The ZDZ-50 and 100 twin are way "different" than the rest of the engines in their line. They have very long exhaust duration and don't do very well (power-wise) on in-cowl mufflers. These engines are very similar to the ZDZ-40 and 80 twin as far as the lower end (same stroke), size and weight. The 40 and 80 twin run very well on in-cowl mufflers because of different porting. The 60 and 80 single also run well on in-cowl mufflers. I think ZDZ should offer a couple of porting options for the 50 and 100 twin - one optimized for in-cowl mufflers.