torque and horsepower

[Ripper]

i'm not sure what section this would belong to but i need to set a few things straight. answers would be appreciated.

well i was debating with my old man about torque and horsepower. first of all, i know that torque = newtons/meter. torque is a kinda of force. another thing that i know is that when you want torque you need to sacrifice horsepower. so here's the thing: when cars go up a hill do you upshift or downshift. i know you down shift to accelerate more cuz the peak horsepower is made by a higher RPM range. but he tells me that you downshift for more torque, then you upshift to gain speed. he said that you downshift becuase the gear ratio is higher.

again, any inputs would be greatly appreciated.

[LexmarkX73]

hehe



actually im not sure exactly what your questin is.

http://www.elec-toolbox.com/Formulas/Motor/mtrform.htm

that should give you a wealth of formulas and relationships between car forces.

[CrazyHawaiian]

Each ratio of the different gears in your transmission combined with your rearend differential ratio will determine how your motors power is applied to the ground. The lower transmission gears (which have higher ratio's) are designed for lower speed, higher load situations like going up a steep hill or building speed from a stop. The higher gears (with lower ratio's) are designed for higher speed, lower load situations like freeway driving. Because torque is associated with "low end" or "high load", this is why people say stuff like downshift for more torque. As far as the motor goes, the correct gear is needed to maintain the powerband (RPM) in relation to the speed and load. On a hill this would be a downshift. So you're both right.

[Ripper]

hey hawaii, thanx for the response. i think Grid covered this once about how torque and horsepower are correspondent. anyway, lex, i did search this but his post never came up, so Grid if you don't mind explaining to me one more time. plz do so, thanx.
to rephrase: what really happens when you downshift. (lo-drive). torque is crucial when you want to acceleration (after downshifting). and taht's where HP kicks in. sorry if this sounds kinda confusing.

[Drift For Food]

It sounds like your mindset is a bit off on this torque/hp, upshift/downshift thing.

Ok, torque and hp is directly related to each other:

Horse Power = Torque x RPM

Torque is basically the static power the motor has at any given rpm. Basically think of torque as the strength the motor has. It's kind of like tightening a bolt with a wrench. You can apply a certain amount of force on that wrech to apply a torque to the bolt for tightening it. It has to do with the physical leverage of the motor, the rotating mass of the components, and even the compression, really all aspects of the motor's design is involved and affect the torque of the motor.

Horse Power adds a second part to the strength issue. RPM gives the engine a second attribute...motion. Something that is moving has power behind it. This is refered to as inertia and involves moving energy called kinetic in scientific terms. It's like a person throwing a baseball. A baseball just sitting there isn't harmful. However, when you throw it, you give that ball energy. This energy of motion gives the ball power, strength.

So, horse power is the combination torque AND RPM to create an end power output that you feel.


Now going to what it means to upshift and downshift. It's all gearing. Your engine always produces the same power. To allow you to speed up fast or have power at low speeds, you gear the engine low. This gives the engine a lot of leverage to rotate the tires more easily. Low gear allows you to speed up fast or pull a trailer, but it doesn't allow you to go very fast. You may rev up to 6000rpm and only get up to 40mph. To go faster, you need higher gears. You shift up to second and change the gearing between the motor and the tires. Now you're in a higher gear and a lower torque ratio to the wheels, less leverage than first gear. You can go to a faster speed, but you don't have as much leverage as first to accelerate or pull something heavy. Your motor has to work harder to do the same thing. As you shift into third and forth you can go faster and faster but you lose more leverage. A comparison would be like riding a multi-geared bike like a mountain bike. You can change gears to make petalling easier or harder. If you're in low gear, you can petal really easy but you can only go 5mph when you're petalling really fast. To go faster, you move up to a higher gear. For the same speed of petalling you can go faster, but you have to work harder. You don't have as much leverage to make the bike go with ease. When you go up a hill, you'll either have to petal really hard or shift down to a lower gear to make petalling easier. You'll go slower, but you can make it up the hill pretty easy.

I hope that helped