Originally Posted By: A_Harman
Yes, Chrysler's small-block Hemi isn't a true hemi, but neither was the 426 or the 392 (331, 354, 392) family that preceded it. None of them had combustion chambers that were half of a sphere. The big advantage of a Hemi is the ability to put large valves in it, thereby enhancing high-speed breathing ability. The dual spark plug, twin squish combustion chamber of a new Hemi gives a lot faster burn than old single-plug quiescent chambers.
Actually, the first-generation "Whale" Hemi's did have true hemispherical chambers. The 426 *almost* did, the big deviation was that the whole chamber was canted so that the valve angles weren't symmetric around the cylinder centerline. That was done to help make the heads narrower, the first-gen Hemi is a good bit wider than the true big-block 426 even though the block itself is more closely related to the Chrysler smallblock. The modern Hemi is more of a hemi-ellipse. ;-)
There's nothing magical about the hemispherical shape in and of itself, the magic is in the valve angles and the fact that the entire circumference of the valve becomes unshrouded when the valve lifts, unlike a wedge head where a part of the airflow bangs straight on into the cylinder wall at right angles, or even a multi-valve head where the flows from adjacent intake valves interfere with each other. The hemi shape does help smoothly bend the flow into the cylinder, too.
Yes, Chrysler's small-block Hemi isn't a true hemi, but neither was the 426 or the 392 (331, 354, 392) family that preceded it. None of them had combustion chambers that were half of a sphere. The big advantage of a Hemi is the ability to put large valves in it, thereby enhancing high-speed breathing ability. The dual spark plug, twin squish combustion chamber of a new Hemi gives a lot faster burn than old single-plug quiescent chambers.
Actually, the first-generation "Whale" Hemi's did have true hemispherical chambers. The 426 *almost* did, the big deviation was that the whole chamber was canted so that the valve angles weren't symmetric around the cylinder centerline. That was done to help make the heads narrower, the first-gen Hemi is a good bit wider than the true big-block 426 even though the block itself is more closely related to the Chrysler smallblock. The modern Hemi is more of a hemi-ellipse. ;-)
There's nothing magical about the hemispherical shape in and of itself, the magic is in the valve angles and the fact that the entire circumference of the valve becomes unshrouded when the valve lifts, unlike a wedge head where a part of the airflow bangs straight on into the cylinder wall at right angles, or even a multi-valve head where the flows from adjacent intake valves interfere with each other. The hemi shape does help smoothly bend the flow into the cylinder, too.