360 Rotating assembly
Overall, the coolant and rad-air have not changed their thermal resistance, but the head has gone to a much lower thermal resistance so the whole system has lower thermal resistance and so higher heat flow; the coolant temps go up a to reflect that. But the head interior surface temps ARE going to be lower because thermal resistance of the the heat transfer to the head surface has not changed. (What I don't have is how the temps would be if the t-stat is out of the system....but it is not going to change the fundamentals.)
Let’s take the radiator out of the picture first.
The aluminum will transfer heat quicker out of the chamber.
The internal heat hasn’t changed. Or very little. Because of that, a good octane is still required. Squeeze is squeeze. On that alone, it will need a certain amount of octane to prevent detonation. An 11-1 engine iron headed or aluminum headed (built the same otherwise) will still produce the same heat in the chamber.
Now the dynamic cylinder pressure is the next issue. Coupled with cam timing, this will produce a certain amount of pressure. As we know, this can and often will change the octane requirements.
Being that aluminum will transfer heat faster, there is wiggle room to allow more compression and octane required. BUT for best results, the octane requirement hasn’t changed because *** amount of PSI will still require YYY amount of octane to run at its peak lower making ability.
All the above is more so from a point of view of maximum return for peak power. Not what you can get away with. As a few member(s) here and one fella I sat down with in my back yard discussing this issue, you can run a aluminum headed pump gas engine all day @ a 11-1 ratio on 93 octane even with small cams.
If you could remove heat as quickly as aluminum with an iron head, it would be the same. But since we can not..... iron gets the bad rap. Take a little heat away from the iron, (drop the compression ratio a point or so - reduce cylinder pressure) and it is fine.
A lot has been written above the two metals and this topic. I decided to use the kiss method on it and not worry about it. *** octane has a certain amount of detonation resistance so the engine build will not push that threshold. While this may leave power in the table, it is far better than having issues when you push it to far.
Everyone can push there pencils, slide there abacus were around and punch calculator key pads all they want and retort with the scientific replies, that’s fine. Sometimes pushing it and running the science is an effort that isn’t really worth the 5-7 street HP you get in return. Race HP is all about pushing it.
