combustion chamber design
Engineers changed the shape of the cylinder heads to try to meet the emission stds starting in the 70's. In the older 60's engines the combustion chamber was a smaller somewhat D shaped area and there was a thin area outside the D between the head and the top of the piston. the thinking was that this area cooled and that the fuel did not burn in this area . some pistons were made dished and some combustion champers changed to be O shaped to meet the new smog regulations. A smaller D combustion shape will have more turbulance which help keep the fuel and air mixed,, smaller combustion chambers will have a faster burn. producing more pressure closer to the top of the stroke. It's gets complicated trying to get more power and economy and burn cleaner.