The theoretical relationship between engine air consumption and exhaust back pressure is derived for an idealized, 4-stroke-cycle engine inlet process, and compared to results obtained by testing a typical automobile engine. The exhaust back pressure of a 1971 Ford 351-W, V8 engine was varied 0.5-1.5 atm, under a wide range of engine speeds and loads. The results show that engine air consumption responds to variation of the ratio of absolute exhaust back pressure to absolute inlet manifold pressure in a manner approximating that indicated by theory, with a strong dependence on engine speed. These data can be useful in the design of speed-density fuel injection systems for automobiles.Data are also presented concerning the effect of exhaust back pressure on performance and exhaust emissions. Increase in exhaust back pressure decreases nitric oxide, due to the increased exhaust gas remaining in the cylinder, as has also been demonstrated by others. Hydrocarbon emissions are also reduced as exhaust back pressure is increased, since the last exhaust gas to leave the cylinder has a higher hydrocarbon concentration.