An engine with a hemispherical combustion chamber (http://en.wikipedia.org/wiki/Chrysler_Hemi_engine#Modern_Hemi). It gives the car a big oomph when you slam down on the gas pedal. I've noticed that when I do that, I can also watch the needle on the gas gauge drop a tick or two.

Do I need 340 horsepower in my everyday life? Nah, but it's fun. Will I bring my father car shopping next time? Definitely no.