[Aussie Mike]

The Hemi head design has been around for a long time. I think Chrysler produced it's first Hemi Head V8 in the 1951.

Hemi is short for Hemispherical meaning a domed shaped combustion chamber with a central plug and valves on either side of the chamber canted towards each other. The idea was to allow the fitment of larger valves and to promote flow into the chamber. They really need a matching domed top piston to make them work.

The standard LS1 head design is what's known as a Wedge design. The combustion chamber is sloped up towards the spark plug. This design has become very efficient over the years. The wedge shaped combustion chamber has good squish meaning as the piston rises to TDC the wedge shape of the chamber forces the mixture up around the spark plug.

The Valves in line actually produce swirl which aids cylinder filling. The intake charge flows in and is then deflected off the side of cylinder wall and swirls in and down as the piston drops. You can kind of see this effect when filling a bucket with a hose. directing the hose straight at the side of the bucket cause a lot of the water to splash back out of the bucket but directing it to one side at an angle makes the water swirl in keeping more of the water in the bucket. This swirl effect continues on the power stroke as the piston goes down giving a more complete burn. As the piston comes back up the swirl continues helping the exhaust gas exit the cylinder. Think of it as how water swirls down the plug hole in the bath.

That's the theory as I understand it.

The Wedge head has taken over as the design of choice for push rod engines because it's simpler to manufacture, and more compact. It allows the use of flat top pistons and produces great results for everyday driving. The Hemi head really only comes into it's own at high RPM where the greater flow from the larger valves helps.

Cheers