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here's a link to previous discussion of this subject, except that I was (and still am) more interested if there is an efficiency loss when using aluminum heads:

https://board.moparts.org/ubbthreads/show...rue#Post2782669

in a nutshell,
what I found was that there does not seem to be an efficiency hit from using aluminum cylinder heads with their higher thermal conductivity, despite what you may commonly hear.

the best article from that previous discussion was:
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Car Craft magazine has put this technical article up
on the subject of Aluminum versus Iron cylinder heads:

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

sample quote:

"Can you guess what we learned? Zilcho. As in zero difference anywhere in the power or detonation characteristics of the iron versus aluminum heads. Even the optimum total ignition timing was the same at 36 degrees. Regardless of coolant temp, rate of acceleration, steady state, or through a sweep, the dyno curves for the two styles of heads were identical. If anything, we could squint and guess and mumble that maybe aluminum heads were better by 2-3 hp. But the one thing we could never say is that the iron heads retained more heat and made more power than the aluminum. Maybe it's different on some engines with a drastically different water-jacket design, but we'll stand up and say that the old bench-racing line just ain't true."

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