Which of the following does NOT contribute to engine detonation?

Retarded ignition timing does not contribute to engine detonation. In fact, retarding the ignition timing generally helps to prevent detonation. When ignition timing is retarded, the spark plug fires later in the compression stroke, resulting in a slower combustion process. This can lower cylinder pressure and temperature, which are critical factors that can lead to detonation.

On the other hand, a lean air/fuel mixture can lead to higher combustion temperatures, which increases the likelihood of detonation. Excess carbon in the combustion chamber can create hot spots that can ignite the air-fuel mixture too early, also contributing to detonation. A stuck-closed EGR (Exhaust Gas Recirculation) valve can prevent recirculated exhaust gases from entering the combustion chamber, reducing the dilution of the air-fuel mixture, leading to higher combustion temperatures and a greater risk of detonation. Thus, among the options listed, retarded ignition timing is the one most likely to mitigate the risk of detonation rather than contribute to it.