Power and torque

Posted on January 29th, 2008 in Hybrid Power,Opinion,Power,Turbocharging by Julian Edgar

torque-curve.gifThe (repeated) articles that we’ve recently run in AutoSpeed on power and torque are vital to understanding how to make your car go harder.

(The series can be found at Power vs Torque Part 1 –  and Power vs Torque Part 2)

And why is this understanding vital? Simply because people who use the terms ‘power’ and ‘torque’ often don’t seem to really understand what the words mean. The vital point to realise is that engine power is worked out by multiplying torque by revs.  And that’s the only way that power is worked out!

So an increase in torque at – say – 2500 rpm will mean a proportional increase in power also occurs at 2500 revs. It’s therefore just plain stupid to say “there wasn’t any change in the power curve but we got an increase in mid-range torque…” as some manufacturers of performance equipment state.

In a given engine, torque depends entirely on how hard the controlled explosion of combustion pushes down on the piston. More pressure from the explosion will result in more torque. So if you fit a turbo which jams into the engine more fuel and air, the resulting explosion will be bigger, and so at those engine revs more torque will be produced.

And if you can keep those cylinder pressures high as the engine revs increase (that is, maintain a flat torque curve), then there will also be more peak power produced.

And, contrary to popular opinion, it’s power not torque which gets a car performing. Why? – because you can always increase the torque getting to the driving wheels by selecting a lower diff or gear ratio, but you can’t increase the power that way….

An engine with a flat torque curve that revs hard generates a huge amount of power, all because the torque at high revs is also high. A poorly breathing engine drops torque quickly as revs increase, and so to have a high power figure, it needs to generate mountainous low rpm torque.

The key point to remember is that you want good cylinder filling (breathing) to give a high torque figure, and you want it to happen all the way through the rev range so that the power figures are also high.

However, for a road car, that explanation isn’t the full story. Because for ninety per cent of the time you’re doddering around at low revs, a car that has a low torque output but which maintains it at high revs (and so has high power), will feel much more lethargic than a car with high torque at low revs – but which drops that torque curve away quickly to produce, in the final analysis, lower peak power.

While I was once very much of the school that states that a flat, low, torque curve that extends to high revs is the way to go, I’d now be much less dogmatic. Instead, I’d say that a small engine with forced aspiration (eg a turbo tuned for low rpm boost, or a positive displacement supercharger), or a small engine with electric motor assistance (ie a hybrid), probably gives the best road car outcome.

2 Responses to 'Power and torque'

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  1. Wayne said,

    on January 29th, 2008 at 6:14 am

    Sorry to divert attention to your comments above. I recently purchased a VY S Series 2 sedan, and so was quite interested to find that “The Zero Cost Holden Intake Mod” was applicable.
    I wasn’t so much interested in the power increase, as i have an 85km commute, 90+% of which ( by distance) is at cruise conditions. What I was interested in was a possible increase in fuel economy.
    My gut feeling was that it would still help, but wanted to know for sure so I got out the trusty Magnehelic Gauge and connected the low to the snorkle path, and the high to in front of the radiator. I found under most conditions that there was slightly more pressure available (.2 – .4 ) inches of water in front of the radiator. This should provide a slight (probably immesurable fuel economy increase under my circumstances. At that point i wanted to see how much positive pressure was being generated so I disconected the low pressure line it showed approx .4 to .6 inches of water. not bad i thought. Maybe it would be higher with the windows open (might be positive pressure in the cabin), so i opened the front windows, it jumped up 1 inch of water. Closing the front and opening the rears seemed to get rid of that inch of water, so i assume that opening the front windows created a vacuum! Opening both rear windows further, caused an almost painful buffeting which quite surprised me. Has anyone else noticed this?

  2. Henry said,

    on January 30th, 2008 at 3:26 pm

    I agree with your comments above that in every physically measurable way a car with a large amount of torque low in the rev range will drive better and generally outperform a car with perhaps the same or greater power but less torque.
    However one important aspect which has not been discussed and is difficult to measure is how the car feels.
    For example. I once owned an EL Falcon similarly modified as your EF and at the same time a standard pular SSS. Now the falcon has roughly double the torque and 80% more power than the pulsar however peak torque is at 3000RPM and there is little point pushing it to the 5500RPM redline. In the SSS peak torque is aroud 5000Rpm and redline around 7500RPM. It was the pulsar that was more entertaining to drive because the power wasn’t easily obtainable, you have to work the gears and increase the speed to stay “on top of the power band”. This was much more rewarding and it invites you to push your limits becasue extracting its maximum potential was challenging. You get this feeling that “next time I can go faster” and even if you can’t its fun trying. The Falcon by constrast was somewhat one dimentional, By having all its torque readily available you tended to use it more in normal driving, thus when you wanted to press on you found the car’s limits quickly and easily. This was eventually uninspiring even if those limits were higher than the SSS.
    In the end the Falcon was faster, more acessable and better for the real world than the SSS. However I have since sold it and still have the SSS, I don’t regret that for an instant.

  3. Ben said,

    on February 3rd, 2008 at 10:51 am

    Ideally low down torque would be a buil-in trait of all engines. It’s where the vast majority of people drive, regardless of whether there is any advantage to it or not. Look at the Kia Sportage. 100-odd kw, peak torque at 4000rpm, but it sounds so harsh at that revs that most people don’t see the far side of 3500.

    A 1.6 with a small turbo would be worlds ahead in low-mid rpm torque, NVH, fuel efficiency, toss factor, upgrade potential, etc without sacrificing peak power. But it would cost a little more to make and the word turbo would instantly mean that cheap insurance won’t happen. Also, I can’t see a turbo option being available without ‘upgraded’ interior, bodykit, and loads of stickers. On a Kia anyway…