Cadillac engines

Back in the late 70s/early 80s, hot rodders began retrofitting their detroit iron with aluminum heads which lowered the overall weight of the vehicle and, due to aluminum’s superior thermal conductivity, allowed higher compression ratios for better hp and torque.

Before caddy introduced the Northstar DOHC v8s they had aluminum blocks and iron heads.

Where’s the logic in that?

Logic? Logic?! Back when my plant belonged to General Motors, I learned that logic was not necessary. When a rookie asked why something was done in such a mysterious way, I’d laugh. “This is General Motors, bub! It doesn’t have to make sense!” I saved myself a lot of head-scratching by learning not to ask why.

Aluminum heads are especially vulnerable to warpage and cracking because aluminum has a much higher coefficient of thermal expansion than cast iron. Consequently, when a bimetal engine with an aluminum head gets too hot, the head tends to swell up in the middle, causing it to warp and blow the head gasket.

Logic? From a company that converted a gas-powered 350 into a horrendous diesel?

You would need to verify this, but I think that the aluminum Caddy blocks had cast iron cylinder liners and a cast iron head interface. The most heat-sensitive parts of the car were therefore all cast iron, but weight was significantly reduced by making the coolant conduit out of aluminum.

My old Vega had an aluminum block and a cast iron head to add strength and rigidity to the block.

Iron heads are also better for combustion. The hotter the combustion chamber and the cooler the air, the better. The only benefit aluminum heads have is that they are light and disipate heat quicker. But in the long run, if you want to make serious power, for long term use, use iron heads. (Unless you have a top fuel car or a sprint car that’s gonna blow the head up regardless of material.)