Roger

The word HEMI is actually short for hemispherical. In a piston engine, the area from the top of the piston to the top of the cylinder head by the valve heads is known as the combustion chamber (this is actually just an area of air!). The combustion chamber on most overhead valved AUTOMOTIVE engines are either wedge shaped (think of a piece of pie) or hemispherical (think of tennis ball cut in half). The cylinder head design dictates the combustion area shape, so when someone says &quot;hemi&quot;, it really only refers to the engine's cylinder head having a hemispherically shaped combustion chamber. <p>Hemi heads have the spark plug located at dead center of the combustion chamber, which makes for an extremely efficient &amp; powerful fuel/air mixing &amp; flame burn (actual igniting of the fuel/air). Hemi heads also have ideal valve placement to help the flow of air (intake &amp; exhaust) through the cylinder head. In simple terms, the head design is VERY efficient, and allows an engine to make excellent power! On the downside, hemi heads themselves usually are quite heavy &amp; large, and valvetrains can be somewhat complex (read: expensive).</p><p>Wedge shaped combustion chambers are what most OPE engines have, as do most automotive engines. The &quot;wedge&quot; shape by design, is not as efficient as a &quot;hemi&quot; due to all the sharp angles that are present, which creates turbulance. The valve placement &amp; spark plug location are less than ideal also. Cost was a big factor that lead to the wedge design. However, a wedge chamber CAN be modified to produce very impressive burn &amp; airflow rates - having said that, IMHO all this &quot;HEMI&quot; talk (car &amp; truck commercials) as of late really doesn't impress me personally. Chrysler/Daimler is really getting a lot of hype &amp; sales out of it though...</p><p>There is another variation of head called a semi-hemi, but it is not widely used. It is actually a cross between a wedge &amp; hemi shaped combustion chambers.</p><p>Marty <img alt="" src="/components/FCKeditor/editor/images/smiley/msn/shades_smile.gif"/>