Which head/combustion chamber is better for performance? Speed master aluminum LA or aftermarket magnum iron head?

rebelcaptain said

Mentioned were Edelbroor Performer, TrickFlow, Speedmaster and ProMax heads. What about the 1970 and 71 stock 340 heads with the large 2.02 valves? Are any of those aftermarket heads better/worse? The literatures says "increased performance". Performance doesn't necessarily effect gas mileage. What's the story?

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Don’t ask me about gas mileage. It’s a subject I never cared about or studied. As far as going quick yes an X or J head can be made to go fast, real fast. My street car ran 11.82 with them back in the 1970’s and that was with junk parts. A friend has a set running 9’s with small valves but it’s a drag only car.