something wicked this way comes
When it hit the streets for model year 2001, the CLK55 was fresh, exciting, and the fastest car in the Mercedes-Benz stable. The W208 was supplanted by the W209, but it grew stagnant and was easily overlooked. Yes, it was improved in some areas, but the main problem was the E55. In 2003, the E55 received THE upgrade; der Kompressor. Adding a supercharger upgraded the E from "a very fast car" to ballistic status. The CLK faithful craved the 469 horsepower / 500+ pound-feet of torque, thermonuclear power plant, but that itch remained unscratched. For some, the real anguish was knowing what the factory could have done.
While the CLK55 grew stale, the AMG division initiated a project to build a new engine. In the past, AMG had always modified standard Mercedes-Benz engines. This was different. It was a project to build a "new from the oil pan up", AMG specific engine whose purpose was immense torque, a spacious powerband, and Mercedes-Benz durability.
For this project, lead engineer Bernd Ramler was enticed to leave Porsche (where he had designed the V10 that powered the Carrera GT) and return to Mercedes-Benz. Four years later, Ramler's newest creation, the 63, was presented to the CLK faithful.
In the CLK63, horsepower increased from 362 to 475, while torque leapt from 376 pound-feet to 465. Finally, the CLK acquired "ballistic" status. The CLK63 didn't simply replace the CLK55, the CLK63 obliterated it. Allow me to take a moment and clarify that. "Obliterate" is a verb. It means "to blot out or wear away, leaving no traces of the original object". If one drives a CLK55, and then drives a CLK63, the memory of driving the CLK55 will disappear.
Why? Many people look at the "zero to sixty" figure. That doesn't tell the story. From zero to sixty, the CLK63 is at least a half-second faster than the CLK55. I say "at least" because both cars are severely limited by traction. Once rolling, traction is not an issue, and that's when you can tell the difference. From five miles an hour on up, the CLK63 simply pulls away from the CLK55. They're both electronically limited to 155 MPH, but the CLK63 will be there a lot faster than the CLK55. This is not to imply the CLK55 is slow. It's not. It's just not "CLK63 fast".
The CLK63 brakes have been upgraded to match the additional power. The front rotors are long wearing, fade resistant, composite material. The rears are traditional cast-iron rotors. This combination works well. Repeated late braking exercises did not produce the fade more commonly felt in the CLK55. Although stopping power remained consistent, there is no pedal feel for the first 25% of travel.
In addition to the guttural sounding, naturally aspirated, big horsepower, bigger torque, vee-eight, the CLK63 received a seven-speed transmission. The Mercedes-Benz 7G-Tronic was upgraded to handle the type of torque spikes that would otherwise break the standard seven-speed. The advantage of the 7G-Tronic is it employs close gear spacing. This allows it to keep the engine in a rev range where optimum power delivery is available both before and after gear changes. The results are "pin the occupants to their seats until the 155 MPH speed limiter kicks in" type acceleration. The transmission has three modes; Comfort, Sport, and Manual. These SpeedShift equipped transmissions shift 30% faster in sport mode than they do in comfort mode. In manual mode, the transmission shifts 50% faster. A pair of steering-wheel attached shift paddles allow the driver to make manual gear changes. Personally, I believe the paddles are the correct approach. There was a time when buttons were offered, and they felt disconnected. The paddles feel more precise.
When driven in a normal fashion, the CLK63 behaves the same as a CLK350. The steering feels weighted without excessive feedback, (with the engine revs low) it's relatively quiet, and the suspension is compliant (again, at reasonable speeds). This is exactly what you'd expect from a luxury coupe. With it's visually restrained appearance, no one would guess what lies beneath the hood. There are a few clues. There are small "6.3 AMG" badges on the fenders and it has an AMG specific front bumper. There are AMG-spec wheels, but nothing that screams Pimp My Ride.
The interior is just as subdued as the exterior. There are AMG specific parts - the door sill panels, instrument cluster, and sport seats, but it all looks like it belongs in a Mercedes-Benz.
Although the chassis doesn't relay every bump in the road, it's not numb. As the lateral loads are increased, the chassis relays increased feel, letting the driver know just how much traction is available. Also, ESP is always there to help out. The flickering light is an indication that traction has been lost, and the system is intervening. When the ESP switch is set to 'off', it's more like a watchful parent in the next room. With ESP off, when the vehicle is nanoseconds away from doom, ESP will awaken and intervene. The " ESP off" feature is different on the CLK63, because it waits longer before coming to the rescue - just in case the driver intended to perform a particular maneuver.
There are some downsides to the CLK63. One downside is a personal issue; I'm not a cab guy. I prefer a coupe. In the U.S., the only CLK63 coupes available were the limited edition Black Series. CLK63 coupes were available in Europe and Asia. If you're in the U.S., and you like CLK cabs, and you can get past the second downside, the CLK63 may be for you.
The second downside is threefold and it's all mechanical. The problems are correctable, and hopefully under warranty. If the car is out of warranty, the problems are correctable for less than five thousand dollars.
1: Early M156s have top end issues. The least of the issues involves noise. Some engines are a little noisy due to cam gear lash. As long as the gears themselves don't show wear, a little too much lash is usually just an annoyance.
2: The second issue is cam wear. When the car is turned off, the camshaft buckets typically bleed down. When the buckets have bled down and the car is restarted, a gap will exist between the cam lobes and the buckets. This is noticeable by the tapping noise that exists until the oil pressure builds back up. The cam lobes are made of a softer material than the buckets and the cams wear the entire time the engine is tapping. The fix is to replace the M156 valve buckets with the M159 (SLS) buckets. The M159 buckets are an improved design that doesn't bleed down like the original equipment buckets. The M159 buckets also have an antifriction coating to reduce cam wear. As of this writing, M159 buckets are less than $1K. The labor will take several hours because the cams have to be removed to access the buckets.
3: The cylinder head bolts are defective. The heads of the cylinder head bolts have been known to break off. When that happens, the head gasket can catastrophically fail. Normally, when a head gasket fails, it's a gradual process and coolant is eventually either pulled into the combustion chamber or the cooling system gets pressurized. When the head bolt breaks, the coolant seal can be instantaneously lost. If that happens, a cylinder can rapidly ingest coolant and hydrolock before the driver recognizes the problem. When an engine hydrolocks while running, it typically bends a rod. This isn't conjecture, it has happened more than once. The good news is; that's not always the case. A head bolt could break and only result in an oil leak. It's not likely, but that could happen. One last thing to consider about what goes on when the head on a cylinder head bolt snaps off; that chunk of metal usually bangs around under the valve cover until the engine stops running. Sometimes it gets wedged between expensive moving parts. If it causes the valvetrain to stop while the crank is still in motion, a full rebuild will probably be required.
CLK55 / CLK63 Technical Data