Originally Posted By: turtlevette
I've had arguments with people about this before. There is nothing that magical about a hemispherical combustion chamber. It's archaic and less efficient. A modern head should create quench as the GM engines do. It acts to create turbulence to get a better homogeneous mixture to burn. So the head should be flat and be within 40 thousands of the piston top at tdc.
GMs LS engines create plenty of power without being hemis. It just sounds macho. There are usually 20 of these in challengers at the show with their hoods open. I just walk right by as there is nothing to look at other than a huge plastic piece covering the whole thing.
The modern "HEMI" moreso resembles a pent-roof than a hemisphere. It has two quench areas (even the ones that "don't have a quench area" by nature of the shape of the chamber relative to the bore actually do) and canted valves that oppose each other. Unlike your GM LSx example, which is a traditional wedge, the HEMI and pent-roof designs offer more valve area. This was also the logic behind the Cleveland and BBC heads, which both feature canted valves, to allow for more area.
A quick look at the chambers of each:
6.4L Chrysler "HEMI":
Ford 6.2L SOHC head:
BMW M62 DOHC head:
GM LS3 "CNC" head:
Like with the SBF and venerable SBC, on the LSx both valves are on the same angle on the same side of the chamber, which limits valve area relative to bore diameter. The valves on opposing angles opposite each other like seen in the HEMI, SOHC and DOHC configurations offer more room for larger or more valves.
This doesn't make a huge difference in stock engines, but does, generally, facilitate greater flow for a given bore diameter in high performance applications. This becomes particularly relevant when dealing with engines that are bore-diameter limited and was evident with the Chevrolet 305 that could not accommodate larger valve heads like its 4" bore sibling the 350 and was subsequently of limited potential when compared to GM's previous 302 (4" bore) or its Ford counterpart (302HO).