Experience dictates that just by lowering the head (more compression) you gain more power especially in lower rpm range, idle quality can suffer, but the engine runs cooler. Also, that a higher head (less compression) will increase top rpm speed on bigger tracks.
A decrease in head shims (an increase in compression ratio) will increase torque because as the compression ratio goes higher, the actual ignition timing occurs sooner. However there is a point of diminishing returns where detonation occurs or engine temps can soar, and if this happens a colder plug can help.
A colder plug will also increase torque, except in the instance of a colder plug; the ignition is slowed until a greater point of compression build occurs.
When you increase head shims (a decrease in compression), top end is enhanced as the ignition timing is retarded and occurs later. Generally a hotter plug is needed to advance the ignition cycle so that timing does not occur to late in the cycle, but at this point you end up over leaning the engine to get it to rev properly and the engine life will suffer dramatically.

A hotter plug causes ignition a little earlier in the combustion process and can have the same effect as “Advancing” the ignition timing and increase overall power output, especially at higher rpm.

There are limits, however, and installing too hot a plug can cause pre-ignition (detonation) and risks of damaging your engine.


AFM