A tale of two Hemis: Chrysler's 5.7 and 6.1 V8
by Eric Bryant
In this installment, we'll take a peek inside the Chrysler Hemi siblings - the garden-variety 5.7L, and the rip-roaring 6.1L version found in SRT8 products.
Inevitably, someone will ask, "are these engines really Hemis?" First, we should define the term Hemi - it's short for "hemispherical combustion chamber", and it relates to the distinctive
half-spherical combustion chamber shape used in certain Chrysler V8s back in the 50s and 60s. The 426-cubic-inch second-gen version dominated stock-car racing and still has its place in drag racing; it wasn't really a great street engine, but regardless it built name recognition like few other engines.
OK, so are these really Hemis? The
modern rendition uses two "squish pads", one each to the front and rear of the chamber; this helps keep the air-fuel charge closer to the center of the chamber and encourage additional turbulence. Ideally, a combustion chamber would resemble a sphere, as this results in the least surface area for a given volume (that means less heat rejection to the coolant) and the smallest burn distance for a spark plug.
Domed pistons in a hemi chamber results in a thin skin of charge; imagine a golf ball sitting next to an orange rind and you get the idea. The squish pads improve the situation significantly, with the only drawback being semantics regarding the trademark name. Squabbling about the chamber shape neglects the most important attribute of the Hemi design - the valve placement. We'll get to the importance of that in just a moment, but suffice to say that this new Hemi deserves to carry the name.
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Source: Autoblog
by Eric Bryant
Monday, Febraury 20, 2006
