I use a 16 x 8 almost exclusively on this size engine, because most of my planes are faster, aerobatic planes. An exception for example, is my Fokker Triplane, on which I use an 18 x 6. The Cub might very well like an 18 x 6, but you will only know after trying both for yourself. Within reason, pitch determines speed, so it will also depend on how powerful your engine is whether a 6 inch pitch will give you the speed you desire.

AV8TOR