No i dont think you can hopefully this can be explained properly by people who understand it.Compression Ratio afaik is the relation between the amount of volume or 'squish area' when a piston is a TDC and the amount of volume there is once the piston is at BDC. So if your TDC 'squish' area is say 50cc and your BDC volume is 400cc the ratio is 8.0:1. Measuring the cylinder compression in PSI or BAR may well(possibly) show a low comp engine but as for working it out i,m not sure if its possible.I like stuff like this but only as an enthusiast who thought he,d have a wee go at answering lolpmsl. If its near standard YB then its most likely near standard compression which on a YB is 8.0:1 ish