Start this discussion around the bar, braai (barbecue), tea trolley or at the car club covering the explanation of horsepower and torque and everyone is an expert. In(s)ane comments such as “Horsepower can drive your car into a wall but torque determines how far you go through it” attempt a cynical, jocular reasoning but just confuse the issue. Explanations will continue to abound however, from the very basic to the inevitable boffin who will detail a mathematical ‘clarification’….but seldom do these really create understanding.. .The role that torque plays in engine related car chat is either misunderstood, completely overplayed…or both.
The surprising thing is that most of us know what is meant by the expressions ‘a torquey engine’ or ‘the engine has grunt’ (euphemism for torque)…these simply express the fact that a driver feels good response between the action of the right foot and the G-force in his/her back. This interestingly also tends to be associated with low/mid speed engine response more so than that at high rpm. (We will get back to the important word response in a ‘mo’). Where the system fails, however, is deciding how torque relates to horsepower in an easily understandable way. That is because the whole thing is currently confused to a point of insanity…to the extent that car scribes who should know better, talk more readily about the massive Nm of torque in a particular vehicle, than referring to the all-important horsepower. They do so of course because most engines these days are turbocharged and as a consequence have massive torque numbers which need to be channelled into the conversation…somehow… even though they are technically irrelevant.
Torque in the real scheme of things does nothing… it is purely a measurement tool from which we can arrive at Horsepower or work.
We have all heard of the difference between ‘talkers’ and ‘doers’…. In this case, ’Torque’ is the talker….’Horsepower’ is the doer. ….
‘Doing’ stuff and by that I mean actually getting the job done, like moving a vehicle, is the exclusive preserve of Horsepower… alone…. torque is a stationary measurement.
Ask the following question of anyone talking about torque….when does torque become horsepower or horsepower become torque?… or better still, what functions do the two perform?…or as they say so often in politics…what is the separation of ‘powers’ ?… and there will be silence, incoherent babble or a propellerhead will start that discussion on how you use a formula to calculate horsepower.
Let me put forth an explanation which has at least some attachment to daily life:
Most of us folk have had the call from our better-halves asking to open that bottle of jam… the difficulty in opening those lids often defying the laws of physics. You arrive at the spot and attempt to open the offending lid with bare hands. After a few attempts of eyeball popping, wrist-wringing, muscle tearing effort applied to the jar…..nothing happens…. all you have really done is applied a large amount of torque, hands slipping (wheelspin) .…and achieved exactly nothing. This calls for reinforcements and in our house, it is the twenty-year-old ‘Tricky Dicky’, a flat piece of tacky rubber designed specifically to increase grip so that the lid gives way before our joints explode. You try again and after a superhuman grunt…eventually, the lid cracks open…you have now achieved something… work has been done…movement…and here is the key to this conundrum….
Torque resulting in radial movement… and caused by applying that torque = Horsepower… that’s it.
Don’t make it any more complicated than that and… horsepower is work done… Torque does not do the ‘work thing’ at all…torque needs to be applied repeatedly to generate horsepower or work.
Taking this analogy a step further. Lots of small jars opened quickly is lots of horsepower (Jam jars per minute, aka rpm), and can be equated to an engine producing good horsepower at high Rpm. Fewer very large jars containing huge amounts of jam, done more slowly and requiring massively more torque to open (jam jars per minute, aka rpm), can be equated to big horsepower at low engine speeds…that’s the simple explanation.
…and here is the good bit…opening lots of small jars quickly can give you exactly the same amount of Jam (horsepower) as opening big jars slowly. This explains why for example, when a diesel truck engine produces 600Bhp at 2000rpm and a tiny petrol racing engine produces 600bhp at 10000 rpm they will both be doing exactly the same amount of WORK. In fact, if you adjust the gearing to suite, that race engine would do the job in place of the engine in the big truck…and sound a lot better in the process.
When we refer to a torquey engine, what we are really saying is that the engine produces relatively good horsepower at low speeds..or as a ‘screamer’, good horsepower at high engine speed. That is all there is to it…we should technically not be using the word torque at all to describe performance…but it has crept into our vernacular, has a certain connotation attached to it and it has stuck. At the end of the day, that’s OK because it does convey a meaning we all seem to understand in a similar way, despite the fact that it is technically misguided.
But let’s get back to that all-important word ‘response’. As car buffs we all like to know the horsepower (Ok Ok… & torque) of things and yes that number gives us an idea of how well a given machine will go but absolutely does not let us know how it will feel when driving it. In my early days as an engineer, I was fortunate enough to get to understand this and able to work on engine response issues. It stemmed from an experience in the very late 60’s driving a 1600 Ford Capri against a 1600 Opel Manta, it is a very basic example but explains the issues that can affect the feel of an engine.