Complex sentence: For the 1999 model year, a new 3.5 L DOHC engine was introduced it was a six-cylinder design based on Cadillac's Northstar V8, which was nicknamed the "Shortstar".
Split into simpler sentences: For the 1999 model year, a new 3.5 L DOHC engine was introduced. It was a six-cylinder design based on Cadillac's Northstar V8, which was nicknamed the "Shortstar".