Take the compression of both cylinders add them and devide by 2, multiply that by the motors max rpm and multiply that by the actual cc's, and then devide that by 6400000 and you should have your approximate HP (very close)
It works on singles/Twins/and tripples if done correctly - with a single just use the compression - with a twin and a tripple use the average compression. I got this formula from one of my snomobile magazines...... When I use it on my 91 Formula 470, I get 85.2 hp before adding the UFO's into it - so somewhere between 85 and 91hp.
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