The Abarth team was given the problem of making the Fiat 600 faster, but after putting the exhaust on it, they couldn't close the engine cover (is it a hod when the engine is behind the rear tires?) because the engine would overheat. So they propped open the engine cover.
So they tested it anyway, and learned that the car was 11 kph faster with the engine cover propped open (of course the maxed out engine helped too!)
Learned on Top Gear, episode 73