TAKE a Maestro or Montego Turbo – put it alongside a Citroen BX 16V and race them from the lights. The result should be pretty easy to predict, really. The Citroen has 160bhp at its disposal, and a rev limit of 7300rpm – the kind of crank speeds that O-Series engines can only dream of unless seriously modified. This compares to 152bhp of the Maestro and Montego. Also, the British pair has longer gearing, and a higher kerb weight.
So, as I say – an easy outcome. The Citroen romps home ahead.
Where the Citroen takes 7.4 seconds to reach 60mph, the Maestro has long since gone, reaching the oft-used landmark a half a second earlier. So why would that be, when in theory, the revvy Citroen with its Mi16 (XUJ9) engine holds all the aces.
The answer lies in something called torque, of which the Citroen has a less than sparkling 133lb/ft against 169lb/ft of the British contingency. But what exactly is torque, and why is it my bum dynometer tells me a car has good torque characteristics without fully understanding what it is.
I do know it’s that less than glamorous commodity that turbo diesel owners tend to bore us all about in pub arguments, using it to explain why their cars are so great at overtaking without changind down a gear or two. Great. But ask any of these guys just what torque is, they soon clam up, avoiding eye contact in the same was as you did your teacher at nursery school when they were chosing the next person to see the nit nurse…
So here’s a challenge to austin-rover.co.uk readers – the person that comes up with the best real world explanation of torque, in language I can understand, and without mentioning bars ome metre long will not only earn my undying respect, but will also win a nice free Vanguards model of the MG ZT in a rather fetching shade of Trophy Blue.
Yup, that’s right – the explanation I most like, and most readily understand will gte published here, and will also win a FREE model for its author. So, come on tech heads… enlighten me.
Try and undo a wheelnut on a car with a short ratchet (say 12″ long), and then do the same with a 3 foot length lever. You can put the same force from your arms/weight into the lever to try and turn the nut (which may need say 80 pounds of weight applied to the lever to move) but the 3 foot lever gives you 3 times the mechanical advantage over the nut as the 12″ lever. This means you only need to “push” 26 (and two thirds) pounds of weight on the end of the long lever to provide 80 pounds/foot of turning force on the nut. One thing you have to remember, you have to move the 3 foot lever further though – as you don’t get the reduced “effort” for free.
It gets a whole lot more complicated when you’re talking about engines, but similar rules “sort of” apply.
I have to say its one of the things that I always got confused about also. The best description I’ve ever heard (and been able to understand) is quite simply :
TORQUE is low engine speed pulling power
HORSEPOWER is high engine speen pulling power
Basically amounts to the same thing but at different engine speeds.
Power is the amount of water you can carry up a hill in a bucket, torque is how fast you go up a hill.
Lots of power is lots of water but will take all day, add some torque and you can fly up the hill (i.e., if you had a 1hp engine developing 10lb/ft, the engine will move 1 bucket of water 10 ft up the hill in 1 minute. 1hp engine developing 20lb/ft the bucket will move 20ft up the hill in 1 minute.
So back to the two cars
BX 160bhp and 133 lb/ft = 0.83
MG 152bhp and 169lb/ft, = 1.11
Therefore the MG is 0.28 more powerful than the BX. Which is why it can go quicker.
To put it very simply: The power delivery of an engine is torque times revs…
So the ideal engine has a flat torque curve with the same amount of torque for all revs leading to a linear rise in power as the engine revs raise. So here’s the solution why the Maestro is head on: For most of the time it simply has more power then the Citroen…
Torque is basically an equivalent word for “Force”, only we use it to describe a rotating force. In terms of an engine it’s the basic measurement of how hard (not fast) the engine can push and once that’s been delivered to the wheels, punch you in the back. the “Torque curve” is basically a graph of “if the engine is doing X RPM, it can push me back into the seat Y”. Which, if you work it all the way back through the drive train of the car (Wheel>Transmission>Crankshaft>Pistons>Exploding mixture) is a measure of how big an explosion you’re getting at any given instant. It’s describing the efficiency of the engine at making hydrocarbons and air go “BANG” at any given RPM (assuming we’re holding the throttle wide-open), as that’s what affects it: various engine designs are better at some RPMs than others.
Or, to use an analogy, it’s describing how hard you can push the pedals on your pushbike. Torque is how hard you can push the pedals, Power is how fast you can keep pushing them that hard.
The Maestro has low-down torque because it’s got a turbo on it. For reasons I’m sure you’re well aware of, this means bigger bangs. until the engine gets to high RPM and it finds it can’t suck in or, or blow out, as much gas as it wants, meaning the top end of the torque curve has fallen off. so it can push the pedals hard, but it runs out of breath.
The BX, being naturally aspirated, never makes bangs as big as the Maestro can make (lower peak torque) but what it can do, is continue to make big bangs when the engine is spinning faster, as the four-valve heads allow it to get more air in and out of it. It can’t push the pedals as hard as the
maestro, but it can keep pushing the pedals quite hard at high speed. The advantage to this is that this ability to keep it up at high revs allows you to run shorter gear ratios, which gains you back at the wheels some of what you’ve lost at the crankshaft- the downside being that the driver has to do more gear changes.
Torque is generally associated with the stroke of an engine, that is the diameter swept by the big end journals on the crank around the main crank bearings. In general revvy engines will tend to have wide bores and short strokes, so greatly reducing the amount of torque versus power they produce – great for getting off the line, but not so good for in gear acceleration.
The longer stroke is good for performance as it makes it easier for the piston to push the inertia of the flywheel around. Go to your front door and open it with the handle. Now close it and try opening it by pushing about an inch from the hinge – much more difficult. Essentially in a car with a very short stroke, the engine is trying to push the car along “an inch from the hinge”, whereas a longer stroke allows the piston to act further along the door if you like, so allowing more of the power to be used in propelling the car forward.
A long stroke doesn’t tend to allow for a revvy engine, as it takes more time for the piston to complete one sweep round the crank. Imagine a gerbil in a wheel, connected to a 600kVA generator to supply electricity. This generator will need a certian amount of power input to overcome starting inertia. If you connect a six inch diameter wheel to the shaft, then the poor gerbil will expire trying to move this thing – it will not have enough torque. If you then provide it with a sixty foot wheel, the (new) gerbil will be producing the same mechanical force at its legs (one brake-gerbilpower), but the much larger diameter wheel will allow it to move the generator albeit very slowly. The principle is similar for a car’s engine / flywheel, with the gerbil as the engine and the flywheel as the generator. The engine finds it easier to turn the flywheel, and so move the car if the stroke is larger (larger wheel for the gerbil), hence it makes the application of power easier.
Is the Editor of the Parkers website and price guide, formerly editor of Classic Car Weekly, and launch editor/creator of Modern Classics magazine. Has contributed to various motoring titles including Octane, Practical Classics, Evo, Honest John, CAR magazine, Autocar, Pistonheads, Diesel Car, Practical Performance Car, Performance French Car, Car Mechanics, Jaguar World Monthly, MG Enthusiast, Modern MINI, Practical Classics, Fifth Gear Website, Radio 4, and the the Motoring Independent...
Likes 'conditionally challenged' motors and taking them on unfeasible adventures all across Europe.
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