In laboratory engines instrumented to measure surface ignition, vibration and noise; various types of surface ignition, even non-knocking, caused obvious roughness. Additives effective in controlling surface ignition were helpful in reducing vibration and noise. However, in clean engines, pure hydrocarbons with normally high combustion rates resulted in obvious harshness and roughness. These effects, comparable to surface ignition, indicated the margin between normal and abnormal manifestations may be quite narrow. A severe and increasingly common occurrence with test engines has been shorting of spark plug gaps by grossly migrating light duty deposits under moderate to heavy stress.