When operating an engine with a CSU and a turbocharger, what increases the likelihood of detonation?

When operating an engine with a constant speed unit (CSU) and a turbocharger, increasing the manifold pressure tends to create denser air in the combustion chamber, which can lead to higher power output. However, if this is combined with other factors, it can increase the risk of detonation. Reducing the RPM can be seen as a measure that might lower the stress on the engine, but it does not necessarily address the risk of detonation directly influenced by air-fuel mixture conditions and cylinder pressures.

The correct answer highlights a condition where the fuel mixture becomes leaner, which means there is less fuel for the same volume of air. This can lead to increased combustion temperatures and faster flame propagation, resulting in detonation. High manifold pressure during lean operation exacerbates this condition, as the increased oxygen content in the mixture can cause combustion to occur more violently, making detonation more likely.

In summary, the combination of high manifold pressure and a fuel-lean setting creates a situation prone to detonation, emphasized by the response to operations like reducing engine RPM that do not effectively mitigate this risk in the context provided.