So last year I got a school bus motor that is a p pump motor. I striped down mo blowed up ve motor to use the head that I had just bought. When I look at the pistons I. Both blocks I noticed there was a large difference in the two. Ve mother had the normal bowl. But the p pump motor had a bowl but had a “mushroom” shape in side the bowl. I wish I had pics but I don’t. I have already reassembled the motor and it runs fine. Just curious as to why they are different. I assume the p pump motor pistons would make more compression because the “mushroom “ takes up a lot of space. Maybe I’m wrong. Anyone have an idea

 

The earlier NON I/C'ed pistons are different than the '91.5-'93 motors, which I believe are the same style as the P-pump ones.

The NON I/C'ed injectors move more fuel and have a slightly different spray angle as well. This spray angle and the piston bowl work together.