There are two different "compression ratios" when it comes to engines. I'll try to clarify the differences between the two most common ones.
The most common one everyone talks about is STATIC compression ratio, or "CR" as it is often abbreviated. It is a calculated number, measuring the volume swept by the piston from bottom to top dead center, plus the unswept area between the top of the piston at TDC and the cylinder head. The number means how much the original combined volume is reduced when the piston moves from bottom dead center to top dead center. It is NOT a measure of pressure. It is a RATIO, like 8.5:1 This value does not take into account ANY environment variables, air density, not even valve overlap or cam profile (compression bleeds off when a valve is open when the piston is traveling UP).
More accurate and useful, yet much harder to calculate, is DYNAMIC Compression ratio. It is basically STATIC compression ratio corrected with pressure bleed-off caused by valve timing. The end result is always lower than static CR. It is still not a unit of pressure, but a ratio just like static CR.
EXAMPLE: An engine is calculated to have 8:1 static compression ratio, and 7:1 dynamic compression ratio. Someone decides that 7:1 DYNAMIC CR is optimal for a good, efficient combustion, and should not be deviated from in either direction. Then they want more power, so they install a race cam that holds the valves open longer than the stock cam, in hopes of making tons and tons of power.. But the valves now bleed off some of the compression, way more than before. Static CR is still 8:1, but dynamic CR has dropped to 5:1 and the engine is a pig to drive, does not respond well to throttle, and has weak low end power.
To correct this, they need to compress the fuel more, and that is achieved by reducing the compressed volume. So they install domed pistons, machine the head so the combustion chambers get smaller, and mill the block so there is less "deck", unswept area above the piston at TDC. Now their STATIC CR is up to scary 10:1, but the dynamic CR is back at a safe 7:1 level and everything is hunky-dory and they have a happy engine.
ACTUAL compression is a pressure reading taken from the spark plug hole with a pressure gauge. It shows true pressure at VERY low rpms, only the starter turning the engine over. Every little leak will reduce this number, and the density of the air does have an effect, however minor in real life.
Clear as mud, huh?
