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Of all the strikes against the 396, the most egregious is its combustion chamber design. Early big-blocks had closed combustion chambers. What does that mean? Compare it with the later, open-chamber heads of a 454 on the left. The 396's intake valve is so buried in the chamber it has very little room to breathe. Air needs room around the valve to flow into the cylinders, and with almost half of the intake valve shrouded like this, it has nowhere to go.