I have about 7 cox engines left, 10 norvels, and 2 AP engines. Cox engines run well at full throttle but there is no cheap effective throttle or muffling system. The AP engines (.09 & .15) are comparatively heavy but have a convient muffler if it can be worked into the airplane design. They throttle well and have reasonable power.
I buy norvels because they are light, quiet, fuel efficent, inexpensive, durable, reliable, powerful, and fit in almost no room whatsoever. Their mufflers are lacking in the way they seal. Some people tap the crankcase to get reliable tank pressure.
In my opinion each engine has its use. I find that Norvels cover more of my needs.
