Originally Posted by F-BIRD'88
Dynamic Compression is rooted in the fact that the intake valve closes after bottom dead center. Since your cylinder isn't sealed right away as the volume above the piston begins to decrease, you do not keep all of the air that is in the cylinder at BDC (Bottom Dead Center). What is compressed is what is left in the cylinder after the intake valve closes. This is why Dynamic Compression is always lower than Static Compression.
this statement is false and misleading.... When the engine is running at speed in its natureal best power rpm range the cylinder pressure is very high. Back pumping is little or non. On a tuned induction system engine.
( a engine with headers, cammed and developed induction ( a powerfull intake) the engine takes in more air , resulting in higher than 100% volumetric efficiently at WOT. cylinder pressure is high.
Back pumping before the inake valve closes only occurs at low rpm, when the engine is running out of its best power band. Not "on the cam".
Real running engine dynamic compression ( when its running "on the cam" @WOT) is not fixed and often and should be higher than 100% volumetrically efficient if you are any good at building and tuning engines.
induction ram effect-exhaust scavedging. A high performance tuned induction-exhaust system engine is not a simple air pump. Cylinder pressure is highest BMEP right around peak torque output rpm.
Air always flows in the direction high pressure toward low pressure relative. and it don't care which way the piston is moving in a cylinder.
In the running engine, running at speed you have a open intake valve, a rising piston and air still entering the engine.
inertia Ram effect.
Thats not possible without some compression. The air keeps moving into the cylinder from the intake port until one of two things happen. 1. the valve closes. 2. the air pressure in the cylinder is equal or higher than the air pressure just under/at the exit of the still open intake valve.
I stand corrected about bleeding air back into the intake. Other than that, nobody is arguing against you with regards to the rest of your post.