[jannes_z-28]

The Hemi heads have the intake and exhaust runners in line, and the valves meets in the semisphere opposing each other. That way you get a straight line for the gases to flow. The shape of the combustion chamber requires a very heavy piston to get any compression and makes high RPMs difficult to reach.

What Chevy did with the Mark IV head was a great compromise where they could reach good flow and also directing the flow in to the cylinder in a better way that the Mopar HEMI head could do. This design was then applied to the special heads for the Small Block. One other design with similar technique were the Ford 351 Cleveland that you could say were a small block engine with a Chevy BB head.

With the good flow from the runners and the shape of the combustion chamber they could run lighter pistons in these engines.

Look at the Pro Stock era pre 500 cu in, the 351 Cleveland had great success with Glidden, Dyno Don and Gapp&Roush. The Chevy guys were good too with de-stroked BB's getting pretty much the same kind of engine combo that the Ford guys had.
Using a 348 crank (with offset grinded taps) in a Mark IV block you could get down to around 330 cu in with a 4.25" bore, which were bigger than the Ford guys could do with the Cleveland block.
These Chevy engines could rev over 10.000 and making a lot of power. Back then the Mopar Hemis were to big and heavy and didn't have much chance... ProStock rules then had a weight/cu in ratio with different factors for different engines. The Cleveland had a higher rate than the Mopars.

Jan