It's actually somewhat simple. Static compression ratio is a comparison of the volume of a cylinder from top dead center to bottom dead center. Say the volume of one cylinder is 10 times as large with the piston at BDC than it is at TDC, the static compression ratio is 10:1.
Static compression ratio is a built in ratio and is unchangeable unless the engine is disassembled and parts are changed.
The dynamic compression (sometimes called running compression) takes the camshaft events into consideration. An engine is a simple air pump. It pumps air into and out of its cylinders to turn the crankshaft and make useable power.
It is how that air is pumped through the engine that can determine how much power and when that power is developed in the RPM curve of the engine. Things like how the engine will be used and what fuel it will be run on effect its ultimate outcome.
If it will be used to drive a street car and run on some kind of available pump gas, that must be taken into consideration. An engine with a fairly high static compression ratio can be tamed to run on pump gas with a camshaft that has the appropriate specs.
The later we close the intake valve for example, can result in cylinder pressure being bled off to suit the needs of pump gas available. An engine with say, 10.5:1 can run on pump gas if the dynamic compression ratio is around 8.1. Most people say 8.5 is the limit, and they are probably right, but I like to play it safe. Things like hot humid days and idling in traffic for long periods can get an engine to spark knock pretty quickly, so 8.1 is my limit for DCR (dynamic compression ratio).
Also too, cylinder head material has to be taken into consideration. Without trying to get into the iron VS aluminum argument, it is generally accepted that an engine with aluminum heads can stand more compression for the same fuel type than an engine with iron heads. For this discussion, I am leaving all the whys and wherefores at the doorstep.
There have been professional engine builders who've run engines with aluminum heads on pump gas with as much as almost 12:1 compression. By using a camshaft with a late intake valve closing, enough cylinder pressure can bleed off to make it possible.
Then you have things like quench that fit into the equation. All quench is, is getting the mixture squeezed into as tight a space as possible and causing turbulence. This turbulence allows for an even higher static and dynamic compression ratio to be had because the turbulence promotes a more complete burn, which has a cooling effect on combustion. More fuel is burned in combustion, instead of still burning going out the exhaust. .......at least that's my take on it. I am sure somebody will be along to argue their point shortly. lol
At any rate, simply put, static compression is that ratio that's built into the engine. In other words, "I have a 10.5:1 383 engine in my Dart."
Dynamic compression ratio (DCR) is that ratio that is effected by the camshaft, cylinder head type, material, quench or no quench and is somewhat "adjustable" if you will through the timing of the camshaft and the camshaft events.
As I said, take all of this with a grain of sand. I am sure someone will be along to tell me just how wrong I am. They always do. Whenever anyone asks a
"loaded" question like you have, I don't see all the critics brave enough to step up and try to explain it. At least I did try.
