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Re: Tractor HP vs ?
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Posted by Steve - IN on January 05, 2003 at 03:35:08 from (12.222.17.160):
In Reply to: Tractor HP vs ? posted by spud head on January 04, 2003 at 23:15:49:
spud head, Current hot rod snowmobile engines produce about 1 horsepower per every 4 to 5 cc's of engine size. That puts 180 hp within easy reach. The snowmobile has to turn 5000 to 8000 RPM to make that number. Your tractor probably turns around 2000 RPM for the same HP number. Besides the longevity / breakdown factor from higher revving engines - there's another difference, that's torque. The basic equation is this: (Torque x Engine speed)/5,252 = Horsepower. So let's say the snowmobile at 6000 RPM actually makes 171 hp. That equals 150 ft lbs. of torque. Now your tractor: At 2000 RPM and 180 hp it's making in the neighborhood of 470 ft lb.'s of torque -- or about 210 per cent more torque than the snowmobile. Quite a difference. Two different design goals. #1: You wanna live fast, die young, and go out in a blaze of glory, or #2: do you wanna pull big loads, go slow, and live long? #1 = snowmobile. #2=tractor. Two different roads to get to the same destination in terms of horsepower. The math involved isn't too bad. 1HP = 550 ft. pounds of work per second (James Watt decided, 150 years ago, that because he observed ponies in Scotland could do 22,000 ft lbs of work / min. (220 pounds of coal lifted 100 feet in 1 minute by rope and pulley) a horse surely must be able to do exactly 50 per cent more (his guess), or 33,000 ft lbs / min. or 550 ft. lbs / a second. -- and from that wild a$$ guess, we get the definition of one horsepower). So it's the ability to lift 550 pounds 1 foot in 1 second OR some equivalent thereof with time, weight, and distance all being variables. In Watt's terms 1 horsepower can be a lighter load faster, or a heavier load slower over the same distance. Like Wayne said, the difference between draft horses and race horses (1 pound of coal lifted 33,000 feet in 1 minute is still 1 horsepower and is defined as the equivalent to lifting 33,000 pounds of coal 1 foot in 1 minute). In the HP formula above, time is no problem to figure out in RPM, but flywheels/speed vary, so we use radians (length of an arc / length of a radius) to get a constant number to deal with. There are 2Pi radians in a revolution. Sixty seconds in a minute so 2Pi/60 = 0.10472 radians per second. The 550 ft. lb. /sec. definition number over 0.10472 = 5252 (550/0.10472), which is where the magic approximation number in the horsepower formula above comes from. In a more practical way, when you dyno'ed your Case, you saw the dyno was actually measuring torque - the ability to turn a shaft - and you got a calculated a value for horsepower based on engine speed and the magic constant 5252 number. So HP is the end result of a calculation -- and simple multiplication tells you there are different ways to reach the same number, by varying torque and engine speed (and by the way, I haven't checked all my math above, but I think you get the general gist of it). Steve
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