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What is Horsepower?

Posted by Auto.com Staff | Jan 3, 2019

Horsepower is the unit of measurement for all things automotive. But what does that horsepower figure really mean? Mathematically speaking, HP = Torque x RPM / 5252. But for those of us less interested in math it can mean much more or, at times, deceive us. For that number to really be meaningful, we should also be looking at another unit of measurement: torque, and at what RPM both of those occur. But first, we will take a quick look at the history.

Way back when engines were new and powered by steam, people wanted a way to measure the power output of this new technology. Enter James Watt, a Scottish engineer who formulated a way to compare the mighty steam engine to the power of draft horses. Watt's new power measurements, metric and mechanical horsepower (hp), have been with us ever since. Because cars use mechanical horsepower, that is what we are covering here.

An engine that revs particularly high may have a very high horsepower number, but comparatively less torque than one that makes peak horsepower at a lower RPM. This is because of the math we were talking about at the beginning. The best driving engine may not necessarily be the one with the larger horsepower number, but one with similar torque and horsepower numbers which delivers power in a more linear fashion.

This usually means it has a wider, flatter, and more useful torque curve which can be achieved in a number of ways. In the past we normally would have referred to this as a ‘square engine’. That is, an engine that has an equal or similar bore and stroke. An over-square engine has a larger bore than stroke which has the benefit of reducing piston speed and allowing for increased engine RPM which comes at the expense of low-range torque. Torque can also be accentuated or reduced in certain areas through more sophisticated engine controls.

Lately, technology has pushed the more economy-minded vehicles toward turbocharging and direct injection along with variable valve timing in order to broaden the torque peak and gain the efficiency needed to meet modern fuel economy and emissions standards. Cars like Volkswagen's Golf R even use twin scroll turbochargers to improve transient turbocharger response in order to more closely mimic the more instant response you would see from a much larger displacement engine, while still returning the fuel economy that you would expect to see from a 4-cylinder engine.