ORIGINAL: marktur
Terry, here's the specs:
OS 55AX:
Practical RPM: 2,000-17,000
Power Output: 1.75 ps/16,000 RPM
Weight: 14.25oz (404g)
Recommended Props: 12x7-8, 13x6-7
Tower 75:
Practical RPM Range: 2,000 - 16,500 RPM
Weight: 652g (23.0 oz) (with Muffler)
Horsepower: 2.2 BHP at 15,600 RPM
Recommended Props: Break In: 12x6 or 13x6
Another option is the ASP52. My Bobcat with an ASP52 flies at around 100mph and is noticeably faster than another I've seen flying with an OS50 so I figure the ASP can't be much less powerful than the 55AX.
The big bonus of the ASP is the price -- around US$60.
This engine has been very reliable and runs like a swiss watch. A great buy and with a 10x7 or 10x8 prop it's a perfect match for the Bobcat.