Propane is 110 octane. You can use any compression ratio up to 12.5:1 if you choose, but realistically, about 11:1 works best. Higher ratios just work the starter to death, and don't really gain much more performance. A couple of modificaztions are necessary to make things work right. One is installing hardened valve seats, at least on the exhaust valves. If the engine was not set up for unleaded fuel originally, it probably won't have hard seats to begin with. While the engine is down for the compression ratio change would be a good time to go with stainless steel valves and hardened valve seats. No, you can't get that much compression increase by milling the head. A good set of high-compression pistons will be necessary.
You will also need a heat exchanger to vaporize the Propane and feed it to the carburetor. Propane vapor displaces a certain amount of the air the engine would breathe otherwise, so if no other modifications wre done, such as the compression increase, just converting an engine to propane results in a 10% power loss due to impaired breathing. Raising the compression regains this loss, and then some. The downside to raising the compression is that even premium gasoline will detonate if you have to use it in an emergency, like running out of propane a long way from a refueling facility.
Jerry
