Two additional terms the engine manufacturers use when describing timing "Advanced" and "Retarded" As shown in figure 1, when the timing is advanced the spark will occur before the piston reaches the top of the engine cylinder (BTDC). on come late model cars equipped with various emission control devices, the timing is retarded so that the spark occurs after the piston has started down in the cylinder (ATDC). Engine timing is changed by adjustment of the ignition distributor.