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It has long been recognized that ignition of the fuel-air charge in Otto cycle engines can be and sometimes is induced by sources other than the electric spark. These additional sources of ignition may be any hot surface to which the fuel-air mixture is exposed. However, since the sources of ignition, other than the spark, which prevail are quite often transient in nature and difficult to reproduce, a systematic study of their effects on the performance of engines actually operating in service is a difficult and time consuming task. This difficulty can be circumvented, however, by employing controlled additional sources of ignition such as extra spark plugs or artificial and controllable hot spots. This paper presents the results of a series of tests made to study the influence of a controlled second source of ignition on performance in a single cylinder CFR overhead valve engine. An independently timed second spark plug was used to simulate surface ignition.