I have a 1994 Impala SS with a 5.7L lt1 engine with cast iron heads. Which is better in horsepower gain? Aluminum heads or cast iron heads? People say cast iron heads downsize the hp from 300 to 260 hp. Also which of the two heads are more durable and less restrictive?Aluminum Heads are lighter, easy to repair,BUT... they warp easily if the engine gets too hot, the sparkplug holes strip easier plus the the plugs are more likely to get frozen in. Cast Iron Heads are heavier but they can handle overheating a lot better (without warping). Harder to strip out the sparkplug holes and plugs less likely to get frozen in. As far as Hp, it depends on specific heads and who makes them. There also can be Intake Manifold- to-head match up problems and which exhaust manifold (or Headers) goes on which head. You need to talk to someone who really knows all about the head choices for your block.If they are exactly the same with the exception of 1 being aluminum and the other being iron the iron one will make slightly more power. The 2 big differences are you can run higher compression with aluminum and there is a big weight difference.only difference is weight and aluminum is easier to repairOther related question