Car manufacturers use different units to measure and express the power of a car engine. Horsepower and torque are among the most frequently used terms, especially when talking about the car’s acceleration. What are they and how important are they?
What is torque and what is horsepower?
Both torque and horsepower are about measuring force. Torque expresses the turning ability of the engine (the ability to turn its flywheel) and horsepower means the total power output of the engine. In very simple terms, torque is the force you feel pushing you back in your seat on acceleration, while horsepower is the speed achieved at the end of that acceleration.
Both horsepower and torque can be measured in a number of different ways, e.g. in Brake HorsePower, Power Standard, Newton Metre, Watts, Pound/Feet and so on. Our preference is BHP for Brake HorsePower (the amount of power produced by an engine minus the amount that gets lost due to friction) and Lb/Ft for torque conversion (pounds per foot of rotation around one point).
In fact, brake horsepower is derived from a calculation that relies on torque: BHP = Torque X RPM / 5252. Defined simply, multiply the torque by the axis’s rotational speed per minute (RPM) and you’ll get the engine’s final power output.
Is torque or horsepower more important for acceleration?
The importance of either torque and horsepower depends on the nature of your needs and, of course, you can’t ever have one without the other, anyway.
Truthfully, there is no definitive answer as to which is more important. A more important consideration is understanding the balance between the two, how they interact or, perhaps, how to influence an engine by giving it more of one or the other. If you want to shred your car tyres, you need some low-down torque, but if you are after land-speed records, then horsepower will always win out.
Engine specifications for better engine power
Not all engines are the same, as even two units pulled from the same line will differ on an engine dynamometer. Even then, it would need to be a fully-controlled environment as the air pressure, temperature and humidity will all affect the power readings of an engine, even more so when we bring forced induction into the equation.
It is possible to shape the characteristics of power: for example, a long stroke engine will generally give more torque than a short stroke engine. On this same basis, we can play with the cam timing to affect the power delivery – advancing the cam timing should deliver more low-down torque, while retarding it equates to high RPM horsepower.
Another element to consider is forced induction (FI). Supercharging or turbocharging the engine is a great way to increase its power, usually in quite a cost-effective way. Here, again, we have two routes, both offering different options. Supercharging, on one hand, will generally give you more engine torque, whereas a turbo will give you extra horsepower.
Of course, we need to add a caveat. Either method increases ‘power’, but there are a lot of other factors you would need to consider. For the sake of simplicity, however, that’s what you could expect to see by taking the FI route.
One further consideration when it comes to engine design, type and power is the choice of fuel. This relationship is becoming increasingly blurred as once diesel was considered to be truck fuel and petrol was for motor cars.
It is no longer the case as diesel cars became almost as popular as petrol ones in the last years, making an enormous impact on the race track. For instance, manufacturers such as Audi and Peugeot only entered diesel cars in the famous 24-hour Le Mans motorsport race because the torque and fuel economy are ideally suited for endurance racing.
The applications of horsepower and torque
When it comes to the torque vs horsepower issue, it really is a question of application; if you’re trying to pull a tree-stump out of the ground, horsepower isn’t going to do it (unless you were to add a 100 metre length of rope and give yourself a run-up). Yet, if it’s something a little more sporty, horsepower has to be the way forward. Most manufacturers have found their balance but occasionally they adjust the figures slightly.
Truthfully, whilst many of us just may know that a particular engine or car is inherently torquey or powerful, very few would be able to distinguish the characteristics of a ‘normal’ road car to any degree of accuracy. The general consensus is that you’d need to have a minimum of around 10% difference to actually feel it, unless your seat of the pants dyno is particularly well calibrated.
The future of torque in electric vehicles
In general, the talk regarding torque vs horsepower, will undoubtedly become a moot point in the future, as it will be replaced by one simple measurement: kW.
Forgetting all of the whys and wherefores, electric power can deliver massive amounts of torque from standstill, meaning acceleration is always brisk, despite the weight penalty of having hundreds of laptop batteries glued to the chassis.
In fact, a Tesla Model S P90D has just taken the record for an all-electric production car ¼ mile sprint, completing it in just 10.9 seconds. As a comparison, a Bugatti Veyron will do the same in 10.175 seconds. It really does seem as if electric power is the way forward, as the gap in performance in closing very, very quickly.
Conclusion
Whether it’s forced induction, long stroke vs short stroke, bore size, compression ratio or camshaft design, it all plays a part in changing the power characteristics of an engine. There is no simple answer as to what is best, so perhaps the simple question is: what suits you best?