Reply to "Head info"

Cuginflyer, the difference in the early (closed-chamber) heads is in the way the combustion chambers were machined. There are two cast-iron 'eyebrows' that overhang each chamber and cause the piston to approach very close to them at TDC. This not only increases compression ratio, it squirts two streams of fuel-air mixture out into the middle of the chamber, creating high turbulence just as the spark plug fires. This turbulence reduces the octane requirement of fuel needed to run knock-free. The result is a good 50-60 bhp increase over the later 'open-chamber' heads, on 91-octane gas. Adding domed pistons to open chamber heads will increase the compression but not the turbulence of the closed chamber type. The result is, open chamber heads in almost any combination of parts will not produce the power of a set of closed chamber heads, and if they do, it will be on 106-or higher-octane fuel. Advise not wasting time trying to improve open chamber 351-C heads, which will be most U.S castings after 1971.

Other factors to consider are- being machined for an adjustable valvetrain rather than hydraulic lifters and sheet-steel rocker arms so one can use mechanical or roller cams, and the use of smaller intake & exhaust valves such as in the '2V'- heads use. So- called 'Aussie' closed-chamber cylinder heads were made with the high compression of the U.S. closed combustion chamber '71 4V heads but with the smaller ports and valves of 2V heads. "Smaller" valves are relative, incidently- 2V valves are still larger than the biggest valves useable in a modified 350 Chev cylinder head. The result for 2V closed chamber Aussie heads is much more midrange power and better drive-ability than is possible with the huge "4V" valves in any head.