Alloy heads
Tinmannz statement above is pretty much the long and short of it. His statement is accurate. Ten years ago I wouldn't have believed that an aluminum head would tolerate more compression than an iron head but it will. Aluminum will suck so much heat energy out of the combustion process it will cause a negative effect on cylinder pressure. Pressure = power. How?
I'm only going to write this once and it is amazing how few truly understand how the force to push against the piston is created. It's heat and the more of it the more pressure. It's not and explosion it is a burn. Air and fuel enter the combustion chamber, the piston squeezes the mixture which helps the heating process, the sparkplug ignites the mixture and the mixture burns which causes the rapid expansion of Nitrogen. It is this rapid expansion of Nitrogen that works against the piston crown. Aluminum can and will reduce the amount of thermal energy available all other things being equal.
Anyone that's ever TIG welded on an aluminum cylinder head would notice that you have to stand on the pedal to get your puddle going but once the head heats up and gets too hot too touch it takes very little amps to produce a nice puddle. This is because aluminum wicks heat energy away like crazy. Same thing in the combustion process. Anyone that argues this is not speaking from experience and lacks critical thinking. J.Rob
Ouch. First off I hold you in high regard and really appreciate your defense of me on the Moparts forum.
My
critical thinking now requires me to disagree.
Having done TIG welding on aluminum and acknowledging the heat required, there
seems to me a large difference between TIG welding on aluminum and hot gas expansion in an aluminum combustion chamber. Electrical arcs and expanding hot gasses could have a very different energy exchange rate with aluminum.
I believe the high frequency used in the AC TIG arc breaks down the resistance in the air gap between the tungsten electrode and the aluminum to begin the arc and the transfer of the heat therein.
In a combustion chamber I also believe there may be a cooler air gap boundary that the heat of the gas has to compress and penetrate before getting to the aluminum, and therefor retards the transfer of the heat to the aluminum owing to the speed of the combustion process in the internal combustion engine. This may be similar to the dead air boundary in the port of a cylinder head.
Theory is one thing, scientific (maybe dyno?) tests are another. Though
I have done no dyno tests on aluminum vs iron, the only tests I have ever read of have not shown a loss of energy, read horsepower, in switching from iron to aluminum heads. The testers used iron and aluminum heads with the same combustion chamber shapes and similar air flows, and in each case found the aluminum head to produce no loss but slightly
more horsepower. They then claimed they didn't believe their own results were accurate. Now there is some critical thinking!
Do you have detonation or horsepower tests that show otherwise? Because I would love to see them!
