Im not talking the kind you measure cranking your motor off with a compression gauge. What effects static compression, Actualy compression of you motor, timing and forcing more air in. 4 lbs of boost in a 10 to one motor makes it basicaly a 11.5 to one motor and timing has to be retarted or better gas has to be used.
If you are talking about dynamic compression ratio, the ratio whose number can result due gasses entering or leaving (also by reversing through intake valve) during the compressin phase , it is always lower , by a large amount , than the static compression ratio which is determined simply by volume in cylincer during compression stroke, affected again by cylinder head chamber and piston shape.
Conservative timing always has a higher dynamic ratio than radical timing.
Cylinder pressure determines dynamic ration; there is an algebraic formuls to determing dynamic ratio.
Timing determines, or stops , gasses being pushed back out of the intake valve, which is why port tuning and scavenging are so important to keep gas volume in the cylinder and not lost.
It is , far more mathemtical/physics involved than this, when actually cylinder head shape and engine timing is concerned but it is still just matter of physics and chemistry which is why fuel, down to actuall heat value of the brand of racing gasoline used can make a huge difference.
In the Engine Master series, engine builders (they were the best of the best) had engines running on 91 octane street fuel with 13:1 compression ratios on carburettors, their knowledge of valve gear material, cylinder head design, dynamic ratios and timing, made it possible to do so without ruining the engine.
True Ford and Chevrolet high performance big block street engines, starting in the mid-sixties were safe to 6,500 rpm and could be over-revved to 7,000 (over revved not held there) without problems, but it was not til lthe 80's that valve gear material became common and they went from steady state 8,000-8,500 in comp. engines to over 10,000 by the turnof the century.
But by that time they were running up to 17:1 static comp. rations in some Trans-Am road racing engines, till the fact that only the teams with money not being a problem could afford this tech. so they were dropped by 4 to 5 points by racing rules.
Going back fifty years in many areas high octane street fuel was not found , easily, and yes if you were going to race, even stop light to stop light, not blue printing ( which cost money but less than a ruined engine ) and was simply making sure your engine waas operating within fatory design specs, not buy special parts, was a problem waiting to happen.
It cost a lot of dudes more money to fix what they broke, than having a competent engine builder take it apart and make sure all was set to best specs.
(Although, and I knew some dudes quite well, they would take the car/engine to the "highly reccomended" engine builder, and he would say , what you really should do is change this , that or the other thing, and regret it greatly later.)
When I had my Boss Mustang, even with a compartively low 10:6 compressin ration , when they stopped selling premium leaded gasoline up here in the late seventies, I started using (buying) various fuels for the engines sake. I used octane booster, power booster (which is exactly what it said it is (it did not contain alcohol) and was illegal in many sactioned racing classes, to finally settling on aviation gasoline mixed with regular alcohol free gasoline.
