[1795]

Originally Posted by olddog

Fuel is the issue. Today's pump gas, at 92-93 octane, auto-ignites at a much lower temperature than the high octane race gasoline. In fact if it was not for the fuel going through a phase change (liquid to a gas - latent heat of evaporation) and cooling the air charge, gasoline would auto-ignite at 10:1 compression. This is the main reason why we take the AFR from 14.7:1 down to 12.5:1 at WOT. We are using the extra fuel, that we cannot burn, to cool the air charge and prevent detonation. The un-burned fuel is also making some extra pressure, as it expands from the heat. Kinda like making steam pressure with fuel. From a detonation point of view, gasoline is a poor fuel choice, but other merits make it the top choice used.

The head material is what matters, not the flow. Aluminum conducts heat much better than cast iron (About 4 times better, if memory serves). Therefore aluminum is much less likely to have a hot spot, and it can transfer heat to the water much better, thus its surface in the combustion chamber is cooler. Additionally aluminum can be machined easier and have a smoother surface. Way less potential to have a spot hot enough to act like a glow plug, and the combustion chamber runs cooler. This make aluminum capable of running higher compression, with gasoline.

Now the flip side is that conducting heat from the combustion chamber to the water is an energy transfer, therefore less power and efficiency. A trade off that is well worth the price.

Ok, that makes some sense. I was wondering what in the material would make the difference and did not think that it was relative to aluminum being stronger than cast iron. Thanks.