Mach number temperature generally decreases with increasing altitude between sea level and , the speed of sound also decreases. For example, the standard atmosphere model lapses temperature to at altitude, with a corresponding speed of sound (Mach1) of , 86.7% of the sea level value. While the terms "subsonic" and "supersonic", in the purest sense, refer to speeds below and above the local speed of sound respectively, aerodynamicists often use the same terms to talk about particular ranges of Mach values. This occurs because of the presence of a "transonic regime" around M = 1 where approximations of the Navier-Stokes equations used
Ford Mustang Mach 1 (for use with 4-venturi carbs) versions. The 351C 4V (M code) engine featured 11.0:1 compression and produced at 5400 rpm. This new performance engine incorporated elements learned from the Ford 385 series engine and the Boss 302, particularly the poly-angle combustion chambers with canted valves and the thin-wall casting technology. For 1971, Ford reduced the content of the Mach 1. The standard features of the model were as follows: In 1971, the Mach 1 was available with three 351 Cleveland engines; the H-code 2-V, the M-code 4-V and beginning in May of 1971, the 351CJ (Cobra Jet or GT engine).