Reasons why for new cc figures.
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Manufacturers have always given max. HP ratings, just like auto makers. But mower engines have a 'speed limit' of about 3200 RPM for safety reasons. So, an engine that produces 6.5 HP at 3600 RPM will produce less at the mandated lower speed. Duh. As long as everyone uses the same rating method, a 7.0 HP rating would always mean a more powerful engine than a 6.0 HP rating. So after a bunch of whiney-cats sued them, they now have to give torque ratings (usable work) instead of HP ratings.
The 'cc' number is cubic centimeters of displacement, or the 'size' of the engine. All things being equal, a larger engine makes more power. Currently, mower engines have grown to about 190 to 195 cc as a result of the horsepower war brought about by the trend toward mulching and self-propelled mowers.
So, if you need a very powerful snow blower, go by the rating and displacement, although the two do not necessarily go hand in hand.