A shortcoming of the Hemi-type combustion chamber lies in its inherent poor emissions capabilities. During overlap, since the intake valve and runner is basically pointing right at the exhaust valve, it ends up dumping a good portion of unburned fuel directly out of the exhaust. Chrysler had a tough time actually making the engine pass emissions guidelines to make it legal. Running dual spark plugs, as blown Alcohol and Top Fuel guys have known for years, allows the tuner to stagger the timing of each plug. He can initially light the fire, then bring in the second plug to really help it or in case the flame was blown out. The second plug can also add a great deal of spark dwell to the burn time, picking up a little power and knocking down a little more on emissions.