Power, Eventually

Weighing more than 4,600 pounds, our all-wheel-drive test car was heavier than many large crossovers. But its engine Ч a 5.5-liter V-8 good for 382 horsepower and an impressive 391 pounds-feet of torque Ч made the car feel downright light. Even loaded with adult passengers, the S550 moves out from a standstill and pulls strong on the interstate. Enjoy it responsibly; the power never seems to run out, and without an eye on the speedometer you'll soon reach triple digits.

The rub, unfortunately, is accelerator lag. Such lag has long been a result of the electronic throttles that are employed across the industry, and it ranges from minor to pronounced. The worst cases display erratic inconsistencies, and the S550 is among them. Press the gas, and the resulting thrust could be relatively immediate, or it might be delayed by a half-second or more. It's vexing, and we've noted it in cars as diverse as the Audi A6 and the now-discontinued Saturn Outlook. Mercedes, for its part, seems to be a repeat offender; we've also noticed accelerator lag in the GL-Class and the prior-gen E-Class.

The S550's seven-speed automatic shifts smoothly, upshifting lickety-split when you press hard on the gas. (The S400, S550 and S63 have a seven-speed auto, but the S600 and S65 employ a five-speed.) It's not the best about kicking down to lower gears Ч the Jaguar XJ's six-speed automatic is better Ч but the V-8's prodigious torque means you can power out of a corner even in a higher one. The transmission offers a Sport mode, which holds lower gears longer and lessens the probability that you'll need a downshift. Should you need to, Sport mode still takes its time, unfortunately.

Four-wheel-disc antilock brakes are standard. Our S550's brakes bit down high in the pedal's travel, which several editors noted gave the process a nonlinear, or abrupt, sensation. Other S-Class variants may improve on this: The S600 has larger discs, while the S63 and S65 get cross-drilled, 15.4-inch front and 14.4-inch rear discs. (That's into the upper echelons of supercar territory Ч just 0.3 inches smaller than the 987-hp Bugatti Veyron's front discs.)

Move up the chain, and you can get as much as 604 hp in the S65 AMG. Both AMG cars have an AMG Speedshift automatic, with significantly decreased shift times over the regular transmission. In the other direction, the S400 Hybrid teams a 3.5-liter V-6 with an integrated motor. It's a mild hybrid, not a full one, so the motor chips in to help the V-6 but can't power the car on its own. It runs off a compact lithium-ion battery Ч among the first lithium-ion applications in a mass-market hybrid. Combined gas mileage is 21 mpg, a 17 percent improvement over the S550. (Most of that comes by virtue of having two fewer cylinders: In Europe, the V-6 S350, whose engine the S400 uses, is already 11 percent more fuel efficient than the S500, equivalent to our S550.)

We drove the S400 last winter, and while it doesn't provide the S550's effortless thrust, it certainly has enough passing power. The brakes, on the other hand, are disappointing: They employ a regenerative function, like most hybrid-vehicle brakes, but the effect makes for a stiff pedal that's more bricklike. Even for a hybrid Ч but especially for a Mercedes-Benz Ч the S400's brakes need work.

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