<p> In this work the effect of spark timing on the spark ignition engines is investigated by computer simulation and experimental test for speeds of (1500,2000,2500,3000 and 3500)rpm at spark timing of (20<sup>o</sup>,30<sup>o</sup>,40<sup>o</sup>,50<sup>o</sup> and 60<sup>o</sup>) before TDC for each speed. This is done in order to find a suitable mathematical expression for spark ignition advancing with respect to the speed of the engine to predict the correct ignition advance as in real engines .The results showed that the method of using a mathematical expression is more realistic and reasonable comparing with the results obtained by other workers.</p><p> <strong></strong></p