A large database of second-by-second emissions and engine operating parameters has been gathered on 1992-1994 model year vehicles as part of the FTP Revision Study. The results provide insight about the impact various engine operating parameters have on engine-out emissions and catalyst conversion efficiency of modern, fuel-injected vehicles. The discussion consists primarily of observations made directly from the data, along with some background information to facilitate understanding by people without detailed technical backgrounds.Air/fuel ratio was the dominant influence on engine-out CO and NOx emissions, as well as HC and CO catalyst conversion efficiency. Brake specific engine-out HC emissions were influenced primarily by the block temperature and by deceleration. NOx catalyst conversion efficiency was influenced both by the air/fuel ratio and by micro-transient fuel control.