Complex sentence: In the 1990s, Julien developed together with Bernard Boyer a 500 cc racing car which achieved an average speed of 222.557 km/h at the Circuit Automobile Mortefontaine in 1997 and thus broke a 44-year old track record for cars of this class.
Split into simpler sentences: In the 1990s, Julien and together developed a 500 cc racing car which achieved an average speed of 222.557 km/h at the Circuit Automobile Mortefontaine in 1997. This broke a 44-year old track record for cars of this class.