The early, closed combustion chamber is more efficient in power production than the later open chamber heads. Tests done during the 50's show a 5% increase in power from a closed chamber to a fully open chamber. On the other hand, the airflow is enhanced slightly by getting that part of the chamber out of the way. The additional airflow raises the horsepower and the net result is pretty much a draw. The reason for the change in shape was mostly for pollution considerations. If you look at the aftermarket race heads, you find that they are produced with the minimal combustion chamber philosophy.
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