For a decade, we’ve been conditioned to judge a vehicle’s potential performance on the basis of its advertised horsepower rating. But that figure has little relevance to the kind of driving most of us really do, unless we’re drag racers.
Think about it: when’s the last time you revved your engine to 5,000 or 6,000 or 7,000 r.p.m. at wide-open throttle under full load, which are the conditions for generating those numbers?
Even when accelerating hard to pass or merge, chances are your tachometer needle never passes 4,000 r.p.m. Especially if you’re driving a typical family car or SUV. And most of the time, it will be at 2,000 r.p.m. or less.
That point wasn’t lost on Mazda’s engineers when they were developing the engine for the new mid-size Mazda CX-9 SUV — a turbocharged, 2.5-litre four-cylinder that replaces a 3.7-litre, V-6 in the current model.
In the process of that development, they came to question just how performance was defined. And what customers, particularly those of mid-sized SUVs, really needed. So they conducted a study of just how those customers drive in the real world.
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Not surprisingly, they found that the bulk of their driving follows the pattern mentioned above. It’s heavily biased to engine speeds around 2,000, even under high loads, with only a rare excursion to 3,500 or above.
Consequently, the engineering focus for the second-generation SkyActiv 2.5-litre Turbo was on achieving strong response in that lower speed range — not just at full throttle but at part throttle, where most driving is done.
It’s not unusual for such engines to achieve peak torque low in the speed range. But they’re not always instantly responsive as turbo lag — the time it takes for the turbocharger to spool up to a speed where it is providing significant boost — causes a momentary lag in acceleration.
Mazda addressed that issue with a combination of a four-into-three-into-one exhaust manifold configuration and what it calls a Dynamic Pressure Turbo.
The former is arranged so that every exhaust pulse follows one from an adjacent pipe in that array. The latter includes an integral flow control valve that routes exhaust into the turbo through a small high-velocity passage that acts much like the airstream in an airbrush.
The combination of the two effectively sucks the spent exhaust out of a cylinder while also quickly spooling the turbo up to speed.
Along with a novel exhaust gas recirculation cooling system, they help reduce both emissions and fuel consumption. And they enable quick and smooth throttle response right in the speed range where drivers need it.
That’s not just a theoretical assessment. I’ve driven a near-production prototype of the new CX-9 and can attest to both its performance and the smoothness of its application.
In achieving that end, the new engine gives up a bit of potential peak power at high speed, which may not look as good as some competitive figures in a catalogue. But, as Mazda development engineer Dave Coleman puts it, “We’re not selling catalogues.”