It seems that the manufacturers of lawnmowers are posting that the rated HP will be significantly less because the engine must meet environmental, safety and operating requirements.
I was curious about this and started looking at the displacement of some of the new motors and compare it to my old 3.5HP B&S motor. For instance, a 6.5HP Tec motor has a total of 195cc's and my old Briggs has a total of 148cc. So I can say that the 6.5HP motor definitely has a larger displacement than the 3.5HP.
I got the manual out for my 15 year old B&S and in the manual there is a disclaimer as well about the total HP the engine will produce. The manual states that even though it is a 3.5HP motor, it does not actually produce that much. Basically the statement says that the motor is rated for 3.5HP at 3600 RPM but because of safety, the RPM's are HP are less for the engine. Further to this they actually state that to improve startablility and performance they have made changes to the motor so that it will not produce 3.5HP at 3600 RPM.
So it looks like the motor manufacturers have been doing this for many years but recently the lawnmower manufacturers are actually stating this on there web pages. I wish they would post what the actual torque and HP is for the motor instead of meaningless numbers.
