In trying to make a buying decision for a new lawn mower I got stumped when I finally realized (it took me 2-3 days) that the rating system had changed.
How can you compare older mowers (before mid-2007), that are rated in HP, to the newer ones (mid-2007 onward), that are rated in ft-lbs of torque, or even worse only give you size in cc's? How do you make a decision on what to buy?
I have decided to summarize the pertinent information I gathered from my research in a very concise way so any consumer can have a starting point for comparison in a SIMPLE way.
First, if you have an "older mower" you need to know its effective or "real" HP based on its operating RPM.
Want to know the actual HP of your older mower?
Older mower is 6 HP. Multiply by 0.85 = 5.1 HP ("Real" HP)
0.85 = 3060/3600 (to convert from tested RPM to actual RPM)
Too hard to calculate in your head on the spot?
Try multiplying by 0.8, then add 0.3 to your answer. This is a close approximation.
Want to know the torque of your older mower?
First, make sure you know the "real" HP of your motor (as above).
Then use the "real" HP as follows:
Older mower is 5.1 HP. Divide by 0.6 (or multiply by 10 and divide by 6) = 8.5 ft-lb.
Want to know the HP of a newer mower?
Newer mower is 7 ft-lb torque. Multiply by 0.6 = 4.2 HP
Remember, this is only to give you simple way to start to be able to do a comparison but it is based on several ASSUMPTIONS. They are not important, just to keep it SIMPLE. Even the "real" HP is NOT the real HP. But that is too much detail to cover here.
If you can AVOID IT, don't bother trying to compare size or displacement of a motor shown in cc's for the simple example that even B & S has the same 190 cc motor that is sold as a 6.75 ft-lb torque model all the way to an 8.5 ft-lb torque model. Unfortunately, in many cases that is all the info that is supplied. Then your only fallback is the following:
Want to know what the HP is for a certain size engine? Use the old rule of thumb:
Engine is 190 cc's. Divide by 28 = 6.8 HP (this is the "rated" HP. Now multiply by 0.85 = 5.8 "real" HP. This is a VERY ROUGH estimate.
Too hard to calculate in your head? Try dividing by 30. (I think this is a slightly better estimate anyway.)
I hope this helps someone. It sure did for me when I had to buy a new one yesterday. (I registered on this forum just so I could share this.)
Want to know more about WHY the rating system changed? Go to these 3 links. Each contains slightly different info to give you a more complete picture:http://www.washingtonpost.com/wp-dyn/content/article/2008/02/15/AR2008021501732.htmlhttp://forums2.gardenweb.com/forums/load/tractor/msg0215391823783.html?26427http://www.inspectionnews.net/home_inspe...-vs-ft-lbs.html
Want to know more about the CALCULATIONS? Go to this link for a good explanation:http://www.revsearch.com/dynamometer/torque_vs_horsepower.html