# What exactly does horsepower do to cars performance-wise?

I often see horsepower rated to cars. I would guess that more=better, either faster, or more powerful, but I don’t know.

If you increased a car’s horsepower by a certain amount, will the performance change in a consistent manner? What would be the general difference between a car that has 50, 100, 200, 400 horsepower assuming they all weighed the same.

I believe that as far as a cars performance is concerned, the actual amount of horsepower the engine produces does not exactly determine its actual performance. I believe that its the actual amount or Torque the engine is capable of putting out that determines its ( the cars) overall performance. In order to get a high level of torque one would need to factor in the gear ratio of the transmission, the gear ratio of the differential, the size,width of the tires, and a great many other things. I am very sure that there are others here that are far better versed with these matters than I, and I am sure that they will chime in and clear this up a bit better for you, but I would go with the torque factor if you want your car to go really fast

Power and torque both have an effect on the performance of a car.

It is possible to increase the power without increasing the torque, just by desiging the engine to rev higher.

Power = torque*revs

When engines are designed, their output characteristics are mapped to suit the intended application.

This might mean that a car is economical, or that it has very high total output power, or that it produces a lot of torque at a certain engine speed.

One way to look at the way power is produced is to plot the power and engine revs on a chart and observe the curve.
This can also be done for torque and revs too.

When you then look at both these charts you can get some idea of the overall performance.

However, the final characteristic depends also upon the engine gearing and transmission losses too, and you can modify the gearing to suit the engine better.

Finally, to answer the OP, if you were to take a vehicle and increase the power by 100%, you would run into problems, the structure of the vehicle is not likely to be able to take this without some modifications,
The clutch would need to be uprated, the gearbox would probably need to be heavier duty, the differential would need to be heavier duty, the engine mounts might not be strong enough to withstand the torque reaction from the engine, and tyres and brakes would probably not be good enough.

This would all mean a heavier vehicle.

If you intended to increase power and see what the effect was, it would also depend upon how exactly you increase the power.

If the engine produces more power by revving higher, then what would happen is that it would not accelarate any faster, but it would accelarate for far longer.The reality is that modifying an engine to rev higher often means that lower rev power is sacrificed to some degree and so accelaration can actually be slower.

If the engine produces more power by increasing the torque, then it will accelarate faster, up to the original rev ceiling.

Increasing the torque may well not increase the vehicle speed by much at all, because the speed is determined by the engine revs through the transmission chain, the engine would still top out at the same revs, however with more torque available it would be possible to change the gearing ratios and this would allow a higher speed.

At the risk of over simplifying:

Horsepower determines the top speed of the vehicle.
Torque accelerates the vehicle.

Horsepower is the amount of energy the engine can produce. The metric unit for horsepower is kilowatt. At high speed you have all kinds of frictional losses to overcome. Wind resistance, tire resistance, losses in the drivetrain, etc. The faster you go the higher the frictional losses (especially wind resistance). More horsepower will let you get to a higher speed.

Torque is the amount of force the engine can apply in a rotational manner. More torque gives you better acceleration. Of course acceleration is also influenced by many other factors such as transmission ratios, vehicle weight etc, but all else being the same more torque = better acceleration.

Going one step further, for daily street driving you are most interested in the torque curve. This is the amount of torque at different rpms. You want the torque to come in at low rpms, and stay up as far up in the rpm band as possible. The lack of low end torque is what gives you a vehicle you will constantly have to downshift to accelerate.

I’m not sure if that makes sense. If an engine produces more power by revving higher, and you use it with a lower gear, isn’t it the same as an engine that has higher torque at lower revs used with a higher gear?

A physicist’s answer is that horsepower is all that counts, and torque is only relevant for choosing the proper gear ratio.

Horsepower is a measurement of power. To get an idea of why this is significant, you could say that doubling a car’s horsepower without adding weight would make it accelerate twice as fast.*
Since it was brought up, torque is a measurement of rotational force and is measured in foot-pounds. It can be measured at the engine or at the wheels. Torque at any given rpm equals horsepower times rpm times a constant. Typically, an engine’s torque will be higher at lower engine speeds. Rear wheel torque will be lower at lower speeds since the transmission is in a lower gear.

*Actually wind resistance would play a part. The car would accelerate almost twice as fast at slow speeds. At high speeds, the car would still accelerate faster, but it wouldn’t accelerate twice as fast. Of course, you could also get the same effect by cutting your car’s weight in half.

Yes. 2x foot-pounds of torque at y rpm and x foot-pounds of torque at 2y rpm with a 2:1 reduction produce the same torque and rpm (and thus the same vehicle speed) at the rear wheels. Power is power.

Can anyone explain why I always forget exactly what torque is? I’ve had it explained so that I understand it before, but I always forget very quickly!

Torque is a unit of force. Think of it as pushing on something without any actual movement being involved. In physics if nothing moves you aren’t doing work. If you exert a force to move something then you’re doing work and power - the ability to do work - is involved. That’s why torque can be measured apart from RPM.

Torque can be measured with a braking type of dynamometer. An engine in a test stand is run with a load on the crankshaft. It can be a disk brake or a eggbeater in a vat of goo, doesn’t matter. Torque is measured by the twisting force the engine puts on its mounts in the opposite direction.

An important thing to remember is the horsepower figures that are usually stated are ‘peak’ power, or the most power the engine makes at one point. Everyone likes to have a high ‘peak HP’ number to brag about, but A car with a lower peak HP number that has higher power accross the rest of the RPM range could easily have stronger acceration.

Here is a graph that shows a 190 HP engine that is stronger than a 200 HP engine.
http://mywebpages.comcast.net/stevtecv6/dynos/VQ30DEvsJ30A1_DynoSig.jpg

And two engines both rated a 240 HP
http://mywebpages.comcast.net/stevtecv6/dynos/VQ35DEvsJ30A4_DynoSig.jpg

As they say on the automotive board I am on, Total power area under the curve ownz joo. (IE - Higher average HP is better than higher peak HP)