torque and horsepower hold hands, and they have a threesome with rpm. unless you compare engines at the same rpm the numbers are nothing more than numbers. the higher torque at lower rpm's will be lost in gearing
This is correct. The HP vs Torque discussion comes up on tractor sites periodically on what’s more important. The statements made above are mostly correct, hp is hp, is true, hp is a measure of how fast an engine can do work, doesn’t matter if it’s an high rpm - low torque engine or a low rpm high torque engine, once’s geared to 540 rpm they are exactly equivalent. So way do people like high torque tractors? What people really care about a lot is the shape of the torque curve more so than than the actual peak torque, or peak power.
I wrote the following years ago, and saved it, and just paste in these threads periodically when it seems appropriate:
I'll put my two cents in on the hp vs torque discussion.
The physics definition of Power a measure of how much work can be done per unit of time. Work is defined to be the amount of force applied in over a distance. In linear forces, Then power = force X distance / time. In rotational forces power = torque X revolutions / time or equivalently, power = torque X rotational speed. In terms of horse power:
hp = torque X rpms X C, where C is some constant that takes unit conversions into account.
This means that at a given rpm, hp and torque, are basically equivalent because if you know one, the other is known. PTO power ratings are given at pto speed of 540 or 1000 rpms, so doesn't matter if they give you torque or power you can compute the other.
This means a tractor with a high rpm low torque motor with the same power of a low rpm high torque motor can be geared down so that they both produce the same rpm, torque and power at the pto and do the same amount of work at the same speed.
But every talks about high torque tractor engines and that they are better and that it you don't need the high power rating as much as high torque. So what's going on? Well what also matters a lot in how well a tractor works in practice is the shape of the torque vs rpm curve. High torque motors tend to have the peak torque at lower rpms and the torque cure goes down at a slope from there. When multiplied by rpms this results in a flat power curve for a good range of rpms. This is the desired characteristic in a tractor and I'll explain why below. Lower torque motors tend to have there peak torque at higher rpms and then drops off quick after the peak. But because peak is closer to the max rpm range there is less of a flat spot at the top of the power curve.
Why does this matter?
Well when your doing a job such as running a chipper or cutting grass, the power demand or required power to do the job is not constant. When your cutting grass you go up hill where the grass is higher the power demand on the motor goes up. If the power demand goes up higher than the available power output at the rpm level you set the throttle, the motor bogs down and begins to slow down. So what happens next depends a lot on your motor’s power curve. If your have a high torque motor with a flat power curve, when the rpms drop, the tractor slows down dropping the power demand down to the available power level. The available power stays the same but the demand went down so you keep going just a little slower. But, if you have the type of motor were the power rolls off more quickly when the rpms drop and the tractor slows down, the available power also goes down also. So there's a race to the bottom and there is more of a tendency for the tractor to stall. This is why torquey tractors "feel" like they have more power, they don't feel like they going to stall if you don't let up on the hst peddle when they get loaded or your slow to open the throttle.
I think this is also why gas tractors tend to need higher power motors, because the power curve is not flat and people tend not to like to have to run them WOT all the time.
But at WOT a higher power motor low torque motor can do more work pull harder than a high torque but low power motor no matter what the engine rpm and torque combination because after the high rpm motor is geared down it is generating more torque at the standard pto speed.
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