Torque Vs Power



Just as R.M.S, a term used in electrical power measurement is often incorrectly used, the correlation between torque and HP or Kilowatt is often misunderstood. A colleague in the motor industry mentions that at least 40% of his clients pass on information which they have ‘overheard’ when purchasing new camshafts without understanding the true role of both HP and torque in camshaft design.  The same goes for engine re-mapping. Clients are eager to see the HP but know very little about torque. Remember the huge problem MotoGP riders had when changing over from their 500cc two-stroke machines to 900cc 4-stroke machines which delivered substantially more torque at lower revs and literally ate tyres coming out of corners. Although the racing engineers know a lot more about these things than a mere layman I recall this very clearly from the first races, possibly still a major problem. The bottom line is you cannot just push HP up without it affecting your torque figures, the reciprocation is just as true.

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HP is the amount of work that an engine must do over a specified amount of time, torque is defined as the force applied at a radius from a given point. One HP is universally defined as the power required to lift 33,000 pounds one foot in one minute. Work is measured in foot-pounds, torque in pounds-foot.



Confused?

  • Power is the amount of work that can be done over a specific time measured in foot-pounds.
  • Torque is a standard used in rotational equipment, such as can be measured on a internal combustion crankshaft, armature of an electrical motor or output shaft of a turbine or the tendency of a force to rotate an object about an axis, fulcrum, or pivot measured in pounds-foot. Using a torque wrench is a great practical demonstration of this.  See note * below.

The crux of the matter:

HP and torque ARE linked.  More horsepower means more torque. More torque means more horsepower.  HP = (Torque x RPM)/5252.  Torque = (HP/RPM) x 5252.

A dyno measures torque and calculates the HP from the formula above.

Some interesting facts:

  • If the engine can rev over 5252 RPM, HP will be higher than torque. At 5252 max RPM HP = torque and lower than 5252 RPM, torque is higher than HP. Look at the diesel engine.
  • Torque doesn’t always mean better pulling power, look at the gear ratio.  Trucks and diesel vans often have more gears to go through than a petrol (gas) engine.
  • Riding at maximum torque does not necessarily use less fuel, often a good read in magazine articles and manufacturer claims. The more explosions per minute and the more throttle going up a hill = more fuel use.

Note *  The torque wrench reads how many foot-pounds of force that you are exerting on the shaft. If the handle on your wrench is one foot long, and you place one pound of pressure on the end of the wrench, you have exerted one ft-lb of torque on the shaft.

Now for something completely different:  Steam Engines Vs Internal combustion Vs Electrical

Try to spin your wheels in 4th gear with a petrol engine. A steam engine can do this. So can an electrical motor. Both steam engines and electrical motors deliver extremely high torque at low revs – try to hold the shaft of an electric motor when it starts up. An internal combustion motor generates a force acting on the piston internally, as the name applies. A steam engine doesn’t – the pressure acting on the piston is generated externally. Like the magnetic forces in an electrical motor. Icebreakers use diesel electric power. Can you imagine a diesel locomotive with a clutch and gears?

Electric cars are known to accelerate very (as in extremely) rapidly. Steam trains can pull an extremely heavy load at very slow speeds. Steam trains fall into the external combustion category whereas the motors covered above i.e. diesel and petrol are internal. External combustion motors are all very high torque low RPM motors. Steam turbines are external combustion. Nuclear reactors are external combustion. A chief engineer once told me “why make something go up and down to make something else go round and round?” Never a truer word has been said.

So with racing cars we push the envelope with HP, work horses push the envelope with torque.  Long life and maintenance free engines = Longevity is more around low end torque than horsepower at high revs as can be seen by steam trains (again) and electric motors (again) and steam turbines (again) – all industrial use.  With the right gearing diesel engined cars have proven to be as good on the race track as petrol engines, especially in endurance racing where the efficiency of diesel has the edge. The disadvantage is obviously weight but with the new BMWs and Audis on the road and racetrack sometimes wonders if they are indeed diesel – huge steps have been taken in the power to weight ratio debacle. The question I pose then is just what does the diesel engine look like after 24 hours around a track?

And the winner is…. electric motors. Huge power to weight ratio. And the loser is…. electric motors…… Low power to weight ratio caused by battery power unless it is an electric train.

Have a look at this site,  electric cars are for girls….

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