangs. Big revs and brisk gear changes also elicit some wonderful cracks.

Vision is also fantastic by supercar standards, in part thanks to the see-through fins on either side of the engine. It's clever stuff and evidence McLaren has thought about the 720 for everyday use.

Speed freak

Yet it's still blisteringly fast courtesy of a 537kW/770Nm 4.0-litre twin-turbo V8.

Matched to a seven-speed twin-clutch auto transmission it makes for a fiery combination.

The dash to 100km/h takes just 2.9 seconds, matching the 720S Coupe to the tenth. It's only grip stopping it from going any quicker.

But that's not where the 720S feels quickest, mainly because it's constantly fighting wheelspin in first gear.

It's second gear where the scenery rushes up phenomenally quickly – traction still an issue as those 305mm-wide rear Pirellis struggle for traction – with third gear not far behind it.

While there's some minor sponginess between what your right foot is saying and what's happening at ground level – blame wisps of turbo lag – things get exciting very quickly, rapidly overcoming any shortcomings.

Stepping up

One drop-top that approaches the Spider's ballistic pace is Lamborghini's Aventador SVJ. The Lambo's 351km/h top speed just shades the McLaren's 341km/h.

Both claims are largely irrelevant, with no race track in the world long enough for you to hit that peak.

The McLaren fights back with its 0-200km/h time, which is a full 0.9 seconds faster than the V12 Lamborghini.

It's also 0.1 seconds faster than the yet-to-arrive Ferrari 488 Pista Spider, the most focused convertible yet from the Italian manufacturer.

So, if it's acceleration you're yearning for – that brutal thrust at any legal road speed - there's no open-top car faster than the 720S Spider.

Prime athlete

But it's the light-on-its-wheels flavour that is always on display, fast direction changes a snip.

Press on the firm brake pedal and brilliantly effective six-piston carbon ceramic brakes wash off speed with supreme confidence.

And pressing the brake pedal does so much more than activate the calipers; it also sends the rear air brake almost vertical – in turn blocking the rear view – increasing the aero drag for a mild braking effect and helping keep the tail of the car pinned for more surefooted stopping. Its impact is negligible below 100km/h, but once you hit triple figures the stopping power is phenomenal.

Braking also almost instantly fires down a gear or two, the slick seven-speed twin-clutch transmission almost telepathic in its ability to envisage what you'll want next.

Roof down

There's a catch with the 720S: To achieve the 341km/h top speed, the roof needs to be kept in place.

With the roof lowered there's more aerodynamic drag, something that lowers the top speed to a mere 325km/h.

Academic, but we're onto the details here…

All in the details

Like the coupe, the Spider has some terrific details, like the carbon fibre instrument cluster that flips through 90 degrees when you dial up track mode, providing a slimmer readout with the basics: speed, engine revs and what gear you're in.

McLaren engines don't usually have that glorious in-your-face red finish of those in a Ferrari, in part because they sit so low in the car, part of a plan to lower the centre of gravity.

But the company has added some flair with the 720S courtesy of red LEDs that send out a subtle glow from the mesh engine cover at night.

Either/or

Whether it's a Ford Mustang or Porsche 911, the seemingly simple task of taking the roof off can have negative effects.

But with the 720S Spider there's not much to detract from the open-air experience. Performance is outstanding and dynamics cutthroat sharp.

There is, however, the slight issue of price. That shift to convertible adds a hefty $66,100 to the 720S price, which is a big whack. Total ask is $556,000, plus options.

That's a massive penalty for some extra wind in your locks, however soothing and brutally effective it is.