Power Equipment Stuff
I have an obsession with outdoor power equipment…have since I was a kid. Things like pressure washers, mowers, tractors, snow throwers, etc…yep, I’ve got ‘em all. Admittedly, I’ve pretty much been a Briggs guy. The parts are abundant and avaiable, they’re bullet-proof motors (if maintained properly), and they’re priced well.
Briggs kind of ticked me off in 2007, though, when they changed their (5 HP and less) engine power ratings from HP to torque. For many years, HP has been the commonly understood metric folks use to gauge engine power output. It’s like Miles Per Hour: it would confuse folks if car manufacturers all of a sudden changed the speedometers to Feet Per Second. We just don’t quite have a feel for FPS as we do with MPH. Fortunately, I can relate/gauge engine torque with power output. Even still, it would have been cool if Briggs stated both HP and torque in their product specs/marketing, similar to vehicle engines.
Anyway, I happen to have a 2007 walk-behind lawn mower with Briggs’ “new” torque rating: 6.75 (measured in pound-feet). I happen to know that most Briggs small engines run anywhere between 3200 and 3600 RPM. I’ll assume mine’s running at 3200 RPM…So with a bit of math, I can calculate that my mower outputs a tad more than 4 HP:
HP = Torque x RPM / 5252 = HP
Or…
6.75 x 3200 / 5252 = 4.1 HP
Where does the ‘5252′ constant come from, you ask? In a nutshell, it’s a series of conversion factors relating to work, force, and time - all bundled in to a convenient number. It’s all well-explained here.
Alternatively, if you’ve eve wondered how much torque your old 3.5 HP Snapper mower engine produces, the formula is:
HP x 5252 / RPM = Torque (in lb-ft)
So…
3.5 x 5252 / 3600 = 5.1 lb-ft