Aluminum heads

HughesEngines said
Compression ratio and cylinder pressure are not strictly results of mechanical factors.

Confusing compression ratio and cylinder pressure is a common mistake and problem for amateur engine builders. They are related, but not the same, as commonly thought. Compression ratio is strictly a result of mechanical influences such as bore, stroke and combustion chamber volume. Cylinder pressure is pressure inside the combustion chamber when the engine is running. Cylinder pressure is influenced by compression ratio along with several other factors – for example:

1. Altitude – The higher the altitude, the lower the cylinder pressure: resulting in reduced performance like lowering the compression ratio, but the compression ratio remains the same.

2. The material the cylinder head is made of effects pressure too. The aluminum cylinder head will dissipate (that means draw off) heat from the combustion chamber much faster than cast iron with no effect on the compression ratio. But it will lower the running cylinder pressure. Lower temperature means lower pressure, and lower pressure means less power. That is why you should increase compression ratio when switching to aluminum heads, or you can leave it alone and reduce your power, all other things being equal.

I am good with most of this until the part that I bolded above. The power result is not directly the result of peak pressure. It is a result of the pressure changes over the volume changes in the cylinder. Anyone who is interested in this needs to look up and start to understand what is called a 'P-V' diagram. Here is a good example; look at figure 2 halfway down the page:
Four stroke engine - Energy Education

The energy output is the area inside the curve (the larger upper part of the curve). You can have a higher peak pressure (the highest point on the curve), but a smaller area, like if you open the exhaust valve earlier and the right hand section of the enclosed area shrinks. Thus the energy per cycle will be lower despite a higher peak pressure.

So there is no direct, automatic relationship between cylinder pressures and power output. It has to be examined as pressure changes versus volume changes. Higher compression ratio means higher peak pressures, but the end pressures will be higher too, so, again, it is pressure changes versus volume changes.

Which gets into other effects of higher CR but we'll leave that alone for now....!