Aluminum heads

RustyRatRod said
Compression ratio and cylinder pressure are both results from mechanical features. Aluminum or cast iron has nothing to do with either, as long as the chambers are the same volume. The only positive attribute aluminum has on combustion is, the quicker heat dissipation, which does allow for more compression, but only through the more effective heat dissipation. The physical compression or cylinder pressure is unchanged from cast iron to aluminum all other things being equal. A compression test would surely be the proof there.
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Let's a tackle things one at a time. The above would be true IF there was no heat transfer between the cylinder components and the compressing mixture. This would be called an adiabatic process (with no external heat transfer to/from the gases), but the reality is that there is substantial heat transfer to the gases during the compression stroke. Most of this is from the cylinder head. If there is a hotter material (AL or iron), and equal heat transfer coefficient (the resistance to heat flow across change in materials) then the hotter material will obviously heat the compressing gases more. If there is more heat input to the gases, then the pressure WILL be higher. There is no avoiding this.... it is a laws 'of physics' thing, expressed in the Ideal Gas law..... PV = nRT. If T goes up then P HAS to go up too.

If there is doubt of the effect of added heat causing increased pressure, then let's look at something that we all pretty well accept: quench. Quench is the rapid movement of the compressed gases across a cooler surface (typically the piston). It lowers the tendency to detonate by lowering the gas temps.... and thus the pressure. So if you believe in quench (and I think pretty much everyone here does), then the belief that more heat into the compressed mixture raises pressure (and raises the tendency to detonate) automatically goes hand in hand with that. They are 2 sides of the same coin, so to speak.

So now we are really only left with the question of whether AL and iron transfer the same heat into the compressing mixture or not. That leads to 2 questions that effect that: