The 2011 Porsche Cayenne S Hybrid is dead quiet as it waits on the Inside Line test track, its start-stop mode engaged. Throttle wooded, the hybrid Cayenne's electric motor hums to life, and a fraction of a second later, the shrill roar of the supercharged, direct-injected V6 fills the air. Only then does the Porsche Cayenne Hybrid punch a hole through the morning haze. Along the way, it sounds like the nicest, most expensive vacuum cleaner we could ever hope to own. Still, the 2011 Porsche Cayenne S Hybrid is quicker and more engaging than any other hybrid SUVs we've ever tested, which is an admittedly low bar.

The real question, then, is not whether you'd buy a Cayenne Hybrid over a cheaper, less personable Lexus RX 450h, but whether you'd choose it over any of the other 2011 Porsche Cayennes.

We Took Out 300 Pounds, Then We Put Them Back

Porsche started work on the hybrid Cayenne six years ago (and indeed we drove a prototype with the old bodywork), but the automaker waited for the SUV's 2011 redesign before rolling out the production version. This was a wise move, because the Cayenne S Hybrid would have weighed 3 tons on the original leaden chassis.

The new unit-body is 150 pounds lighter, and altogether the base Cayenne V6 and V8-equipped Cayenne S weigh 300 pounds less than their 2010 counterparts.

Of course, the hybrid components add back all that weight, but at 5,165 pounds, our 2011 Porsche Cayenne S Hybrid test vehicle is just 50 pounds heavier than the last Cayenne Turbo we tested. So there's that.

Like a Honda, But Better

As hybrids go, the Cayenne Hybrid is pretty simple. Like all Honda hybrids to date, it's a parallel hybrid. There's a 47-horsepower electric motor sandwiched between the longitudinally mounted, supercharged, 3.0-liter V6 and an eight-speed automatic transmission that drives all four wheels through a conventional all-wheel-drive system. A nickel-metal hydride battery pack is mounted behind the rear axle.

But Porsche has gone one better than Honda by fitting a clutch between the 333-hp V6 and the electric motor. The clutch fully disengages the engine during electric-only operation and coasting, rather than allowing the pistons and crankshaft to keep cycling, and this reduces frictional losses. (On Honda hybrids, only the engine's valves close.)

That makes the 2011 Porsche Cayenne S Hybrid different from series-parallel hybrids like the Prius and RX 450h is that it can't operate its gas V6 and electric motor at different rpm and blend them together for maximum fuel economy benefit. To do that, the Cayenne would need a second electric motor and a continuously variable transmission. This approach would have cost more and risked an un-Porsche-like driving experience.

Quick When It Has a Full Charge

Although it's technically possible to drive the hybrid Porsche Cayenne on electric power alone up to 37 mph, it requires tiny throttle inputs and considerable patience from other motorists.

Mostly, the electric motor is just an assist for the V6, and when the batteries have a full charge, this combination (380 total hp and 427 pound-feet of torque) delivers you to 60 mph in 6.1 seconds (5.9 seconds with 1 foot of rollout as on a drag strip) and has you at the quarter-mile mark in 14.5 seconds at 93.9 mph. Among hybrid SUVs, only the V8-equipped BMW ActiveHybrid X6 (claimed 5.4-second 0-60 time) could beat those numbers. For comparison, the RX 450h takes 7.6 seconds to reach 60 and 15.6 for the quarter-mile, while the Cayenne Turbo is on a different planet (4.6 and 12.8, respectively).

The battery charge depletes quickly, though, and on Interstate 5's Tejon Pass, the Cayenne's V6 feels pretty ordinary and sounds as if it would rather be motivating a tractor. The eight-speed automatic, though, is always on its game, moving between gears so deftly that we barely notice shifts. The transmission's sharpness also keeps us from being annoyed by its aggressive overdrive gearing, which has the engine at 1,800 rpm at 70 mph.

This, along with the Cayenne Hybrid's ability to shut off its engine when coasting at up to 97 mph, explains why the hybrid Porsche is rated at 24 mpg on the highway versus just 20 mpg in the city. We easily lay down a 24.2-mph highway-only tank, and on our city-intensive fuel economy loop, we get 23.2 mpg. Our average over 1,400 miles is 21 mpg.