Mercedes themselves rates the CLK63 AMG Black Series at 500 horsepower and 478 pound-feet of torque. The C63 AMG Black Series gets a 10 horsepower bump on paper to 510 horsepower. The C63 AMG Black Series weighs slightly more yet also traps slightly more in the 1/4 mile than the CLK63 AMG Black Series stock for stock. A difference of 2/10's and 2 miles per hour as tested by Car and Driver. The only way for this to take place would be if the C63 AMG was making more power, which the dyno results below support.
Let's start with each CLK63 Black Series run from each car, car one through six:
The highest peak horsepower figure is 406.9 wheel horsepower. The lowest is 390.0. Average wheel horsepower works out to 400.2 wheel horsepower.
Compare this now to the six runfiles for the W204 C63 AMG Black Series from two cars:
The highest peak horsepower figure is 437.3 wheel horsepower versus 406.9 for the CLK63 AMG Black Series. The lowest is 432.6 versus 390.0 for the CLK63 AMG Black Series. Average wheels horsepower for the C63 AMG Black Series comes out to 434.8 wheel horsepower. Compare this average figure to the CLK63 average figure of 400.2 wheel horsepower and it becomes abundantly clear there is a horsepower discrepancy between the two cars.
This discrepancy makes sense as the C63 AMG Black Series has been shown to tarp higher in the 1/4 mile stock for stock. Additionally, Mercedes themselves rates the car higher. Furthermore, the M156 V8 in the C63 AMG Black Series features stronger internal engine components than the M156 in the CLK63 AMG meaning a more aggressive tune is possible which Mercedes clearly applied and insinuated with the higher horsepower rating.
All runs are in SAE correction on a dynojet. Thank you to Renntech for the graphs.