Just wondering why, in these discussions, people always say the former and not the latter.
In: Engineering
No. Horsepower does not change when geared up or down. Or, at least, peak horsepower doesn’t.
This is because horsepower is more or less torque times speed, and horsepower is conserved while torque and speed can be exchanged.
Horsepower is a calculation which includes torque and rpm as inputs. If you gear up, torque goes down so HP doesn’t go up.
The torque of the engine does not change when gearing down, it is the output that is multiplied. The engine is producing the same amount of horsepower and torque, but by using gears you can multiply that force on an object (usually a rotating shaft). This is why we talk about engine torque and torque at the wheels when discussing about the power of trucks.
Simple way to understand this, you on a bike. You are the engine, by using a bigger rear cog when climbing a hill, you the “engine” are not magically producing more power and force, you produce the same, but that force is now multiplied at the output allowing to climb easily.
No. If you gear down you get more driving force at the rear wheel at the expense of top speed at engine’s rpm limit. If you gear up you get a higher top speed at the expense of driving force. Which is why you use a transmission in the first place.
Get on a bicycle with multiple gears. Put it in the highest gear. Try and accelerate rapidly. It will be very difficult, because the driving force at the rear wheel is reduced due to the gear ratio. Now put it in a low gear and you’ll find acceleration is much easier. Don’t shift up, and see how fast you can go in that low gear. You’ll spin out at a pretty low speed. Shift up to increase your top speed.
The horsepower of an engine is a function of its physical parameters – displacement, stroke length etc, plus operational parameters like friction, intake resistance etc. It doesn’t change based on which gear is selected.
Torque, on the other hand, is horsepower divided by RPM (times some coefficient). So if you keep the engine at the same horsepower but choose the gear that ends up with wheels rotating twice as slow, the torque will be doubled.
Power is constant.
Speed and torque can be exchanged for each other.
Think of it like two gears, a big one and a teeny tiny one. One can be the source and the other the output. One way you’re gearing down, spinning the little one to make the big one slowly turn a hand crushing torque. Or you’re spinning the big one to make the little one ZIP extremely fast into a blur…which you can stop with your pinky finger.
That’s gearing. Total power remains constant, torque and speed change depending on gearing.
I recommend reading The Way Things Work by David Macaulay. The first chapter will really hammer home how simple mechanics works. Enough to make my first year physics course almost unnecessary.
Torque and horsepower are not the same thing and the terms are not interchangeable. Horsepower is the product of torque and speed and remains constant (simplifying a bit). If you increase the torque, the speed drops and if you drop the torque, the speed increases. The product will remain constant.
I’ve always wonder this but does this mean when a 4wd vehicle goes into 4 low that it would output higher torque numbers ?
Horsepower measures how much “work” an engine can output.
Gearing ratio changes how that “work” manifests itself, e.g. rotation speed vs force (or torque).
Say you are capable of moving a ton across from one side of a football field to another in 30min that’s your horse power.
The gearing ratio is how you’ll do it, you can either drag it very slowly in one go or in few very heavy chunks, or you can pick up small parts and sprint across the field.
The end result is still 1 ton over 30min.
Horsepower is more a measurement of raw power regardless of what combination of speed and torque that is.
High torque can be geared to low torque higher speed.
High speed can be geared to low speed high torque.
Horsepower doesn’t change.
Latest Answers