Strangely, the engine used in the previous C32 AMG was heavier than the current 5.4 V8 as found in the C55 AMG... Then again, the difference in capacity isn't as huge as between the F40 and Viper.
Plus the Viper engine is constructed using the US's metal and not Europe's Tinfoil, so I'm also guessing the Viper's unit is heavier.