I have a 1947 Oliver 88 that I pull with. I run propane . The engine is a stock 231ci with 6.75 to 1 compression. I don't want to change it to a 265 ci . I would like to increase horse power with a little more compression .  I need MORE POWER ,can anyone come up with ideas that doesn't require a sleeve and piston change .   Thoughts on milling the head ,running a richer load of propane ,ignition timming are what I am thinking about . Anyone have ideas -----Thanks  MIKE