Horsepower Vs. Torque - Corolland: the Toyota Corolla Forums

This is a concept I've been studying and studying and still can't come up with a definitive answer to. What is better, torque or hp? Here is what I have read up and learned so far:

1) Horsepower is an old pre-metric measurement created by James Watt, measuring the power of a coal pony (moving 33,000 lbs of coal 1 foot every minute). Therefore, 1 HP is 33,000 ft-lbs of force a minute.

2) Torque is the rotational force an object emits. In the US, it is measured in ft-lbs.

Now understanding the relationship between torque and hp is where I get lost. As far as I've read up on definitions and relations, both torque and hp are concepts of mechanical work. If a force (like HP) is allowed to act through a distance, it is doing work. If torque is allowed to act through rotational distance, it is doing work as well. Power is the work per time. Power = torque x angular speed. Therefore by definition, torque can never reach its peak at a higher RPM than the peak power (horsepower). The problem I'm facing is that all these definitions are just facts which in themselves make sense to me, but together don't answer my questions:

1) What does it mean if an engine produces alot of ft-lbs torque, but not alot of hp? (like in diesel engines)

2) What does it mean if an engine produces alot of hp, but not alot of ft-lbs torque? (like in the XRS engine)

3) For acceleration, is it better for your car to produce alot more hp, or alot more ft-lbs torque, or is it best if the two have relatively equal numbers?

4) If the XRS has to push its engine to such high RPMs to achieve power and torque, wouldn't that be a disadvantage and mean it doesn't accelerate as fast compared to other cars of equal hp and weight?

Sorry to have to throw such scientific questions out at you all, but they have just been annoying me for years and no matter how many times I read up on textbooks and wikipedia articles, it just doesn't answer my questions (as you can tell, I'm not majoring in Physics in college).

You can read to your eyes bleed and still not understand something. Sometimes, you just need to experience yourself. A car like a Corolla XRS or a Honda Si gets moving by gearing, and high revving engines that work with variable valve timing to make the car move, and having a light weight car.

A engine with toque like a diesel or a V8 has grunt off the line, but generally have a much lower rev limiter then a performance 4cyl.

Most people prefer to have torque over HP because with a car like a Honda, you really gotta rev them out to make them move. With a torque engine, you can cruise at lower RPMs and still get down the road. One is not always better then the other. A Civic Si or XRS could probably take on a lot of v6 cars that have a higher torque rating and maybe even HP. Weight and short gearing and high revs play a huge roll. With some mods, that NA 4cyl could probably even take on some V8s. The downside is, they have to be revved out to the rev limiter, and most people don't like doing that just to get a car to move quickly.

It gets more confusing then just HP and Torque because the car has a power curve. It can be seen on a dyno read out, but it can also be felt when driving. It might take awhile for a 4cyl to get to it's peak power, but with all the nice advancements now days, that engine might be able to hold out it's power curve till 8K RPMs. A engine with more torque will have a higher peak in power, but due to a low revving engine, a cam usually made for low end power, the engine can puke out past 5K RPMs.

I think the Lotus Elise is a wonderful example of what a small displacement engine can do in a light weight car. 190HP 138' lb Torque 0-100MPH in 12.9 seconds.

In the past Torque was the most popular form of power. Tire shredding torque to move pony cars. Even going back to the 70s a small group of people tunned Honda's to beat V8s in the 1/4. Now days, that is common practice.

try this

I found it fairly understandable.

jim

Hey thanks for that website. Augenstein (the website creator) explains it SO MUCH better than wikipedia, or my old high school textbook.

You can think of two extremes:

- high torque: Semi Tractor

- high horsepower: F1 car

Which would you like to be in?

gvr4ever hit it on the head - it all boils down to gearing and weight. Some people swear by torque, others by horsepower. Bottom line, it depends on what the vehicle was designed to do and what you expect out of it. Coupling the appropriate transmission and chassis will yield the best results. How to chose the correct gearing and weight will be determined by the engine's powerband. What makes it even more confusing, is that you cannot measure HP, only torque, HP is calculated from torque measurements.

Another good link here:

http://www.allpar.com/eek/hp-vs-torque.html

I think I posted something in the past with more detail (more physics), but only useful for people that like equations and derivations.

I've often heard that torque is what a engine can do and horse power is how fast it can do it. That isn't a true statement when it comes down to technicality, but it sort of makes since.

If you get behind the wheel of a WV TDi, the engine has enough torque to spin the tires with only 90HP. The engine revs out slowly, but the much higher torque number (around 180 I think) gets the car down the road. Even with only 90HP, it moves pretty well. If a car only had 90HP and 90ft lbs of torque, it would be slow as dirt.

Power is not a force. Power is the rate at which work is done. Work is a measurement of a force over a distance. So in the classical sense of the definition (the horses), horsepower is the rate at which a horse can drag something (that has mass) over a specified distance.

Torque is a force. Torque is a measurement of the amount of force that acts on an object and causes that object to rotate. If we are talking about two objects that are connected and are perpendicular to one another (like an axle shaft is connected to a tire), then the torque applied to the road by the tire is the force applied to tire from the shaft times the radius of the tire.

So really, these are two different kinds of measurements.

Here is a good website that helps to explain general physics concepts:

http://science.howstuffworks.com/fpte7.htm

Now, I don't know which is more important, but it is interesting to notice the rims of professional cyclists. When they want to accelerate faster, they reduce the weight of their rim. When they want more rotational inertia, they increase the weight of their rim. This rotational inertia is also related to torque and is similar to heavy or lightweight flywheels bolted to the crankshaft of your car.

Hmmm interesting topic. What would that mean then if two exact same cars both produced 200 HP, but Car A produced 150 ft.lbs torque and Car B produced 200 ft.lbs torque?

Bikeman982

Hmmm interesting topic. What would that mean then if two exact same cars both produced 200 HP, but Car A produced 150 ft.lbs torque and Car B produced 200 ft.lbs torque?

It would mean that the car producing the greater torque with the same HP was more efficient.

Hmmm interesting topic. What would that mean then if two exact same cars both produced 200 HP, but Car A produced 150 ft.lbs torque and Car B produced 200 ft.lbs torque?

Depends on where the peak torque occurs - all in the powerband. Manufacturers know that numbers can help sell cars - HP and TQ are important, but it is more "useful" to know what HP/TQ values are at known rpms along the powerband.

Example:

Car-A with 200HP/200TQ vs Car-B with 200HP/150TQ - Car-A certainly looks "better" on paper. But if the peak torque is generated by Car-A is at 8000RPMs and only has 20% of peak torque available off-idle compared to Car-B that has peak torque occuring at 3500RPMs and 67% of peak torque available off-idle. Car-B will spank Car-A in most acceleration runs starting from a standstill and up to a speed threshold - depending on gearing. Car-B, eventhough less powerful on paper, already has the powerband advantage over Car-A - effect would be greatly enhanced if gearing is "tall" (benefits from the greater torque available at low revs). Only chance that Car-A has would be have the gearing so short, that the additional torque off-line would be less of an influence.

Good example of powerband, graphically at this link (a little bit different, as one engine has more HP, the other more TQ - but nice to see pics default_biggrin ):

http://www.straightdope.com/mailbag/mhorsepower.html

I was recently shopping around for a new Mazda MX-5. At a local dealership, the guy was bragging how the new MX-5's are immensely faster than the old ones. So I decided to throw him a curve ball and asked him what's the difference between HP and torque. He actually gave a pretty inciteful answer.

According to him, think about torque as the "pushing" power, and hp as the "pulling" power. You need torque to push you off the line and push you into motion, but you need hp to pull you along that motion and continue your acceleration. That's why heavy vehicles like trucks and buses use diesel engines, because they make a helluvalota torque which they need to push their huge mass into motion. However, because they don't make enough HP to pull them along, its why you never want to drive behind them (accelerate snail slow). Another thing the dealer told me was that looking at HP vs. Torque can give you an idea of whether its a high revving engine or not. Most car engines rev to about 6000 rpms for max power and have pretty even hp to torque ratios. My 01 Corolla has exactly 125 HP and 125 torque and revs to 5800 for its max power. If you look at a car like the 2005 Celica GT, it produced 140 HP but only 126 torque. That car's hp maxes at 6400 rpm. The MX-5 I was looking at made 170 HP vs 140 torque and had a max power hit at 6700 rpm. Now take a real high revving car like the XRS which has 170 HP, but only 126 torque. It maxes at 7600 rpm. you can see the trend going on here.

So what does this all mean according to him? Torque is fun because it gives you the in-your-seat feel when you step on the pedal, but HP is the bragging rights because its what the rest of the cars around you see when you pull ahead of them.