Maybe this will do it. Force per unit area. The coin sinks into the plasticine further when pushed end-on because it has more pressure with less the area.
Get it now Turk or are you too dumb???? Get it now 273???
Say the combustion is 1000 psi, you got a 1000 psi pressing against all the surfaces inside the chamber which includes the total top of the piston aka piston area, so each square inch of the piston is getting a 1000 psi.