Audi RS4 BMW M3 Mercedes-Benz C63
Layout Longitudinal front engine, rear-biased Longitudinal front engine, rear wheel drive Longitudinal front engine, rear wheel drive
Transmission 6-speed manual 6-speed manual 7-speed automatic
Suspension Multi-link (f), double wishbone (r) MacPherson strut (f),multi-link (r) MacPherson strut (f), multi-link (r)
Brake diameter (in.) 14.4 (f), 12.8 (r) 14.2 (f), 13.8 (r) 14.2 (f), 13.0 (r)
Wheels and tires 19x9, 255/35 8.5x19 (f), 9.5x19 (r) 245/35 (f), 265/35 (r) 8.0x18 (f), 9.0x18 (r)
235/45 (f), 255/35 (r)
Curb weight 3,967 lb 3,649 lb 3,814 lb
0-60 mph 4.8 sec. 4.8 sec. 4.3 sec.
Top speed 155 limited 155 limited 155 limited
Base price $66,910 $53,800 $53,800
Price as tested $71,535 $66,375 $63,930

Under the hood
*Looking at horsepower alone, you might think all three of these engines would be very similar under the skin. Once you start looking deeper into the stats you can see just how different they really are.

If you look at torque, you can see something different is going on with the C63. AMG relies on the adage of "no replacement for displacement" to get its power. Using 50 percent more displacement than the other two engines has its advantages. The Mercedes gives up horsepower per liter at the high end for down-low torque. Even with today's variable camshaft lift and duration, it's necessary for the engineer to decide what rpm range the engine should be optimized for. Obviously, Mercedes chose the lower end of the range. The result is a car that pulls hard at any revolution so the driver never feels caught out exiting a corner.

BMW and Audi both chose the upper end of the rev range, but went about it in different ways. The BMW's bore is much larger than its stroke, making for an engine more comfortable at high rpm. The swept area of the piston is greater, but the pistons have to cover less distance. This results in lower torque numbers but higher horsepower. The BMW also uses individual throttle bodies for each cylinder. This allows for faster throttle response, better optimized airflow to each cylinder, and greater resonance charging effect at high rpm.

The biggest advantage is direct fuel injection. Spraying fuel directly into the cylinder allows for better fueling along with better cylinder cooling, which also results in a more condensed and more powerful air/fuel mixture. The extra cooling effect of direct injection also allows Audi to run the highest compression ratio in the group, which results in greater power across the rev range.

All three engines have four valves per cylinder, which has shown to be the optimum layout for both power and longevity. The table also shows that all three manufacturers have chosen to use both aluminum heads and blocks. Unlike aluminum blocks of the past that required liners, the new generation of alloy engine blocks is made from a combination of aluminum and silicon. The honed cylinder bores are made of silicon crystals, which are much harder than even an iron liner. The C63 takes things a step further by using low-friction coatings on all bearing surfaces inside the motor. -MF

By Michael Febbo
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