[mr0077]

If the Viper weighed 3500 and the Cobra weighed 2500, it would take 40% more power in the Viper for it to have the same power to weight ratio (and acceleration, all else equal, which it's not) as the Cobra. If your friend has a 550 HP Cobra, the Viper would have to have 770 HP to have the same potential for acceleration (assuming no wheelspin and similar traction).