One cannot rely on one fixed static compression ratio for any fuel, there are too many variables.
Common logic would dictate that a fuel with an octane rating of 100 would work best in an engine with about 11:1, but here we go with race engines running 14:1, and they are making lots of power. A friend of mine is a mud racer (big engine, little truck, 4 wheel drive) running a 540 CID with 14:1, on alcohol. Another friend is a local drag racer and on 104 octane racing fuel, he chooses 13:1.
Most engines use a lower compression ratio for emission controls and engine longevity. My lawn mower engine uses 7:1 as an example, and it runs, and runs, and runs, longer than I would care to admit, with minimal maintenance.
When we work on the engine in a dyno cell and are setting the initial timing curve, we work on a best torque limited curve, before detonation. Increased compression will usually require a lower total advance and less agressive curve before detonation, at best torque.
Propane behaves like the other fuels, but as mentioned, since it is a dry fuel with practically no heat absorbtion or charge cooling. When we have detonation from compression, reducing ignition timing will have little effect.
On a valve-in-block engine, one can tolerate higher compression ratio than a modern valve in head engine, due to a less efficient combustion process. In this case, its the engine, not the fuel that is dicatated by compression.
Personally, on propane, in a modern engine since the early 1990's, 10:1 is a good starting place, balancing engine longevity and power. 11:1 would give a little more power but the fuel mixtures and timing will have to be monitored more closely, and 12:1 needs constant vigilance on the above.
Franz
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