What are advantages and disadvantages of Rotax and Lycoming engine designs? I am wondering what are advantages and disadvantages of these two designs, e.g. Rotax 912 vs Lycoming O-235, compared to each other. At first glance they are pretty similar (both are 4-cylinder, horizontally opposed, with carburetors, naturally aspirated, with similar horsepower output etc.) but there are also some differences (engine displacement, maximum RPM, preferred fuel, oil system and consumption etc.) Can someone explain pros and cons of both designs?
The biggest difference between these two engine designs is that the rotax contains a reduction gearbox which allows it to turn faster and hence develop more power per cubic inch of displacement, and the lycoming is a direct drive engine limited to a rotating speed that maintains the  prop tips subsonic. This gives the rotax better power-to-weight ratio  but causes it to run hotter and shortens its time-between-overhauls. In addition, the gearbox itself requires overhauling which adds to the overhaul costs. In practical terms, air-cooled &amp; geared engines for GA aircraft have not proven as popular as direct-drive engines in similar applications; in fact it is common to replace geared engines with direct drive engines with larger displacement to improve reliability and reduce rebuild costs.