I've written this article to explain the differences between measuring BHP (power) and torque, and illustrate the effects of power and torque when comparing petrol and diesel engines.
I'll try and start with the very basics, and build up the technical stuff until you get lost! Don't worry, I'll be using crates of apples soon...
In order to simplify a complicated subject, I've made quite a few broad assumptions, many of which are not entirely true to life, but have been ignored for simplicity.
To most, power dictates how fast a car will go, and torque is something to do with "pulling ability". Well from now on, open your mind and forget what you thought! Torque is in fact the easy bit, power is the stuff of physicians...
What is torque?
Well forget power for the time being. Gasses and stuff are exploded in your engine, and they make a very high pressure above your piston. This pushes the piston down, which then pushes your car along the road. Simple! The amount of push, is torque.
So, double the push (torque) and you accelerate twice as fast!
So, lets look at how much torque a theoretical engine makes at different revs:
Ah! This engine has 250 ft lb torque from 1000 rpm to 4000 rpm. So lets imagine we open it up in third gear, starting at 1000 rpm. We feel a constant push in the back right up to 4000 rpm. That because the amount of torque = amount of acceleration.
Well it had to get tricky at some stage! Let's do the same as before but in fourth gear. How much acceleration do you feel compared to third? Less. In fact you know you get most acceleration in first gear and least acceleration in sixth. Why??
Well, the engine turns fast and the wheels turn slow. So we have to reduce the speed of the wheels by a gearbox. If you reduce the speed of the wheels to half that of the engine, the torque at the wheels is doubled!
Bit tricky to understand - but you can imagine it. Imagine we attached your engine to a special gearbox, so that when the engine was going at 2000 rpm, the output shaft of the gearbox was turning so slow you could see it turn. Attach a mole grip to that slow turning shaft. Are you going to stop it? No way. In fact could you ever grip it hard enough with any device to stop it? No. That's because the turning force (torque) is very high. Now imagine your engine also at 2000 rpm, but this time connected to another special gearbox with the output shaft turning at a trillion rpm. So fast you think the world is going to end. Now grab the shaft and you could slow it down. There's not much torque there now!
So back to the car. In first gear, the wheels are turning slowly, so there's lots of torque. Great until the engine can rev no more - so we have to select second gear. Now there's less torque at the wheels, and so less acceleration. And so on up the gears. Here it is, visually:
See, loads of go in first, and less as we go up the gears. At some point, the force in our wheels is equal to the wind resistance force, and we can't go any faster. In this graph, it's at about 135 mph. So torque is related to top speed as well as acceleration! Let's use torque forever more!!
So why do we not measure cars by torque alone?
You may have noticed that the car above is a diesel. Lots of torque but only 4000 rpm available. So what, you may say. Well imagine you're having a race against your mate in an identical car, except it's got less torque but the ability to rev to 6000 rpm (i.e. a petrol car).
You both start off in first gear, and you race ahead (you've more torque at the wheels than him).
Then you run out of revs at 4000 and have to change gear into second. Now you've got less torque at the wheels as he continues to race up to 6000 rpm, still in first gear.
But then he has to change gear into second and now has less torque than you and you catch up.
But now you run out of revs and have to change gear, and watch him scream up to 6000 again.
And so on.
Lets look at a picture of the amount of torque (thrust) at the wheels as the two of you drag race:
You are in the purple car (nice choice!) See how after 2.5 seconds, you have to change gear and he (in the blue) has more thrust than you do in second gear.
Damn! Pretty obvious that you need torque and revs! Both are important. The two cars above in fact in a drag race are equally matched. So with torque and revs alone how do we easily compare cars?
The diesel car above has crates of 60 apples. But just 2 crates of them. The petrol car has smaller crates of 40 apples. But it has 3 crates. Yes we want large crates and lots of them!! So we multiply the number of apples in a crate, by the number of crates. Easy. Both cars have 120 apples, and so both are even.
Can you see that we also need to multiply torque by revs ('cos we want lots of both of them too), in exactly the same way to see how much go we have in total? Well guess what:
Apples in a crate x number of crates = Total number of apples.
Torque x revs = Power! Yep, power is the same as number of apples!!
So power tells us, overall, how well the car is going to go. Yippee! Don't try to understand it too much, just remember how I got here and it all makes sense. More power = better performance. But that's by no means the end of the story, and it was based on theoretical cars...
Well, my imaginary diesel (above) had a flat torque curve. So that's equal thrust all through the rev range. In reality a diesel has a low peak torque, which gradually declines as you rev higher:
A petrol car has a torque curve that peaks about 2/3 of the way up the rev range, but it has torque figures that are less all the way along the curve:
This makes sense with what we feel when we drive the cars. The diesel seems to really accelerate well at low revs, then it fades out a bit and then we have to change gear at only 4000 rpm. The petrol car seems to increase in acceleration as we climb up the rev range (though it does fade a bit higher up), and we can keep going all the way to 6000.
So what's the problem? I now know I want power! Well, the problem is this. We only look at peak power. So that's normally right up at the top of the rev range, where we have both lots of revs and still a fair amount of torque. This tells us very little about what happens in the rest of the rev range, where we spend 99% of our time driving.
I think the best way to show what differences there are, is by doing an experiment.
Take one car.
Change nothing except the engines.
Diesel, 190 BHP, with a flat torque curve at 250 ft lb
Petrol, 190 BHP, typical petrol torque curve (193 ft lb at 4500 rpm)
Diesel, (tuning box style) 183 BHP, but with a big torque peak of 291 ft lb at 1900 rpm
Do some performance runs! (Luckily, I have the software to do this for me...)
Drag race: First past the post is the flat torque diesel, with the petrol very close. The peaky torque curve car suffers a bit. Note how the petrol is first to 100, but the diesel is first to do the standing quarter. See how gearboxes muddle the situation horribly?
In-gear acceleration (i.e. overtaking). At low revs, the peaky diesel makes use of the peak. But at high revs the flat torque diesel makes better use of its higher torque figures. In all cases the poor old petrol, that did so well in the drag races, is suffering badly. In fact unrealistically so. Why's this? Well these figures are correct if we did the experiment as I've done it. But in reality manufacturers have to put different gearboxes in diesel cars, simply because the rev ranges of the engines are different.
So if the petrol car changes from second to third gear at 60 mph (at 6000 rpm) then the diesel does too (but at 4000 rpm). To do this the diesel gearbox isn't reducing the wheel speed as much, so the diesel car has less torque than it would have had, if it had had the petrol engine gearbox in! Oh no!!
So we'd better pop in the proper gearboxes and see what happens:
See how the thrust is now very similar between the petrol and flat torque diesel cars?
Now lets do the full analysis, complete with the peaky tuning box style diesel (less power, more torque):
See how there's not much in it? Drag racing times are very similar. In-gear acceleration times are similar at high revs, with the diesel getting the edge at low revs (high gear). The tuning box diesel, with it's high peak torque figure, is slower in a drag race but does a pretty good job at higher speed when the revs are near its torque peak.
Peak power figure is a very good way to compare performance. Both 190 BHP petrol and diesel cars are an even match. This may explain why people have been using power to compare cars for years and years...
In-gear acceleration times are affected enormously by gearbox type. Comparison can only be made if the gearbox and final drive are identical (which they never are).
A 250 ft lb diesel with a flat torque curve goes better than a 291 ft lb diesel with a peaky curve, in most situations.