Complex sentence: Several variants were used in Bristol buses during the late 1930s, the engine going through several versions from RR1 to RR4, which had a power output of 145 hp at 2900 rpm.
Split into simpler sentences: Several variants were used in Bristol buses during the late 1930s. The engine went through several versions from RR1 to RR4, which had a power output of 145 hp at 2900 rpm.