Ok, I get a decent amount of questions about what torque is and how it relates to horsepower. Then, there are the arguments on here about which is more important (impossible argument since they are not even comparible units) and a slew of explanations that are very far off base from the actual physics involved. The most common is people thinking that HP is all based off of engine parameters and tq is "what moves the car". Peices of that are somewhat correct, but the relation of the two is completely off. So I thought I would put out a simple, fairly low physics content explanation of what each one is. I wont go into whether you want a car with more of one than the other for X application, you should be able to determine that from the explanation. So lets not mess this up with that argument. And really, if you see how it all works, there isnt really much argument, it is all in the application.


Lets just make it simple and take it from the start of the process to the end.


1) Air and fuel enter the system
2) Air and fuel are compressed
3) Compressed mixture is ignited
4) Force is applied to the piston which in turn moves the crank


That is all that is really needed to see what tq is. Torque is the effect of the force (created from and regulated by the combustion process) applied at an offset to the center of the crank (the crank journal). This is absolutely no different than applying force to the end of a wrench handle or the pedal on a bicycle. So very very simply, that is what torque is.


Horsepower is not some "trade off" to torque. It is the MEASUREMENT of the tq applied at a given rpm, period, nothing else. Not some mixture of the engine parameters or anything else. It is very simply stated as HP = (tq*rpm)/5252. So a given amount of tq at a certain rpm will equate to a certain amount of HP. You can use HP numbers to see how an engine is using its tq and rpm.

Keep in mind that larger displacement engines can typically create more tq, so many times getting a higher rpm limit out of them is harder. On the opposite, smaller engines can usually be made to rev higher without as much work, but start with less TQ, so they arent producing the power down low.

Now, kind of a side note that most people tend to ignore. Peak HP numbers really mean very little. Many get drawn into thinking that a 200HP car will automatically outrun a 180HP car without looking at the curve. The area of the power curve is really what you want to see.

I think there have been so many arguments over this due to many not really understanding fully what each term really means, which is evident in some of the explanations I see. So hopefully this will help some.

 

I tried explaining that to my dad yesterday but I couldn't word it properly. Thanks..... now i can go school him.

 

This is how I explain it to my customers:

Torque = leverage

HP = force exerted on the leverage.

then I follow with an example that makes sense.

 

That is actually incorrect though. The force is the force applied by the combustion process. The "leverage" is created by the distance that the rod is offset from the centerline of the crank. This force and offset creates torque on the crank. Horsepower is a unit of power, which is work applied over a given amount of time. Horsepower is not a force applied to anything. There is a force applied to the crank that creates torque. The resulting work done over a given time interval equates to power.

 

I know it's technically incorrect, but in everyday language, I use the phrase "it's like". Any other explaination is useless on the common folks.

 

Ok. without getting all scientific:

HP basically equates to how fast your engine can spin its crank--roughly translating to how fast your car can go. Obviously there are limiting factors such as drivetrains, gearing, wheels/tires, suspensions, etc.. But the HP output of an engine is basically telling you how much power it can make, and that tells you how fast the crank can rotate, which tells you how fast the wheels will be able to spin, which tells you how fast the car will go.

TQ is essentially a measurement of the rate of rotational force being applied to the crank. Or the rate at which the engine builds power..roughly translating to acceleration and engine rev speed.