Originally Posted by F100cleveland
Another thing to remember is Ford fitted some of the open chamber 4v heads with 2v sized valves. Must have been an emissions reason behind it or something.
I don't think that it was emissions. Rather, I suspect that it was probably that they performed better on street engines than the large valve heads. Also, the cost was probably a bit lower. In a company where a few ounces of metal is money to them, small valves equals less "iron" to produce/purchase. A few thousand units times 8 intake and 8 exhaust and suddenly you have a cost realization.
The emissions situation of the period may have been the genesis of the change, though. You can almost imagine the thinking that must have went on at Ford corporate engineering. Open chambers, retarded camshafts, unleaded fuel, heavy cars, power robbing transmissions and massive 4V ports on a "performance moniker" that was definitely sagging in the performance category...combined with the changes from the "old" brake horsepower figures to the SAE net figures. We went from 335 HP in 1971 to 210 in 1972. Even if the engines were identical, the marketing problem issues with a perceived 125 HP drop in "rated power" were significant.
The challenge was to improve seat-of-the-pants performance, which nearly always means low-end torque responsiveness. I'm not an expert on the small valve 4V heads, but it isn't hard to guess that they get better on the low end with the smaller 2V valves. I would hazard a guess that the massive ports don't even much affect the status quo (engine total performance) when equipped with the small valves, but that the marketing benefit of the higher performance aspect of the "4V" continued as a result. The fact that lower priced parts and larger volume purchasing (considering that most engines were 2V'd) further completed the "deal" to make it a small, but engineeringly-significant win-win for Ford and the public.
This all combines to suggest that a well-equipped 2V-headed 351 CI engine will perform better at "factory" RPMs than a 4V, especially in regard to low end torque and responsiveness. Of course, we already know that a good 2V will work better at lower RPM than a 4V Cleveland.