[OSC911]

Static compression ration means zero without knowing what camshaft you have and when the intake valve is closing.
See post below about similar question:

When people talk of compression ratio, they usually are talking about static compression but that is only half the story. The most important compression ratios metric is that dynamic compression ratio. You can have an engine that has 11.8:1 static compression that will run nicely on pump fuel with little or no pinging or detonation. Toyota Yamaha headed engines (engines with GE in the engine code e.g: 4AGE 20v) have been using these high compression engines for years. Obviously these are 4 and 5 valve engine which helps, but they have low dynamic compression. The camshafts are designed to close the inlet valve quite late - quite a ways up on the compression stroke past BDC. This allows for a lower dynamic compression - usually 8.0 to 8.5 is ok for pump fuel.

I know I should be comparing apples with apples but this is a factor that is often overlooked in most engines - not just a 911. I know that the piston/cylinder design on an air cooled 911 engine isn’t ideal with higher compression - hence twin plug heads. Static compression is fixed measurement when combining pistons, rod stroke, combustion chamber volume. Dynamic compression is a more accurate measurement when thinking about detonation and pinging. Calculate your dynamic compression also when determining fuel type and or twin plug head requirements and go from there with your decision.