The operating temp of the engine probably went up because aluminum soaks up ALOT more heat than cast iron. Iron heads deflect alot of the combustion heat into the actual exhaust whereas aluminum heads tend to soak quite a bit more up. That's why you can run higher compression ratios with aluminum heads and use lower grade gas. Just one thought, i've heard that when you get aluminum heads, you also want to upgrade the cooling system. Just some ideas, hope this helps. Good luck!
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'84 Mustang 5.0 LX
My car