well..they wouldnt work if they was closed chamberd heads i dont think....no room for them....the reason they pop up is too make up for the space missing between the head and the piston...if it had closed chamber heads it wouldnt even turn over...it would ding all the valves or ding the spark plugs...or just bottom out and not start or move at all untill the heads were unbolted...would be like sticking a peice of metal in there...pop up pistons are great for boosting compression levels in open chamber heads if you really want to run high octang gas.
[edit] M-codeThe 351C 4V engines produced in 1970 and 1971 used this code. Engines varied in compression ratio; 1970 engines were 11.0:1 compression and produced 300 bhp (224 kW; 304 PS) at 5400 rpm, while 1971 versions had a slightly lower compression ratio of 10.7:1, and a reduced power output of 285 bhp (213 kW; 289 PS) at 5400 rpm. Ford owner's manuals for these engines recommended high octane gasoline (100+ octane in 1970) which was at the high end of the leaded gasoline available at the time. However, with the mid-1970s introduction of unleaded gasoline and lower octane ratings, and subsequent disappearance of the super high octane leaded fuels required to power these high compression engines, motorists were either unaware of potential damage or simply unable to find this kind of fuel any more. As a consequence, many of these otherwise durable engines met with an early demise due to the destructive effects of severe engine knocking caused by using low octane fuel.