As someone who has played with Holden 6s longer than most I know that the down draft 12 port Irving head was accepted as being the most powerfull head available for the red motor.
I was forwarded an Ebay add for a Irving head. The claim was only 2 were made. Not true! BUT maybe it was as this one was a heron head which I have never seen or heard of before. So requires special pistons to use.
Hughes Brookfield produced these heads [designed by Phil Irving] in the early 70s and they were regularly used in speedway sedans, 1 or 2 on 149 midgets. And many were used on Holden Sports Sedan engines and later improved production.
The ones I first read about in a Speedway Racing news came with head, rockers, rocker cover, stubby intake and triple 48mm Dellortos. Not webers. These had a 'normal' combustion chamber. Though they were more often used on Sports Sedans. They made very good power increases though you needed a good bottom end as it was easy to 'run over' the bottom end.
I wanted one for my then 202 powered Sports Sedan, but they were all too expensive so after being told it would not work I fitted a 350 Chev. Which worked quite well!
These heads are not too be confused with the similar down draft Duggan head from Duggan engineering, also a good piece but at least in the early days had a reputation for casting porosity. These were often used in Drag Racing with success. And are now/were marketed in Sydney as a J Zed head reputedly with improvements and better castings.
The Holden red/blue/black engines had two basic styles of factory head, the 9 port used 63-79 then the 12 port [with lifted intakes] from late 79 -85. Many have modified these heads to make big power gains. GpN Toranas are making very big HP with 9 ports. In the past port dividers were used to make the 9 port into a 12 port which made the heads more responsive. Not legal for GpN however.
Yella Terra [Perfectune] also made both iron and alloy versions of the later 12 port which were FAR better cast and finished than the Holden version. The iron version was marketed by Holden in the 90s as a replacement item. On a street stock speedway engine they did make more power right through the range.
Over the years several have fitted Falcon 6 heads. There is a Torana running at National level Sports Sedan with an XE Falcon head and it goes fairly well. Damian Johnson.
Wayne Gower here in Adelaide and his son initially did a 3.9 Ford head on a Holden and later a twin cam BA head with success. The Falcon head is to a degree a bolt on. normally fitted backwards as the Ford cam is on the opposite side to a Holden.
