Designers of engine controllers have usually used an experimental trial-and-error approach to determine the best reference basis for compensating the engine dynamics. This paper investigates the nature of engine dynamics, in particular whether the dynamics are time based or crank-angle based. First, available time-based dynamics for a 5.7-L V8 TBI engine and a 3.1-L V6 PFI engine are reviewed. Next, transformation formulae between time based and crank-angle based dynamics are derived. Crank-angle based dynamics for the two engines are computed using these formulae and the results are evaluated relative to the time based dynamics. Lastly, the reference basis for a 2.5-L L4 TBI engine operating in the idle-speed region is examined. All dynamics, except fuel dynamics, are less varying in the crank-angle domain than in the time domain.