Aluminum heads with a low compression 440 will loose compression? Huh?

the problem is there is no obvious way to compare like for like
if the heads were identical in all dimensions iron and aluminium, then the operation of the motor would change becasue aluminium is a better conductor of heat energy and iron is a better insulator this has an impact on the decay of the surface temperature of valves the chamber surface and the spark plug..
the conductivity of cast aluminium is about double that of cast iron. hold a rod of each in a flame and aluminium hand gets burnt faster because the heat travels faster.
auminium frying pan..cast iron frying pan etc etc


you basically changed some parameter that does have an imapct on combustion.

but you are not going to run a carbon copy clone
the auluminium head you run will have a different chamber shape the thickness of the aluminum between chamber and water will be greater in some areas the total volume of liquid aluminium to cast that head will potentially be greater than used for the iron head due to the differing properties of the two metals so no like for like comparison is possible.

so if you were prone to pre igntion with the iron head due to exhaust valve or plug glowing hot..you might not have it with the aluminium head for the same static CR and Cam because of a small change in the ability of the latter to move heat away from the hot areas to the cooler areas.

for most of us school was a long time ago ,but if you did anything about specific heat capacity, and any experiments with chunks of material where you fill a bore with hot oil or put in a small electric heater, and measure the outer suface temperature aginst time, plot it on a graph etc , at school, in physics, its an easy phenomenon to illustrate which is why its on the syllabus for 12-16 year olds.

There is something to "see here" but the vast majority strap on new ally heads as part of a wider rangeing build and its not the only thing that is changed, the motor needs a new tune afterwards so nobody is comparing like for like..... all you can observe is that you built a motor and set it up and it is now better than what you had previously.

Its not bullshit, its part of your high school certificate, but the chances of anyone, from a background of building engines in a decently equipped shop to a guy scrabbling in the dirt under a shady tree, proving one way or another is low. you'd need to compare like for like, and have some serious lab equipment. like a car manufactuer.

the reverse of this can also be used
a set of headers made from cast iron to the same internal and lengthdimensions as your favorite steel tube headers would work better, iron keeps the heat in the gas better, its considred a better insulator than steel tube, so the gas is hotter and it travels faster, and the extraction effect from the chamber is greater. exhaust wrap wouldn't need to exist if we didn't use steel tube.

in industrial setting, power generation, running a turbine off a nuclear reactor, etc. the steam pipe is made from iron with iron fittings for a bloody good reason that goes beyond physical strength, not aluminium tube or steel tube or copper... the steam might not be steam by the time it got there if they were.

Just exmaples where the material has an impact on the retention or extraction of heat energy

not bullshit, just not generally measurable, or easily observable becasue we are not doing experiments where we vary only 1 parameter when we are building old motors

all you can do is put the heads on and see... work from your new baseline

i've yet to come across an air cooled motor with an iron head....They might exist but i haven't seen one, and there must be a none bullshit reason for that i either didn't look hard enough or its something else

Dave