Shimming: How do you know compression is right?

flyjsh wrote:HAZAAA!  Okay, I finally put down the balsa on my current build and took the PT-19 out for a spin.  Fiddled with shims and was able to go from mid 13s to high 14s/low 15s (according to my crappy phone app tach)!  Quite pleased with the first attempt.  Now I guess I need to buy a good tach.

As mentioned previously compression, fuel, and prop size affect timing. With lower compression you can vary prop size quite a bit and still run well. As you add compression (remove shims) you will need to play with prop sizes and nitro content to optimize.

Different plugs also affect timing. Bee type engines usually came with a dome shaped combustion chamber. For TD's combustion chambers were usually conical or fluted, often with a squish band to aid combustion at higher RPM while using higher nitro and usually smaller props.

If you are going for performance in addition to a tach, make sure you have a piston reset tool and learn how to use it. The more stress you impose on an engine, the more maintenance it will need.

Enjoy!

George