I made my first variable compression glow head maybe 10 years ago when I wanted to find the ideal compression ratio for zero nitro fuel and used my Enya 60X as the test engine. The original idea was just to find the optimum compression then make a head with the ideal combustion chamber volume but it worked so well I just left it on the Enya and kept flying with it. This isn't such a bad thing because if I ever wanted to try using nitro, change props or experiment with different heat range glow plugs I could very quickly adjust the compression to find the best for any combination. With the Enya on zero nitro I was able to boost the HP by 20% compared to the standard compression.

Later on I made a variable head for my ST G51 in a CL stunt plane because I was having problems with far too much power when it broke into a 2 stroke.

With large engines like these the only problem is that the compression can only be altered by cutting the throttle back close to idle because at full throttle there's just too much combustion pressure to overcome using a Cox spanner similar to what Andy used with his Norvel. With a modified OS 21 I could adjust it at full throttle though.