What could excessive machining of the cylinder head cause?

Excessive machining of the cylinder head can cause issues like pinging and detonation primarily due to changes in the combustion chamber's geometry and volume. When the cylinder head is machined too much, it can lead to higher compression ratios. Increased compression can enhance performance up to a limit, but beyond that, it may cause the engine to experience pre-ignition or detonation under load because the air-fuel mixture ignites prematurely due to the elevated temperature and pressure conditions in the combustion chamber. This undesirable condition can lead to engine knocking, which adversely affects performance and can even cause engine damage over time.

The other options suggest effects that are typically not associated with excessive machining. Improved fuel economy, better power output, and reduced engine noise may result from proper machining but are not guaranteed, particularly when machining leads to negative effects like detonation.