Compression Ratio=Cranking pressure. If you are 150 or lower pump fuel is fine. 160 on up you need to be careful, time for higher octane to combat DET. This is also timing based, and this part works both ways. If on the border, high compression can be offset with retarded timing, and visa/versa. 20cc may just be fine-depends on these other variables. If you think you are close keep the timing stock (don't advance it). Keep in mind 20cc is the border and can go either way for you at your elevation. The above suggestions are awesome when you are not sure- "Better safe than sorry". If you can't get 21's, then get 22's and then advance it like 4 degrees-will still run just fine for you.
