ScottY (4/10/2011)
......I would assume too though, that all these headers had the square port flanges still wouldn't they, and if so, surely none would have been large enuff to uncover/restrict, the ports?The fenderwell headers were the only set of headers of the three sets used in the round port head test that actually had round ports at the header flanges. The round ported heads themselves were actually ported back in their day to match up to this particular set of headers. The EMC and Sanderson headers have rectangular ports that were actually smaller than the ports at the heads and as a result, both sets had some flow obstructions taking place as a result of the mismatch at the heads. But to look at the torque graph, the EMC headers outshined the other two sets of headers in the lower rpm band regardless of the port mismatch.
But as usual, there’s much more happening here than just a head to header flange port mismatch. The large dip in torque seen with the fenderwell headers has since been attributed to the collector size on that particular set of headers as being way too large. Had this been recognized at the time of the round port head test, the fenderwell headers would have performed much better with downsized collectors.
Since doing that particular cylinder head test, the fenderwell headers have been tested again with a different set of heads on the engine but also with collector extensions that brought the collector sizes down from 4” to 3”. With that change, much of the pronounced dip that was evident with the 4” collectors was removed from the lower rpm band of the curve. Here’s the torque graph where the round ported fenderwell headers were tested on the mildly ported G heads that have rectangular exhaust ports.

Lorena, Texas (South of Waco)