engine compression?
 

how high can i go with single plug ign/with carbs 911sc 3.0 motor

9.5:1, but if you have it apart, why not dual plug?

Race gas is an option but pricey. Dual plug is a better idea but carbs? ITBs and programmable management system with knock sensors.....I like spending OPM.

Even on 9.5:1 you are at risk if say you get bad gas . Best is 9.3:1 stock piston for USA. You'll get more from a nice set of headers and chip,,or carbs or ITBs

It actually depends on the dynamiic ratio rather than static. Camshafts with increased duration of overlap will result in lower dymamic compression ratios than say a CIS design camshaft. Talk to a knowledgeable engine builder for proven combinations.

engine builder
 

i have asked henry at supertec if he would have a suggested build sheet for 3.0sc motor such as which cams,pistons, on another note tho i just pulled my cylinders off today and discovered mahle are these common for 79sc motor, splitting cases on tuesday the motor is locked up solid must be something in the case,to bad the motor was solid no oil leaks it just stopped at a traffic light. ive been picking away at strippping it down about 2 weekends so far

My euro '83 SC 930-10 is 9.8:1 and we only get 91 Octane in Az. I do run ACES IV booster but have had no issues in up to 110 degree ambient heat. Runs cool.
Had no problems with the 9.3:1 930-16 US spec '83 for 2 years in the same environment.

I think it is better to be on the safe side. twin plug safe fix to 10.5:1.