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What Exactly Is Horsepower and Torque

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Old 12-02-2013, 04:31 PM
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Default What Exactly Is Horsepower and Torque

A OnPoint Dyno explanation on what horsepower and torque is. These guys know there stuff. Very interesting read. ENJOY

What is Horsepower? Torque? An Explanation You Can Actually Understand.

SGSash | December 2, 2013
Recently I wrote an article for PRN Ignition (link here) about Horsepower, Torque and Gears. This article was fairly basic – and I want to take this time to expand on the subject as I continuously run into people that don’t quite understand how the three are related, and how to correctly tune an engine and drive-train to maximize acceleration. I will go over a few examples to bring everything together – hopefully by the end of this article you will have a complete and clear understanding of both Horsepower and Torque. You should also understand how gearing comes into the equation, and how you can compare engines with different power-bands and displacements.
The first and most important thing you need to know is that Horsepower and Torque are linked. If you look at Horsepower at any particular RPM point, you can calculate the Torque value. Likewise, if you look at Torque at any particular RPM point, you can calculate Horsepower.
Torque tells us the twisting force the crankshaft is applying on the drive-train. If we use the analogy of a bicycle, torque is how hard you are pushing with your foot on the pedal.
Horsepower tells us the energy the engine is producing. The energy the engine produces is the torque multiplied by the engine speed.
In other words, we can get the same amount of horsepower either by spinning the engine very quickly with a small amount of torque (think motorcycle engine), or by spinning the engine slowly but while creating a lot of torque (think V8). The energy (or hosepower) produced can be the same, even with vastly different torque numbers.
To make sense of this you need to realize that leverage can be used (through gears) to take a fast spinning shaft, and turn it into a slower spinning shaft with much higher torque. In other-words, torque is not important. We really only need to look at horsepower.
Here is a simple example:
A V8 producing 250hp at 5252rpm is connected directly, with no gears to the wheels of the car (obviously this would never occur, but it makes the math easier to visualize). The resultant torque on the wheels of the car is 250lb-ft of torque, and the wheels spin at engine speed of 5252rpm.
An Inline 4 engine also produces 250hp, but at 10504rpm (exactly double the V8, to make the math easy), and rather than being coupled directly to the wheels, there is a 2:1 gear between the engine and the wheels. The result with the 2:1 gear is that both examples have the same wheel torque of 250lb-ft, and they spin at the same speed of 5252rpm.
As you can see, the power is the same, and as a result the wheel torque when speed is matched by the gear, is also the same.

What the example is saying is that torque alone, is irrelevant. Gears can make up for torque. What makes power, and the ability to accelerate your vehicle, is the combination of torque and rpm. Since all of our vehicles have gearboxes, we must really only concern ourselves with horsepower. Not only peak horsepower, but average horsepower across the desired powerband.
So to recap – Torque can be multiplied up or down, Horsepower never changes. In the end, Horsepower is what we use to analyse acceleration and performance.
Now that we have some understanding of how torque and power are related, let me show you how to use that information to compare different engines. Normally you might look at the below graphs and say the engine with the most torque will be the fastest (as they both have the same peak power), but for the example below, both of these engines will perform identically if they are geared to have the same top speed in each gear. Since Engine #2 can rev 5.9% higher than Engine #1, it can use a gear that is the same 5.9% shorter. This adds 5.9% torque to the wheels, which results in both engines performing identically. That is represented in the second graph, where you can see that at any specific speed the engines have the same torque at the wheels (axle torque – or torque after the torque multiplication from the gears), and the same horsepower.
Magic? No. Just Gears.

I hope this makes sense. This example gives you an idea of how the torque numbers you see are not necessarily a good representation of the acceleration of an engine, but the power curve is. Huge gobs of torque really don’t mean all that much – what we need to do is look at the horsepower at different RPM points to put it all in perspective. Here’s a graph that gives you an idea of how much torque an engine must produce at various RPM points to be making 300HP:
So then, what is torque used for? I know I am making it out to be meaningless – but that is really not the case. When we are looking at whole vehicle acceleration, we are mostly concerned with Horsepower, yes. But torque is very useful and provides us with information that is not clearly seen from a Horsepower graph alone. The torque an engine produces tells us about the load it is producing at that RPM point, and the efficiency.
The peak torque of an engine is also the point where the volumetric efficiency is the highest (or how much air will ram itself into the engine). Torque gives us a snapshot of how the VE is changing. When torque is flat, the VE is staying the same. When torque is falling, it indicates that the engine is starting to struggle to flow air, and that volumetric efficiency is falling. Torque will also drop with RPM simply because engine friction will increase with RPM. So the torque gives us a good picture of how the engine is flowing air. – The torque is also reflected inside the ECU tune: The injector pulse-width, or how long the injector is open for each injection cycle, and the Engine Load measured by a mass air flow sensor, which can be expressed as Grams/Rev. Naturally, the highest torque requires the most air per combustion event, so the air and fuel required at those RPM points per cycle will be the highest. Injector Duty cycle and MAF flow per second can be used to get an idea of engine power. Note that these power values relate to time, where-as load based measurements relate to each cycle of the engine.
Not to get off on a tangent, but when tuning a racing engine we typically want peak torque to occur at the lower end of the powerband, so that it keeps the average horsepower up across the RPM area we are racing the engine under. This can sometimes make for a “dead” area outside of the powerband with a highly tuned NA engine, but that’s not supposed to matter. Sequential gearboxes that allow the powerband to be quite small, give the engine builder freedom to move peak torque closer and closer to the RPM limit of the engine, which should result in higher peak power and higher average power (across a narrower band).
Let’s look at an engine with a narrow peak power band, and one with a wider band and see how torque and horsepower relate:

This graph is a classic example of where someone might comment that “power isn’t everything” and “torque wins races.” The reality is that the blue engine lacks both horsepower AND torque below 7000rpm, not just torque. You can observe that power has fallen off a great deal below 6500rpm. Our red engine has a flatter torque curve, with the tuned peak torque around 6000rpm, with the motor suffocating a slow and painful death thereafter – which results in a nice flat (albeit low) peak power curve.
Our Blue engine on the other hand, has quite a bit more top end power, but at the expense of a wide powerband. This engine would be best suited to a car with a close ratio box, drag racing or super speedway racing. This is just an example graph, but basically the bump in the torque at 7000rpm suggests an intake or exhaust event timed for that RPM, causing a surge of power. These strong wave tuning practices also often result in a bit of a dip of power on either side of the tuned point, which makes the power drop off at 6500 even worse.
Realistically, the most ideal engine setup for most road race cars would likely be something in between the two, but this above example shows you how the torque and horsepower relate, and how yes horsepower isn’t everything. What you need to consider is average horsepower – not torque. Especially if you’re comparing engines that have different RPM limits. The same ideas apply with a turbocharged car. A large turbo will spool later and make more top end power, while a smaller turbo will spool earlier, but as the exhaust back-pressure builds up due to the smaller turbine, the airflow across the engine will decrease and the engine will lack top end power. It’s a balancing act deciding how wide your power-band needs to be, and tuning the engine accordingly.
The last thing I am going to discuss is shift points. It should be fairly obvious at this point that you want to keep the engine operating in the RPM area as close to peak power as possible, which means that you want your gear-shift to always be after peak power, so that when you re-engage the next gear, the engine RPM is slightly below peak power. The easiest way to determine shift points quickly is to use this rule:
The RPM you engage in the next gear, should result in the same power you left the previous gear at. This is true in all cases except when you hit the rev limiter first, in which case shift at the limiter. Please don’t blow your engine up! Here is an example:

This example shows three different example situations, based on how far apart the two gears are. The red 1000rpm drop suggests a very closely spaced gear set, where-as the pink line is more along the lines of a factory gearbox. You can see that all points equally straddle the peak power, and the always land at the same power they left at. You can see in the above graph, if the RPM limter was 9000 rather than 9500, both 1500 and 2000rpm drops would result in an gear shift at the limiter.
You can calculate RPM drop based on your gear ratios with any generic gearing calculator. Our Laptimzier APP also has a gear ratio calculator that will automatically tell you the RPM drop based on your gear ratios and the RPM limit you set. Keep in mind that the RPM drop will change based on when you shift (i.e. the drop will be half as much if you shift at 4000 rather than at 8000), and also obviously based on the gear ratio difference between the two gears. For these reasons, your ideal shift point can be different for each gear.
I hope this article helps some of you out there understand the relationship of torque and power. I often see people thinking that 500lb-ft of torque means a car is ultra fast, even if it only has 300hp. I also see people claiming that torque is more important than power, and are reluctant to tune out high torque numbers in exchange for higher power (going to a larger cam, for example), even though their peak torque occurs well before the vehicles powerband. Now that you understand how torque, power and gears relate to vehicle acceleration, you can determine the best parts for your engine.
 
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