Complex sentence: These were almost always turboprops, which had fuel economy on par with piston engine designs, but had far lower maintenance costs and often the time between engine overhaul periods was five times that of the best piston engines.
Split into simpler sentences: These were almost always turboprops, which had fuel economy on par with piston engine designs, but had far lower maintenance costs. Often the time between engine overhaul periods was five times that of the best piston engines.