This is the 73-77cc open chamber (low compression) found on all 2V heads, all 1972-74 4V heads, and the High Output head. Although this chamber is larger, with reduced compression, it is not a desirable one. By design, it aggravates detonation by creating opposing flame fronts under hard acceleration-hence the pinging. This is the chamber to avoid if at all possible. You are limited to 4V heads with huge ports if you want wedge chambers. The Australians got it right by combining conservative two-barrel ports with smaller wedge chambers.