I was reading the recent thread on torque and horsepower, and it brought me to this question.
I understand the concepts of torque and horsepower. I think
But what is horsepower a useful measure of? Why would I be interested?
I suppose my thinking is this: by gearing an engine, you can produce whatever revs you want at the final drive. So who cares about horsepower? Why isn’t the only relevant factor peak torque, since horsepower is just a factor of that and revs, and if you want higher or lower revs at the final drive you can always use gearing.
I think you got it backwards. Torque can be changed by gears. If you use a 1/2 reduction gear, you decrease RPM by half and double the torque. But horsepower remains the same, because horsepower is RPM multiplied by torque. So horsepower is a more meaningful measure of engine performance.
I’m not sure you have an understanding or force and power. Torque is a unit of force only, something which can cause or prevent motion. Power is ability to do work divided by time. Maybe some examples will illustrate.
If you hold up a barbell you are exerting a force equal to the weight of the barbell but you aren’t doing any work in a physics sense. That’s the same as saying an engine has torque equal to a force of 220 pounds at the end of a one foot lever.
Work is when something actually moves. If you do a lift you’ve exerted force over a distance. Lift twice as much weight or two two reps and you’ve done twice as much work.
Power is work divided by time. Let’s dump the barbells and get some useful work done. Two people have a case of cling peaches to pick up from the floor and stack on a shelf. One is your frail grandma and the other is a certain Austrian muscle man. Mr. Muscles will be able to put the case on the shelf in one motion while grandma will have to open the box and put the cans on the shelf one at time. They both did the same amount of work but grandma took more time because she has less power in her thin arms.
When you know those properties about an engine you know what might be the best gear ratios for a given application. A big fat low RPM torque value is good for accelerating from a dead stop with a heavy load. Think truck with a big block engine and a three speed tranny. A sports car might have less low end torque in favor of a high peak horsepower in a narrow band. In order to make that power useful the sports car may need a six speed transmission so RPMs can be kept in the band of maximum power.
Actually I posted this question just before going to bed, and then as I lay there trying to go to sleep I suddenly realised the obvious point that I had been managing to overlook, namely the effect on torque of gearing. :smack:
Thanks for your answers. This thread is an embarrassment.
