ORIGINAL: GPitts
Howcome Tower claims this engine to have 2.2hp at 15600rpm? Also why do engine manufacturers rate their engines rpm/hp these much but when you actully tach them the rpm is much lower. If what tower claims or other manufacturers are true, then how do these rpm ratings get so high?....how small is the prop they are using?....plus with a prop small enough to turn at 15600rpm how is that reaching 2.2hp? Are these ratings actually true or just something they use to decieve people into buying their product?
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The manufacturer probably gets those rpm figures by running the engine on a dynomometer with no muffler in place. It is a legitimate means of obtaining a useful measurement
of power. However, the figures can be misleading for every day, practical, real world, engine operation.
Most likely, Tower is repeating the specs that their supplier gave them. Who else would be better qualified to provide such figures?
I run 12x6 props on .50 size four-strokes. It stands to reason that a .75 glow two-stroke should be able to spin substantially larger props for better efficiency. Don't get upset if your engine can't swing a practical size prop at the advertised peak horsepower claimed by the manufacturer. Your car can't produce all of its horsepower while mounted in a car that is being driven on the road either.
A 13x6 is a good all around size for this engine, regardless of tuning. Milder tuning does permit one to run larger props without a problem, assuming one keeps preignition under control by varying nitro percentage in the fuel, the type of glow plug being used and the amount of compression the engine has. The latter can be adjusted by increasing or decreasing the number of head shims (old term - gasket) that are used between the head and the cylinder.
Don't worry much about the horsepower claims. Marketing does its best to promote the product. Chances are they are technically correct, but you will probably never see such horsepower being generated while using a practically sized propeller. Go fly and enjoy.
Ed Cregger