Infiniti's QX50 is powered by the world's first mass-produced variable compression ratio engine. How well does this clever piece of tech suit the luxury SUV?

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Now here’s an oddity: the cubic capacity of the Infiniti QX50’s engine shifts between 1970cc and 1997cc. It’s the first mass-produced car in the world to feature this strange characteristic, and that’s because it has the world’s first mass-production variable compression ratio engine. 

Besides being of slightly fluid internal size, the compression ratio of the QX50’s all-new four cylinder varies between 8.1:0 and 14.1:0. Altering the compression ratio of an engine has long been the holy grail of internal combustion engineers. 

The VC-T motor is certainly smooth, although this advantage tends to be overshadowed by its slightly gritty sound at low revs

The compression ratio is the multiple of units of atmospheric pressure pumped by the pistons, and the new-found ability to alter this ratio provides another means of fine-tuning mixture for an optimal, efficiency-enhancing fuel burn. In fact, Infiniti claims that its approximately 2.0-litre Variable Compression-Turbo (VC-T) engine combines the power of the V6 that it replaces, and the fuel consumption of a similarly sized V6 diesel.

What does a variable compression engine feel like on the road?

It’s certainly a pokey engine. Peak power is 264bhp, and maximum torque a stout 280lb ft that’s consistently available between 1600-4800rpm. In the QX50’s case, it has a not insubstantial 1753kg to propel, but it certainly feels brisk on the road. And, as the revs rise, increasingly unusual too. 

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The VC-T motor is certainly smooth, although this advantage tends to be overshadowed by its slightly gritty sound at low revs, this swelling to a light whine until you hit 4000rpm, when a keen throttle produces a noise like an old-school twin cam four pounding out its best effort. It sounds slightly artificial, which it is, in fact – noise-generating and cancelling technologies adding to the aural mix. 

The rorty sounds tail off as 6000rpm approaches, or immediately you drop the throttle, the fall in crank speed accentuated by the fact that the engine is hooked to a continuously variable transmission. These ingenious, steel belt-driven gearboxes are certainly efficient – hence the reason for this transmission choice, say Infiniti - but as any Prius owner knows, you end up with the soundtrack of an indecisive power tool user.

This is particularly true on a twisty road, the rise and fall of the throttle as you enter and exit each bend matched by the rise and fall of this two-piece engine and transmission ensemble. You can lessen that effect by using the QX50’s paddle shifts, which introduce artificial gears and better allow you to exploit the grippy chassis. The Infiniti slices bends pretty effectively – its on-demand all-wheel drive adding stability as well as quelling momentary torque-steer squirms – but this is no sports SUV despite the sometimes racy sounds emerging ahead of you. The cabin in any case suggests otherwise, what with the leather and maplewood decor provided by the range-topping Essential version. 

What's the QX50 like inside the cabin?

Pleasingly soft quilted hide, a finely trimmed facia and that classy open-pore wood add to the upmarket ambience, as does a suede-effect headlining. Much of this Infiniti’s interior looks tastefully expensive, but closer examination reveals slightly cheap-looking, old-school instruments, a low-rent steering wheel boss and a sat-nav display that could use more clarity-enhancing definition. And the strange, suede-like inserts to the dashboard and doors resemble an anti-rattle material. 

It’s an interior that’s impressively practical, however. One benefit of downsizing from a V6 to a transverse four cylinder of tight-packed ancillaries is the release of more cabin space, the QX50’s rear room generous even if you can’t snuggle your feet beneath the front seats. The rear seats slide to increase the volume of an already generous boot, this the result of efforts to reduce the bulk of the multilink rear suspension.

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Plenty of thought has been invested in this car, from powertrain to packaging to style, a further benefit of the VC-T technology being best-in-class fuel consumption in the US that bodes well for Europe.

A shame, then, that the QX50 lacks the ultimate polish of BMW’s BMW X3, for instance – its refinement, intrusive transmission and patchy low speed ride a too-obvious contrast to its potency and roomily distinctive interior.

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