JB- power is limited by the amount of air you can get in. Air is limited by the port/valve flow and the cam duration. The cam duration is limited by drivability/ practical rpm needs, as well as diminishing returns as friction as parasitic/intertial losses mount exponentionally with RPM. This isn't just me, but engineers working with heads like those at Edelbrock, AFR or Canfield, as well as consultants like David Vizard and Superflow Flowbench engineers. Superflow say you need an intake flow of .27 to .28 cfm/hp/cyl on race gas even given unlimited compression ratio and cam duration. I figure a rule of thumb setting the cylinders at a constant 4 is hp=intake flow, everything perfect, on pump gas, and have posted that and used it for a while as an absolute limit. This would mean .25 cfm/hp. I am now suggesting that it may be better to work backwards from exhaust flow, as many OE castings are limited by exhaust flow, or exhaust residuals. In other words, if the cycle was starting fresh everytime, your pumping losses would be less, and your mixture to burn pure enough, that you'd make more power than you do having to deal with the 'problems' left behind by an exhaust volume that is unable to clear itself. Anyway, I'm now using exhaust flow divided by .79 equals hp (for a 4cyl). I actually was posting an IM to QwkSwede on here earlier tonight and my figures matched jr-55's fairly closely. Mine were a little more pessimistic.
BTW, Kenny's Burn Vs. Quench Vs. Sqirl thread from last year is a recommended read from the archives. The 2 cent summary: Often getting a good burn outweighs the need for flow.
The flow figures above are of a 405 I had tested to back to back compare my modifications to stock. ... and not to brag, but I had the bench testing done by Neil Fedderly. Neil's brother Bernie (might be cousin?) is the crew chief for John Force.
I've examined 405 and 398 heads and cannot see an exhaust port difference, even when Matt has explained via email where he is seeing it.
I tend to think the 405's large intake shortside radius is all important.
I don't think you can match the big port head with a small port head, just based on that alone.
I do, however, look at the 405 as a compromised design, wherein the design brief probably included a need for it to match the intake manifold and gasket design already set into production for the 160. Especially at the entrance, it looks as if it's blended to round where it would have been ovoid/D shaped given a free rein.
