ProComp/Speedmaster aluminum heads

318willrun said
Then I can make it simple with these questions. One word answers.
  1. Does aluminum dissipate heat faster than cast iron?
  2. Does air being contained (cylinder, balloon, tire) lose pressure in cooler temp?
  3. Does less cylinder pressure create more or less horsepower?
  4. So IF aluminum creates less cylinder pressure, does it allow more compression to be run to make up for the loss of cylinder pressure?
Again, how many different engines somebody has built or what credentials they have, it's really this simple to me unless somebody can explain it with better logic. I'm not saying I'm right, but until my simple logic is debunked with better logic, I have to stand by what I think... for now. I'm always open to learning.
Click to expand...

1: no - alum CONDUCTS heat about 2x the speed of iron, but also weighs 1/2 as much, which means two identical parts made from each material has about the same thermal performance (because thermodynamics relationships are based on mass).

2: yes, but it doesn't matter. The amount of energy is dictated by the mass of the air and the mass of the fuel. Being at a higher density and lower pressure will not matter. Once ignited, the temperature and pressure will rise rapidly. An aluminum head will not be able to extract enough heat to have a measurable effect on power - otherwise switching to alum heads would make an engine MORE prone to overheating not less as is often the case.

3: less, but irrelevant, see #2.

4: begging the question.

My understanding, which may not be correct, is that more compression is run with alum heads because alum is a weaker material and deforms under load to a greater extent than iron. This means that the chamber effectively 'stretches' as air is compressed, reducing effective compression ratio. This is why race oriented engines use more headbolts than OEM applications.

Deformation can be overcome with good design. A well designed cylinder head which doesn't deflect as much would reduce the difference between iron and alum. Since mopar heads have shaft rockers, it's incumbent on the designer to ensure a reasonably stiff head to prevent binding or failure of the valvetrain. There's probably plenty of junk heads made in the ancient past for brand X motors that deflected more (especially flathead engines) which were the genesis of this +1.5 myth.