A Smart Roadster
A modern piston engined plane of your choice
An Alfa 156 1.6 Twin Spark
What is it?
They’ve all got two plugs per cylinder. For safety in the plane (a separate ignition system for each set of plugs), more power in the case of the others.
That’s an issue in the Smart, because you need to take off the rear bumper to access the lower set and it’s not uncommon either not to bother or not to know. Those thinking of the future cut a discreet hole under the number plate to provide easier access.