340 Static Compression Numbers and Test Procedure

yellow rose said
Compression ratio doesn't care what the head is made of. There is no way the the cooling system can pull the heat away fast enough to make a difference. Aluminum may (and does) conduct heat away quicker than cast iron, but the only way it can carry away that heat is through the cooling system.

I not going to to the math, but how many BTU's will it take to raise the temperature of the coolant 1 degree? I'd say it's a bunch.

When the system is relatively stable, and you run into detonation, the aluminum won't be able to dump that heat any quicker into the coolant than CI. At least not enough or quick enough to make a difference.

It actually can make a difference but only at really low RPM, like under 2000 and it's still a very very small effect. The cycles in a typical gas engine happen so quickly there simply isn't enough time for any measurable amount of heat transfer to take place from the chamber into the head, to the point where the combustion temps are cooler with aluminum heads; just doesn't work like that. Remember out of the 4 strokes in an Otto cycle only 1 of them puts heat into the chamber and the rest of the strokes take it out. Now just a hunch but I think where some of that myth came from was due to the surface finish of aluminum vs. cast iron heads. The irons are usually a lot more coarse and grainy on the unmachined parts (chambers) and any sharp edges in the chamber will act like a glow plug and cause pre-ignition. Not as big of an issue with aluminum because even if there are sharp edges the heat will transfer away quickly enough so that part won't get super hot, it will stay the same temp as the rest of the chamber.

Ever tried to weld aluminum? Same issue, the heat transfers away from the weld quickly enough that basically the whole part needs to be pretty hot or you'll be holding the welder on one spot forever waiting for the aluminum at that spot to melt.

When all is said and done, the only thing we need from an engine is pressure in the cylinder. Heat is just a byproduct and a way to increase pressures because more heat = more pressure from a given amount of air/gas. If you're curious where the future of internal combustion engines is headed look up Homogeneous Charge Compression Ignition...