The whole big deal about noise reduction in Europe is to run a larger than normal prop to keep rpm down. This serves three functions. One, the prop noise is less, and two, there are fewer exhaust impulses to contend with, and three, it makes the generated sound of a lower pitch. Whereas a 14-6 is the normal prop for a 91 engine, something like a 15-6 would be in order.

Enjoy,

Jim