I'm not convinced that the biggest valve that will fit is always the best, Jay. But, I'm not a cylinder head porter, and I do believe flow numbers are meaningful, especially if tested on a bore tube that matches the cylinder bore. And like you said there are other considerations like shrouding, notching, and leaving room for an effective exhaust valve.
In general, it seems like the trend is towards using the available space for a larger intake valve and a smaller exhaust valve. At least compared to 20 years ago.
paulie
Paulie, I didn't mean to suggest that the biggest valve that you can make fit is always the best approach. What I was trying to say was the biggest valve that results in a meaningful flow increase over the next smaller size is the best way to go. This is why I buy single valves of different sizes and have my local shop flow test them on test ports before I make a decision on the valve size. For what its worth, I have not seen one case where the bigger valve did not provide a meaningful increase in flow. But, bear in mind that I always try to run the maximum bore size on my max effort engines, like the high riser (4.375" bore) and the SOHC (4.500" bore). I doubt that a 2.300" intake valve would work well on a 390 bore size.
One other comment is that I believe you are correct regarding the trend towards bigger intakes and smaller exhaust valves. On my SOHC, I'm leaving the 1.900" exhaust valve in place while increasing the size of the intake on my new heads. My tests over the summer with header primary tube sizes really convinced me that smaller is better there, so sticking with a smaller exhaust valve and increasing the size of the intake to maximize flow makes a lot of sense to me. I still can't believe that my SOHC made nearly 1000 HP with 2" OD primary tubes on the headers.
I'm with ya on the big valve thing, Jay. I know you have some big bore motors. I would too, if I could swing it.
That's interesting about the exhaust sizing, both valves and header primaries. It's not that the exhaust is not important, but I think in general it's less critical to have it optimized than the intake side. 385 series Fords have horrible factory exhaust ports, but very capable intake ports, and they easily run with BBC's despite the BBC's much better exhaust ports. I guess I feel like if your exhaust port/valve/header is off of optimum by say, 20%, it only affects the power by 5%. But if your intake port/valve/manifold is off of optimum by 20% it affects the power by 20%. I'm making up numbers and speaking in generalizations, but that's kind of how I feel.
I wonder if testing exhaust ports at a different vacuum than the intake ports would be beneficial? The exhaust port is helped by all that combustion pressure so maybe testing it at 28" is not the best way. Maybe testing exhaust ports at a higher vacuum would give a clearer picture of it's capabilities. Heck, I believe even intake ports see a much greater maximum pressure differential than 28".
JMO,
paulie
