Complex sentence: To improve his French Pomeroy had been studying a text whose French author had been stressing the importance of high piston speeds and the necessity of good breathing through large valves and a free-flowing exhaust system.
Split into simpler sentences: To improve his French Pomeroy had been studying a text. Its French author had been stressing the importance of high piston speeds and the necessity of good breathing through large valves and a free-flowing exhaust system.