Harley engines introduced in 1984. The EVO (short for Evolution) was the most radical change Harley motors saw. With aluminum cylinders as well as new aluminum heads designed to decrease oil leakage. Side-squish combustion chambers and flat pistons. A new lower end designed for electronic ignition for better spark control. All these changes came about as a result of higher big-brother emission standards, which the shovel just did not seem to be able to handle.