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Actual real world tests show very very little if any power loss from iron vs aluminum. I have seen several dyno tests of heads in iron and alumnium even as can be like CNC ported and there was no real world differance in the power, 2 or 3 tops on a 350 chevy. The aluminum head allows more compression, it does NOT require more compression.



I also remember reading an article where they used an identical set of aluminum and iron heads on a chevy build and they dynoed within a couple hp/torq of each other.

But, going from stock small port heads to bigger port aftermarket heads could have costed you some low end power due to a drop in port velocity. Also your new heads could be flowing enough more that you're leaning out under WOT.