Review: 2010 Mercedes-Benz S400 Hybrid and Lexus LS600hL

And that's where hybridization comes in. The electric motor in the S400 contributes up to 118 lb-ft of torque, more than making up for the Atkinson sacrifice. In fact, total system power is quoted at 295 hp and 284 lb-ft, topping the 268 hp and 258 lb-ft ratings in a C-class fitted with the 3.5-liter V-6.

As a result, the S400 Hybrid, Mercedes says, reaches 60 mph in about seven seconds, but it can't come close to matching the sports-car-like S550, which takes only 5.4 seconds. The payoff in fuel economy, though, is substantial. Mercedes estimates that the S400 will achieve 23 mpg in the city and 33 mpg on the highway. The latter figure sounds optimistic to us, but even if the S400 earns an EPA combined 25-mpg rating, that's a fuel savings of nearly 50 percent over the V-8. And, let's face it, how many luxury sedan owners really need all of a V-8's prodigious thrust?

Lexus would say all of them - and to that end, the LS600hL loses none of the performance of the nonhybrid LS. Total system output is a robust 438 hp, sufficient to slingshot the LS to 60 mph in 5.5 seconds, according to Lexus. That's only 0.1 second slower than the nonhybrid version, but the more expensive Lexus achieves a combined 21 mpg in EPA tests, beating the all-wheel-drive LS460 by 3 mpg.

The Lexus suffers in one area that the Mercedes doesn't - cargo capacity. The large nickel-metal-hydride batteries and the extra air-conditioning system needed to keep them cool reduce trunk space from 18.0 cubic feet to only 11.7. Mercedes was able to locate its smaller and more energy dense Li-ion battery pack, which consists of thirty-five cylindrical cells supplied by the French firm Saft, in the engine compartment, leaving the S-class's 16.3-cubic-foot trunk intact.

On the road, the Lexus somehow manages to be lithe and nimble; it's an uncommonly responsive supersedan despite its supertanker weight. It combines an ultrabrisk throttle response with very strong brakes and a reassuring blend of grip and traction, although it's let down by a somewhat detached chassis, androgynous steering, and a ride that seems to vary almost at random between brittle and cushy.

The biggest benefit of the Lexus hybrid system is its smoothness. The two electric motors and two planetary gearsets allow the big V-8 to loaf except during hard acceleration. Flat-out, the LS600hL might not be any faster than the regular LS, but it doles out power far more effortlessly.

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