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RUMBLE SEAT / DAN NEIL

E55 AMG: Twist and shout

Plenty of pound-feet -- that's torque -- help make the ultra-performance version of Mercedes' E-Class one of the best cars in the world, if not the best.

September 24, 2003|DAN NEIL

The engine of the 2004 Mercedes-Benz E55 AMG produces 516 pound-feet of torque between 2,650 and 4,500 rpm. For a lot of people, this sentence means nothing. What, after all, is torque? What is a pound-foot, and is 516 of them a good thing or bad? "Pound-foot" seems like nonsense verse, like early Andre Breton or late Snoop Dogg.

You'll forgive my being didactic, but the E55 AMG -- the ultra-performance version of Mercedes' E-Class -- can't really be appreciated without some grasp of automotive mechanics. Most cars: They go, they stop, they drink gas and poop exhaust fumes. What's to explain?

The $76,200 E55, on the other hand, is the most potent production sedan on the planet. Among its parlor tricks: 0 to 60 mph in 4.2 seconds, quicker than a Ferrari 360 Modena, Corvette Z06 or Aston Martin Vanquish, according to Road & Track magazine. The E55 also blitzes a quarter-mile in 12.4 seconds, as fast as that purest of sports cars, the Porsche 911 Turbo. And although the AMG's top speed is electronically limited to 155 mph, the true top speed is, by my calculations, more like 185 mph.

What makes this ordinary-looking, 2-ton luxury grocery getter such a monster is none other than the oft-misunderstood torque, pouring out of the car's 5.4-liter supercharged engine like the business end of Hoover Dam.

Torque is, simply, twisting force. Grab a doorknob, twist -- voila, torque.

Torque is expressed in pound-feet (or in the metric system, newton-meters, but let's not go there, OK?). As Archimedes well understood, a lever multiplies force. Imagine you are loosening a rusty bolt. If you use a foot-long wrench and put 100 pounds of pressure on one end, you are applying 100 pound-feet of torque to the bolt.

The E55 engine's output shaft turns with a maximum force equivalent to 516 pounds of pressure on that same foot-long wrench. It's really pretty simple.

Horsepower -- that familiar unit of power, reassuring in its equine obviousness -- is anything but. The term was coined by Scottish engineer James Watt, who reasoned that a strong horse could raise 550 pounds 1 foot in one second. Trouble is, his unit of measure is foot-pounds -- the converse of torque's pound-feet -- and it describes linear, straight-line force, while torque describes rotational force.

These days, horsepower is calculated as a numerical product of measured torque multiplied by engine rpm, divided by 5,252 (a bit of mathematical housekeeping that cancels out minutes and seconds and turns straight-line into rotational force units). In the E55, the engine produces peak horsepower of 469 horsepower at 6,100 rpm, which is about 404 pound-feet of torque.

It all seems so innocent, like chalkboard arithmetic you might remember from high school physics. But for car enthusiasts, these numbers are, well, scary, with the kind of dwarfing immensity one associates with thermonuclear footprints and ICBM throw-weights. Imagine this scenario: You are merging onto the 10, and there's a break in traffic. Feeling frisky, you mat the throttle ... one-one-thousand, two-one-thousand.... Count to five and you are commuting home at 120 mph or more. How do you look in an orange jumpsuit?

Here's a little gearhead dish: While peak horsepower has a certain marquee value, it's not especially relevant outside of top speed. Acceleration -- the sensual, guilty, giddy gestalt of tramping the gas pedal and feeling yourself shoved into the fast-forward scenery -- is the product of engine torque pitted against the mass of a car.

I'll risk one more physics equation: F = Ma. Fun equals mass times acceleration. The E55 has F in abundance.

AMG -- based in Affalterbach, Germany, not too far from Stuttgart -- is the wholly owned mischief maker for Mercedes-Benz. As a matter of company policy, an AMG-tuned model is the top offering in each of the model lines.

Obscene power is AMG's calling card, delivered by highly developed engines, each hand-built and signed by the technician who assembled it. The E55 engine starts life as a 5-liter V-8 casting, which is then endowed with longer connecting rods ("stroked" is the term of the art) so that it displaces 5.4 liters. AMG uses high-performance engine internals, including matched pistons, a reinforced crankshaft and lightweight single-overhead cams to actuate the engine's three valves per cylinder. Ignition spark is provided by twin coils over twin spark plugs.

A vast amount of binary code from the Bosch engine management system minutely adjusts the fuel-injection spray and timing for each cylinder.

All of which would make for a very healthy hot rod, but AMG goes on to add an enormous supercharger to the engine, plumbed with an air-to-water intercooler (the air consumed by the engine is cooled, making it denser and creating more power in combustion).

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