The stock SOHC ports all seem to have that stall, and I think it is just the basic port design. Factory SOHC cams had lift just shy of 0.600", and of course the intake had to fit under the hood of a Galaxie for NASCAR purposes, so the ports have more of a curve in them than would be ideal. The port designs I've worked with raise that intake port significantly, and that helps a lot with flow at the higher lift.
The Ferrea intake valves are a completely flat valve on the back side, with no taper or tulip shape at all, whereas the Manley valves that I've used have either a 7 degree or 10 degree taper or tulip on the back side. I think this is the primary difference between the valves, and the reason for the marginal flow of the Ferreas.
Back at the end of 2008 when I was getting serious about the port work on the SOHC I got the following intake flow numbers from a raised port head with the 2.300" Manley valve:
0.100" - 83
0.200" - 176
0.300" - 280
0.400" - 361
0.500" - 412
0.600" - 434
0.700" - 443
0.800" - 453
This was on a SuperFlow SF-600 bench and using a 4.500" bore size, which happened to be the bore size I was using on my 585" SOHC engine. The engine ended up making 960 HP at 6700 RPM, using a Hilborn mechanical fuel injection intake that had been converted to EFI, and raised on the heads with 3/8" port plates so that the ports would line up. The engine was handicapped somewhat with the 2-7/16" throttle plates in the intake. Port cross section was about 3.5 square inches (round 2-1/8" diameter port, which is just a hair over the stock size of 2.060"). This engine ran 9.50s and a best of 9.48@143 in my 64 Galaxie, which weighed 3935 with me in it, during Drag Week 2009.
Since that time of course I've made other improvements to the heads, and they flow a little more now than they did before. I'm hoping to incorporate those changes into the Australian heads that I'm working on, to make a really high performance SOHC head available.
