My friend just installed a 3 inch catback on his 90 Formula 350. He has the dual cats. He had the "Bullet" muffler installed also. There is a little noticeable increase in power. The car seems to run better and stronger. He had a 3in install from the cats out. It is a single exhaust but it works! The car is a lot quieter than before. He removed the Flowmaster 80 series. The interior noise is also much quieter.