It dont matter if its 7-1,8-1,9-1 If a engine is 7-1 than when it reaches the 7-1 compression it is at full compression pressher lets say 170psi.Now another engine that is desined to compress at 8-1 at full compression that engine is at a maximun of 170psi (if both engines are in spec/new)Yes 7-1 compression is less than 8-1.What compression pressher is,is the (amount of force that pushes against the cyl.walls,piston rings,bottom of head)wich no matter what the compression is,in the engine class its desinged for. The max is the 170psi for optimal operating range set by the manufacture.Now as a engine wears out over years of use,each cyl wears at differant rates so each cyl will have differant readings,that should not drop lower than 128psi what this means is much lower than this and the volume(air/fuel) would not hold and one would be left with a barly running engine as the compression would be lost .Now lets say on a engine that is 170psi if you put 3xs the uel into the cyl the volume would rase the pressher to a point that the engine would not turn over or the engine would rip it self apart (pop its top)Also com.presser is, wether its 7-1/8-1/9-1/when the compression is reached the pitons rings/valves will hold 170psi max in that cyl this is a safe guard as pressher raised the piston rings would fail at what point i dont know hope this helps its still not exact to what i want to say