Torque vs Horsepower the myth and the reality

High Torque Engine

"Torque is for acceleration and Horse Power is for speed”, I’m sure everyone who gets involved in technical discussions has heard this statement one day or another. Probably the most attractive side of this statement is its simplicity and its clear black-white categorization of the subject. But no matter how common, easy, and memorable this statement is, it is actually very wrong. Unfortunately, the right statement that would links Torque and Horsepower to speed and acceleration would need at least a couple of long paragraphs that are more complicated and much harder to understand than the above simple and wrong statement. 

To make things easier, start by defining each term by itself:

Engine Torque:

is the twisting force that the engine produces at a certain rpm, this amount changes continuously over the engine RPM. Many factors affect the produced torque in any certain RPM including the Intake/Exhaust geometry, valves timing, piston weights, friction ...etc. Usually the engine torque goes up gradually (but not consistently) with the RPM until it hits a high point that’s called “Torque Peak” then starts going down gradually until the point where the engine is not able to breath enough during the intake stroke and the friction becomes so high, at this point the torque goes down sharply.  The Torque peak is what’s usually considered the engine torque and reported in the engine spec sheet with the RPM it gets produced at.

One thing important to remember here is that this Torque at the engine’s Crank-Shaft is not the same as the torque at the wheels although they are directly related.

Horse power: 

As its name suggests, it’s a measure of power or the amount of energy produced per untie of time. Engine power is  the rotation speed of the crank-shaft multiplied by the torque at that speed, therefore Horse Power curve is directly related to Torque curve, it just scales up with the increase of RPM, for example a torque of 100ft-lb at 1000rpm generates 19hp while the same torque at 2000rpm generates 38hp. Moving along the RPM axis, both torque and RPM grow up until we hit the Torque Peak at which point the Torque will start going down, however, if the ratio of decrease in Torque is still less than the ratio of increase in rpm then the Horse Power will still go up. The Horse Power curve will only go down when the ratio of torque decrease becomes higher than the RPM increase, at this point we reach the maximum output power the engine could produce.

Gearing:

The output of the engine is connected to wheels through a number of gears in the transmission and the differential. The ratio of these gears determine the output torque and rpm at the wheel without changing the horse power (in reality some horse power is lost through the drive train as friction, but theoretically speaking we could ignore this). For example if the engine is running at 2000rpm and producing 100ft-lb and the total ratio is 1:4 then the rpm at the wheel will be 500rpm and Torque will be 400ft-lb.

How does all that relate to speed and acceleration at the wheel:

Acceleration: The car acceleration is related directly to torque at the wheels, we always want to get as much torque at wheel as possible. So if it is all about torque why do we care about horsepower? First, the torque we care about here is at the wheel not at the engine, second when we gear for the best possible torque at the wheel we have to consider the speed of the wheel in the equation, and here is where horsepower becomes the most significant number.  To explain it better, let’s have an example; let’s take a car with an engine that has a torque peak of 200ft-Lb at 2000rpm (which means that the HP at this point is 76hp*) and highest HP of 150hp at 5000rpm (which means that the torque at that point is  157ft-lb). Let’s list the two peaking points again:

P1: at 2000rpm           Torque = 200ft-lb       HP = 76hp      (torque peak)

p2: at 5000rpm           Torque = 157ft-lb       HP =  150hp   (HP peak)

 Say at a certain car speed the wheels of the car are turning at 1000 RPM, we pause the scene and go tweak the ratios of the transmission to get the highest possible Torque at the wheel to get the best acceleration. Let’s try to gear at both peak points and see which one will give us the highest torque at the wheel

P-1:     To get the engine running at 2000rpm while the wheel is turning at 1000rpm, we need a ratio of 1:2 which will make the output torque double the input torque:

            Output Torque = Input Torque * 2 = 200 * 2 = 400ft-lb at the wheel

P-2:     To get the engine running at 5000rpm while the wheel is turning at 1000rpm, we need a ratio of 1:5 which will make the output torque five times the input torque:

            Output Torque = Input Torque * 5 = 157 * 5 = 785ft-lb at the wheel

Which means that for this engine the peak of HP point will produce almost twice as much torque at the wheel as the peak of torque. You could also notice that the HP at the HP peak (5000rpm) is almost twice as much as the HP at the torque peak (2000rpm) and that’s not a coincidence, basically for any given wheel speed the wheel torque at the output is proportional to the HP of the engine produced at that moment regardless of RPM and torque.

Why don’t we keep the engine at the peak of the HP if it is the best for acceleration?

We would if we could, and this is exactly what the CVT equipped cars do (see Types of Transimssions), but for normal transmissions, if you select the best ratio for a certain wheel speed then you will quickly need to change that ratio once the wheel speed changes. Ideally, if you would continuously change the ratio of the transmission with the continuous change of wheel speed to keep the engine at the peak of HP, then we could get the best possible torque at the wheel along the whole range of speed (again check how the CVT works). The higher number of gear ratios you have the closer to the HP peak you could keep the engine revving.

So what’s the point of having high torque peak if it’s all about HP?

Having higher peak torque means that torque is produced at lower RPM which also means that the HP at the lower RPM is still high (although not as high as the peak)  and that gives a good continuous power all over the engine RPM making it smooth and responsive. This translates into two benefits:

1-         If you do not have a CVT (and most cars don’t) then you are forced to drop the RPM to a point far down below the HP peak every time you up-shift. If you don’t have enough HP at this point then your acceleration will drop. This usually happens with cars that have low number of ratios and low torque engine. If you have High Torque Peak then usually that means you have good enough horse power on lower RPM.

2-         In every day traffic driving, sometimes you need an immediate acceleration, and if you don’t have a low RPM torque you are forced to down-shift to get the RPM back to the HP peak, and that is not always convenient especially in the city traffic.