Our best group tests - McLaren 675LT vs Porsche 911 GT3 RS

The higher reaches of the Porsche’s rev range feel so special, though, suffused with incredible pedal response and with a visceral power delivery still building until beyond 8000rpm, that you want to stay among them whenever you can. The whipcrack changes of the excellent seven-speed PDK dual-clutch automatic gearbox make it possible to do just that.

Although it’s close on swept volume, the 675LT’s engine might as well be from a different planet. This is a compression ratio of 8.7:1 versus the Porsche’s 12.9:1, twin turbocharging versus Stuttgart’s atmospheric aspiration and 175bhp per litre compared with the GT3 RS’s 123bhp per litre.

Among more mechanical changes to the 650S’s V8 than we’ve time to list here, Woking has actually taken a little bit of mid-range torque away in order to add high-rev drama – which I applaud.

Drama is what McLaren’s sports cars need. But there is still more than a hint of turbo lag to be accounted for here at low and medium revs, just enough of it to notice when you flatten the accelerator suddenly. Perhaps not so much that it’ll be a factor on the road. We’ll see.

Once the engine has responded to that flattened pedal, though, you may not care about the preamble. The 675LT is obscenely fast – and more unconditionally so than the GT3 RS.

Like the Porsche, the McLaren revs beyond 8000rpm, but unlike the 911 it can catapult its bulk into seriously urgent motion with less than half as many revs on the tachometer. There’s enough torque to work those Pirelli P Zero Trofeo R rear tyres up into two or three successive flurries of electronically quelled wheelspin through both second and third gears – all in the dry.

Thursday, 2.19pm: MIRA’s handling circuit

Time for a couple of fast, physical and illuminating hours on MIRA’s Dunlop handling circuit next, before we strike north for the evening. I start out in the GT3 RS, in anticipation of big things. But however high your expectations of this car may be, I doubt you could get out of it with your mind and your senses intact. It really is that good.

The crispness and immediacy of this car’s cornering manners are truly astonishing. Unexpected, too, because the Porsche rides with 
a gentleness that you just wouldn’t imagine possible.

The RS flits towards an apex with a magnetic kind of nonchalance; its every response comes more quickly than those of the regular GT3. On the limit of grip, it has to be mastered with a quick pair of hands, but it’ll tolerate any driving style you like: fast and smooth, or more wild and unfettered. You decide. Want to nudge those 325/30 ZR21 rear tyres into a 70mph, third-gear sweep of lift-off oversteer? Just think it, lift your right foot and it’s happening.

The 675LT feels a bit less at home on the tight, technical Dunlop circuit. In its own way, it’s still brilliant and fearsomely potent, but it’s neither as obliging nor as adjustable as the RS.

McLaren’s rebalancing of the 650S’s suspension and steering systems has paid dividends and sharpened the car’s reactions to a fine point. There is a limpet-like, confidence-inspiring front end to lean on, along with huge reserves of lateral adhesion. After back-to-back drives in both cars, I’ve now no doubt that a P Zero Trofeo R is a stickier tyre than a Michelin Pilot Sport Cup 2. Which is saying something.

But here on track, when you’re likely to be after a more varied driving experience than full-bore qualifying pace alone can provide, even the 675LT feels slightly one-dimensional – much as the 650S did. A softness in the accelerator pedal prevents you from tapping into those huge reserves of horsepower with quite the precision that you’d 
like.

Meanwhile, McLaren’s decision to stick with an open differential continues to restrict your options during hard cornering and can adversely affect the 675LT’s controllability on the limit. It may even be that there’s simply too much grip here for the car’s own good.

Read the full Porsche 911 GT3 RS review

When Woking launched the MP4-12C, it said the car didn’t need an E-diff. We disagreed. It said the same about the 650S; again, we disagreed.

Now along comes the 675LT, still without the telling item of mechanical specification now common on sports cars at less than half the price asked for this one. Hear us this time, please, McLaren. Because a torque-vectoring ESP system is still no substitute for the uniquely rewarding, relationship-building sensation of a sports car being both driven and steered by its rear wheels.

Monday, 8.39pm: A169, Saltergate Bank, North Yorkshire Moors

A long day juggled between two steering wheels is drawing to a stunning close. England’s M42, M1 and A1M motorways haven’t been quiet, but 120 miles on them has provided a few insights into what real-world use of these cars would be like.

The first is that, even when battling through a sweaty rush hour with no air conditioning, questionable all-round visibility, an unintuitive navigation system and bucket seats almost as tight as your three-year-old’s safety seat, driving a lime green supercar can only ever make you smile.

A confession: Prior and I have been playing to the crowds a bit. Two kids in the back of a Volkswagen Camper got grandstand seats to a momentary traffic jam skit, whose cast was 
an orange bespoilered Porsche that suddenly 
made a lot of noise as it came by, followed by a 
snot-coloured McLaren which sort of waved as it passed.

The Longtail McLaren can do that: sort of wave. Flipping its active aerodynamic functions on and off makes that extra-long rear wing waggle up and down. Children, it turns out, love it.

A while ago, the busy motorways began gradually giving way to quieter and quieter stretches, each giving our cars more space on which to stretch their legs.

A mobile speed camera van on the A64 east of York came as a welcome reminder of the need for restraint when driving cars such as these on roads such as those we have in the UK. No custom for him tonight, though.

Now, we’re north of Pickering and treated to our first sight of the rolling, bleakly beautiful North Yorkshire Moors. A full 21 years ago, the bumpy, jinking roads that criss-cross this area were the ones chosen by Autocar’s testers at the time to sample the McLaren F1. Viewed from a downhill hairpin on the A169 cut into Saltergate Bank, with the sun slowly disappearing over the horizon, the scene takes your breath away.

I glance in the Longtail’s rear-view mirror to check that my colleague in the Porsche is being treated to the same spectacular view, only to realise that he’s fallen a little way behind. Later it’ll become clear why.

Maintaining a fast pace in the GT3 RS isn’t hard, but it needs some commitment to the idea by staying in lower intermediate gears and allowing the engine to add a fair bit of mechanical thrash into a cabin already humming with plenty of road roar. This Porsche is very much a ‘go hard or go home’ sort of car.

The 675LT is a slightly quieter-riding car, and also a much easier one in which to cover ground. A-road overtakes are possible with one downshift and without finding the end of the accelerator travel. When the road opens up ahead, you can flex your right foot and pick up what you imagine will be a brisk but responsible stride – then often scare yourself silly at how fast you’re travelling.

As clearly as the GT3 RS distinguished itself 
on the track, it’s becoming clear that the McLaren enjoys at least as big an advantage over the Porsche as a road car – and we haven’t really started with the road driving yet.

Tuesday, 10.43am: blakey road, 
North Yorkshire Moors

Dawn on the slipway at sleepy Sandsend was almost as pretty as dusk over the moor, but we didn’t stay long. Before 8am we were in convoy over blissfully quiet roads that sometimes stretch before you for miles into the distance, before closing in again over crests and around blind bends. Up here in North Yorkshire – where three ramblers and a local in a Land Rover count as the rush hour – the roads have everything.

Having spent yesterday’s journey in the 675LT, I start the day in the GT3 RS, mainly to challenge the impression forming in my head that it may not be as brilliant on the road as I’d like it to be.

The RS suddenly seems less stunningly fast after the 675LT, instead just a bit long-geared and short on accessible torque. For the record, though, I suspect that a Saturn V rocket would feel less stunningly fast after a 675LT, but after jumping from one car into the other, it’s still a factor.

Steering that seemed expertly weighted and brilliantly tuned for the circuit is just a bit hyperactive over a bumpy B-road. Two hands are needed on the Porsche’s downsized steering wheel at all times – and even then the car can still be tricky to guide.

The RS’s suspension doesn’t quite perfect the compromise of body control and bump absorption for fast road work, either. Leave the dampers in soft mode and you can bottom out the rear end as your speeds rise; choose the firmer setting and things are better, if a little stiff-legged. So there’s no mistaking it: on the road, the Porsche is a bit of a fish out of water.

Not so the McLaren. Woking’s engineers will tell you that, even at the conception stage, the 675LT was always a road car first and a track car second.

That may seem ridiculous for something on cut slicks, dressed from head to toe in lightweight materials and capable of licence-losing velocities 
in a heartbeat, but it’s readily apparent, because 
the Longtail’s ride remains uncommonly compliant for a proper supercar, even on the most challenging of surfaces. And yet its body control is all but flawless on the road.

The gearbox could be better – quicker shifts in manual mode, smarter when you want to drop several ratios all at once – but seldom do you notice, because the engine’s remarkable muscularity makes up for the deficiency, and then some.

As we wrap up with our photos and think about turning for home, it’s a relief not to be ranking these cars in relative terms and not to have to pick a winner. The GT3 RS may well be the best track-day special in the world, ready for driving to and from the circuit as often as you want.

The 675LT is evidently one of the most rapid, capable and exciting performance road cars you can buy, and stands ready to entertain you on a circuit as and when the opportunity arises. They’re more different than we’d have believed 36 hours ago – and all the more brilliant because of it.

And yet it doesn’t save you or I from having the same internal conversation that any enthusiast would have before drawing a line under the story: which one of these cars is for you?

The Longtail is a harder car to connect with than the GT3 RS – and ultimately it’s probably the less rewarding to drive. The McLaren’s chassis doesn’t communicate in the same direct, vivacious, plain-speaking terms. You have to trust that it’s going to look after you as much as knowing that it will. But it will. Will it ever.

Before thunking his driver’s door shut on increasingly inclement moorland weather, Prior makes his decision. “I think I’d rather be going that bit less quickly and having the time of my life,” he says. I know well what he means.

The Porsche 911 GT3 RS may not have as much breadth of dynamic ability in its locker, but its high notes are incredible. Inimitable, too.

And yet, with 130 miles to drive home, the McLaren 675LT edges it for me. It’s still a work in progress, but the way it combines huge, easily won pace and fluent, infallible composure on the road would be ideal for the kind of use I’d give it. Some wider sports seats for my not so sporting backside wouldn’t go amiss and, as I may have mentioned earlier, if there was a limited-slip diff option, I’d tick it. But I’d tick it a very happy man.

Read Autocar's previous comparison - Jaguar XE versus BMW 3 Series

Porsche 911 GT3 RS

Price £131,296; 0-62mph 3.3sec; Top speed 193mph; Economy 22.2mpg; CO2 emissions 296g/km; Kerb weight 1495kg; Engine 6 cyls horizontally opposed, 399cc, petrol; Power 493bhp at 8250rpm; Torque 339b ft at 6250rpm; Gearbox 7-spd dual-clutch automatic

McLaren 675LT

Price £259,500; 0-62mph 2.9sec; Top speed 205mph; Economy 24.2mpg; CO2 emissions 275g/km; Kerb weight 1403kg; Engine V8, 3799cc, twin-turbo, petrol; Power 666bhp at 7100rpm; Torque 516b ft at 5500-6500rpm; Gearbox 7-spd dual-clutch automatic

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