Aluminum heads lower compression because of heat transfer??

I ran across this sentence when reading about Ford 302s. I've never heard of this before, but I'm always looking to learn about cars so let's prove or debunk this sentence.

Aluminum heads are said to drop between 1/2 and 1 full point of your effective compression because they draw more heat out of your combustion chamber (the one place you actually want some heat).

source: http://tomak3.tripod.com/page9.html

(It sounds very suspect to me, but I love to be proven wrong, because that means I learned something new)

I've heard the same, but said differently: You can run a higher compression ratio with aluminum heads because they help draw heat out of the combustion chamber.

Its a commonly misunderstood concept. I don't like he phrases things like "effective compression." I would prefer "dynamic compression".

All things being equal, if you swap from an iron head to an identical aluminum head, peak cylinder pressures will drop. The speed at which aluminum accepts and rejects heat does reduce heat of combustion. Many guys say that you can get away with half or one more point of compression with aluminum heads, but the truth is that to maintain the same BMEP you need to increase static compression to compensate for the extra heat that gets soaked away.

Aluminum heads real benefits are weight reduction and ease of porting. As an as-is, bolt-on performance thing, they don't really make sense.

And that leads to the question, what is the effect of combustion chamber/piston coatings? Some possible routes of exploration:

1) Calculate the temperature of the air/fuel charge due to compression, assuming a perfectly insulated container.

2) Calculate the temperature drop of the charge due to heat conduction into iron vs aluminum heads, accounting for the fact that the time ranges from 100 mS @ 1200 RPM to 20 mS @ 6000 RPM.

3) Calculate the temperature drop of the charge due to heat conduction into aluminum pistons.

4) Calculate the effect of heat conducting into the charge from the iron vs aluminum heads before compression raises the temperature.

5) Investigate the difference (if any) between the thickness of the quench layer (unburned charge adjacent to the combustion chamber surface) for iron vs aluminum.

Anyone have access to the SAE library? Bound to be a ton of literature on the subject.

Carter

SkinnyG wrote: I've heard the same, but said differently: You can run a higher compression ratio with aluminum heads because they help draw heat out of the combustion chamber.

This I completely agree with.

There have been a couple magazine articles about this very thing, normally always using the Chevrolet 350-style V8 since there are so many heads available for it. Car Craft did a basic test of such in 2006 and found no difference in actual power output, which I think helps disprove the whole "compression changed with head material" theory. That total ignition timing for best power was the same between both also seems to debunk this in its own way.

http://www.carcraft.com/techarticles/ccrp_0602_iron_versus_aluminum_cylinder_heads_test/index.html

i remember an interview with the head of the School of Automotive Machinists several years ago where he talked about this. he said that you can hold just as much heat in the chamber if you cast the head so the chamber walls are thicker and run a higher water temperature.

he also said that most aluminum heads make more power because they cast better ports and chambers into them compared to a similar iron head.

In reply to novaderrik:

A change in ports and chambers would be a different part, making them dissimilar.

Yeah, like I said, the big benefit of aluminum is weight. The net difference is very hard to measure.

pres589 wrote: In reply to novaderrik: A change in ports and chambers would be a different part, making them dissimilar.

they can still be "similar" without being "identical". but no one ever makes an iron version of a head that is totally identical to the cast iron head they are copying. they always tweak them in some way, to make them better.

i live in the world of small block Chevy V8's, where you have a pretty wide array of choices for heads. you can get aftermarket heads with 180cc intake runners in either cast iron or aluminum with combustion chambers, valves, and exhaust runners that are the same size. on paper, the look identical but they aren't- they will generally put slightly better flowing ports in the aluminum versions because when people are paying the premium price for aluminum heads, they want better performance. the iron heads can be ported to flow like the aluminum heads out of the box, but then you end up with the same power but with more weight.

in that example, the iron heads are "similar" to the aluminum heads but not the same.