Horsepower vs Torque - Which is more important?
It’s a debate that’s been ongoing for decades but has rarely come to resolution: which is more important, Horsepower or Torque?
The answer to the question fundamentally depends on what you are trying to achieve. Firstly, one common misconception is that an engine always produced a certain amount of Horsepower, and a certain amount of torque. The reality is that the figure the manufacturer gives you is only achieved at Peak Horsepower and Peak Torque respectively, and doesn’t tell you the whole story.
Torque is a measurement of rotational or twisting movement. It defines how much work can be done. It is often described as how strong a vehicle is. It can be calculated by Force x Distance. Imagine a wrench turning a bolt. It you have a wrench that is 1ft long, and you apply 1lb of force at the end of the wrench to turn the bolt, you have applied 1lb/ft (pound-feet) of Torque on the bolt. If you applied 2lbs of force on a wrench 1ft long, you have applied 2lb/ft of torque on the bolt. Similarly, if you apply just 1lb of force, but this time use a wrench that is 2ft long, you are also applying 2lb/ft of Torque. This is called using leverage. To achieve more Torque, either increase the Force, or the Distance.
In a combustion engine, the cylinders combust (by igniting a fuel/air mix) which forces the cylinder downwards. Each cylinder is connected to the engine’s crankshaft. Torque multiplies the amount of Force generated in the cylinders, by the Distance from the crankshaft to generate the Torque. This is the same concept as when you apply a Force on a wrench to turn a bolt. The more Force, or the more Distance, the higher the Torque. The more Torque, the more rotational movement that can be achieved on the crankshaft, which means the engine can do more work.
If the vehicle is heavy, it will require more work to turn the axle (and so the engine’s crankshaft) to get the vehicle moving. Having more Torque means this movement is easier as it can achieve more work.
Horsepower on the other hand is a measurement of how long it takes to do a certain amount of work; or Torque x RPM / 5252. (If you're interested in why 5252 specifically, then watch this video.) This aspect of time is all important when it comes to measuring 0-60 times and acceleration in general, as it is a measurement of how quickly the required work can be achieved. If you have more Horsepower, you’ll be able to go faster.
Does this mean horsepower is everything?
The two are fundamentally connected. Since HP = Torque x RPM, then if you have a Torque heavy engine, then you will be able to produce a lot of Horsepower at lower RPMs, which means you won’t need to rev the engine as much to go faster. This usually means you get good 0-60 times as Peak Horsepower can be achieved earlier.
On the flip side, if you vehicle is low in Torque, then you will need to rev the engine very high to get to the Peak Horsepower. This usually means it might not accelerate that quickly from a standing start, but once the engine gets going then it becomes a very fast car indeed.
A great example of this is the E92 BMW M3 that had that amazing naturally aspirated 4 litre V8 engine. It produced a massive 420 Brake Horsepower but interestingly only 292lb/ft of Torque. So you really had to get the revs high to get the most out of the car, but when you did it was an absolute beast. In some way it’s a shame, but the more modern M3s now come with 3 litre straight 6 engines, but they also have two turbochargers. And this means MUCH higher Torque levels. As a result it can achieve much higher Horsepower from lower down the rev range, meaning quicker acceleration. There is nothing quite like the sound of that old V8 though, the straight 6, or a V6, no matter how good the exhausts are, just don’t sound as raw...
Moving on, weight is also very very important. Big trucks and heavy vehicles, for example, will require much more work to move than a small car. This is why they require engines with very high levels of Torque. Large trucks have no requirement to drive particularly fast and so having high levels of Horsepower is not fundamentally important. Without the strength of a torquey engine they simply wouldn’t move.
Similarly, small cars are light and can be moved very easily by comparison, and so they won’t need to produce anywhere near as much Torque as heavier cars.
Is Horsepower the same as Brake Horsepower?
No. Brake Horsepower is measured at the engine’s flywheel and determines how much Horsepower the engine is actually generating. In the real world some power is lost by the time this power is transferred through the car to the wheels. The power generated at the wheels (as measured by a dyno) is known as Horsepower, or Wheel Horsepower. When you take a car to get dyno’d, they calculate Wheel Horsepower and then convert this back to Brake Horsepower for simplicity.
Essentially, if you want to go quickly you’ll need a good balance of both Horsepower and Torque. Having just one of the other might win you a game of top trumps with a friend, but it won’t do you many favours in the real world.
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