Power versus Torque? What to choose?

Hi,

I am planning to purchase a car pretty soon. I am reading reviews and studying the details about the engine and other information required to make a decision. However, I always get confused between the torque specification of a car and the engine specification

Can somebody please explain me that is 40 N-m@5000 rpm better for a car or 50 N-m@4000 rpm better? How much does a high x N-m@y rpm affects the performance of the engine, and it's fuel efficiency?

Another question, on similar lines, is that 40 bhp@5000 rpm a better engine or 50 bhp@4000 rpm a better engine?

Whenever I read a car review, I really get confused with these terms.

Would be grateful if somebody can explain the difference in them. Also, what role does gear ration play in these rpm decisions?

Harsh

Do not even bother yourself with these figures,they mean nothing to me.But focus on the performance figures like 0 to 60,0 to 100(only for highway use),20 to 80 acc in 3rd gear and 40 to 100 in 4th gear.Also see fuel economy for city and highway use as per your needs.I feel Autocar Magazine is the best place to do this study

@Harsh,

Generally speaking greater N-m (or bhp) at lower RPM means that the engine has better low-end torque/power - this means that it can pull heavier loads (or go up slopes) at low speeds without having to press the accelerator hard. It also means that the car will be easier to move from stand-still.

When you compare cars, first of all, compare those with power ratings specified for the same (or similar - within +/-100) RPM. Then simply divide the power (or torque) figure by the car's kerb weight. Choose the one that gives the highest value (some might tell you to divide the car's kerb weight by the power/torque figure - in this case choose the one that gives the lowest value). What we have done here is basically calculated the power-to-weight ratios of the cars and compared them. In essence a more powerful engine in a lighter car means a better (or faster) car - and it also has the potential to give better mileage.

ideally, higher torque at lower rpms is more useful. the torque vs rpm curve should be as flat as possible

T O R Q U E

Hey Rohit,

Thanks for your reply. It helped a bit in clearing the confusion I had. 

One thing I would like to ask is that do we really have terms like low-end torque or something in car terminology?

Other thing in my mind is whether gear ratio plays any part in delivering the power generated by the engine to the wheels? I read an article a few days back, which mentions the the gear ratio involvement while considering the torque and engine power.

http://scienceblogs.com/startswithabang/2009/04/what_does_torque_in_a_car_do.php

The discussion about gear ratio is present in the comments section of the article.

Would really appreciate if you could clarify on the same.

Thanks,

Harsh

For the most part we need not bother about gear ratios when it comes to power or torque - it is de facto that the 1st gear transmits maximum torque to the wheel and hence is used to move the vehicle from stand-still. Power or torque determines the car's load carrying capacity - but not necessarily speed (this is determined by the gear ratio).

From an operation view-point as the gear ratio increases from 1st to 5th (or higher) less of the engine's torque and more of the engine's RPM is transmitted to the wheels for speed. Thus you will see that once we reach a certain speed while accelerating, we need to shift to higher gear (and vice versa) in order to keep the engine's RPM more or less constant.

Putting these into perspective, an engine with great low-end torque for a given weight will have great pulling power but not necessary good acceleration. However, put that same engine in a lighter car witha modified gear box and you can get superior acceleration - a faster car.

@ Harsh Arora

This is a very interesting and exciting discussion.  But deeper, we will go, more confusing it will become unless one is a rocket ...  In a complex machine like modern car with so many interacting components, simple rule of Physics become very complex.  To a large extent, discussion provided in the link of Harish is fair.  Here is my two cents worth of automatic confusion.

Gear Ratio plays  very crucial role, in whole scheme of things in an automobile and is hidden under the garb of gear box.  Engine output power is not directly fed to front wheels but through gear box, which acts like a mechanical engine output power magnifier (just like Amplifier in your stereo or hydraulic system of your car brakes).  To move a standing car you need lot of torque and first gear magnifies the engine output torque.  Remember, Fist gear has the highest gear- ratio and fifth gear has the lowest gear-ratio and you may find # in your car's manual.  You can see why low-end torque (torque @ lower rpm) is so important.  But gear box as such has no power or torque.  So, one time you drove 200 Km @3,000 rpm and other time you drove 100 km @4,000 rpm, When did you get better FE?  Car enthusiats and FE can never live together happily - just like ride and handeling.

For simplicity, take an example.  Say car-1 generates torque of 100n-m@1,000 rpm and car-2 generates torque of 150n-m@5,000 rpm.  You guessed it right, car-1 will accelerate faster initially, if all other parameters are same, like weight etc.  This also proves, why power-to-weight ratio may not give the complete picture of car dynamics.  This is why Hyundai i20 (1.2L) will accelerate faster than Honda Jazz (1.2 L), though power-to-weight ratio of Jazz is higher.  Though I tried to make everything vary simple to have basic understanding  and I hope all of you got it   .  In real cars, everything is so damn complex and we may end-up in endless argument and hair splitting.  So what is the solution, drive it and feel it or measure it.

Another example, Hyundai i10 Kappa has same eninge as Hyundai i20 (1.2L) but i20 is heavier by about 150 Kg. to i10.  So people conclude that, obviously, i10 will accelrate faster than i20.  Wait a minute, not so fast.  Does it mean, if I add two average person in i10 rear seat then i10 and i20 will accelearte at the same rate.  What will you say? 

@Amit,

Quoting you...

"Another example, Hyundai i10 Kappa has same eninge as Hyundai i20 (1.2L) but i20 is heavier by about 150 Kg. to i10.  So people conclude that, obviously, i10 will accelrate faster than i20.  Wait a minute, not so fast.  Does it mean, if I add two average person in i10 rear seat then i10 and i20 will accelearte at the same rate..."

If you add the same weights to both cars then the relative differences will still be there ASSUMING that the RPM range over which both cars are revved happen to be in the flattest part of the torque-RPM curves for both (as they have the same engine). Yes gearbox is important - but to a layman I guess its beyond comprehension (as it is for me). Hence as a rough guide we consider the car's power-to-weight ratio to determine which is better. The point of all this is that an engine that produces lots of torque at lower RPM stands a better chance of giving performance/mileage based on the gearing - the low-end torque can be used to accelerate better (put out more speed to the wheels) or to pull heavier loads (put out more torque on the road through the wheels), but not both - as the gearbox comes into the picture.

Let us not complicate things.

Power = Torque x RPM and Torque= Force x distance

So, its the power which is the final requirement when it comes to engine. The torque for the wheels could be played with gear ratios and differential ratio. 

Just FYI: Engine torque can be increased by increasing the stroke length.

@ Topic starter: Gear Ratios help to vary the torque at the wheels.