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Engine torque explained (also power, speed and the connection between them)

When discussing torque, do you scratch your forehead? Think about horsepower? Other topic of conversation?
It can indeed be confusing. Although torque is the turning force that causes acceleration , it is not necessarily that the engine with the higher torque will have more power or be faster.

I’ll try to explain about torque and his other friends, the power, acceleration, and speed in the following article.

What is torque:
Torque is a turning or twisting force and is a measure of an engine’s ability to rotate the wheels. Torque is measured either by Foot X Pound or Kilogram X Meter. The torque is initially produced by the piston. The force is then delivered to the crank and then to the transmission and on to the wheels.

Every vehicle will accelerate accordingly to it’s torque curve. Every engine has a different torque curve.
Did you ever see two cars compete when one accelerates faster but then the second car begins to close in, and then passes the first? It means that the first car probably had more torque in low RPM.
What more, when accelerating, we can only feel the torque, while the HP (horse power) is only a figure to measure the total work for a given time. 

How is torque created?
In an piston engine, the combustion forces the piston to go downwards, pushing the connecting rod that forces the crank to rotate. Although this happens fast, what that is is quite. A good analogy will be your hand trying to open a bolt with a ranch. Your hand is the piston, the ranch (leverage) is the distance between the crank center (shown as axis) and the center of the big end connecting rod or the length of the ranch you’re holding:

How to produce more torque in a given engine:
There are several ways to do that.  The bigger the diameter of the crank OR piston travel/stroke (which are all similar in length) , the more torque will be generated.
Another method is raising the compression ratio. The more compression you have in the combustion chamber, the higher the force on the piston and the higher is the torque. That is exactly the reason why diesel engines, which compared to gas engines have almost as twice compression ratios, have LOTS of torque produced in ridiculously low RPM.

There are more complicated methods of raising the torque. Some engines use variable valve timing, but this method is more of a way of not loosing the potential torque than it is of creating more torque. The basic idea is that in order to produce more horse power, you have to rev the engine higher (horsepower=work X time).
The faster the engine spins, the less time the fuel mixture has, to enter the combustion chamber, so these engines have long opening valve timing. But, what’s optimal for high RPM, isn’t necessarily optimal for low RPM, so variable valve systems, vary valve opening length according to the engine’s RPM. One motorcycle that has this kind of system is the Kawasaki GTR1400 Concours.

Another variable that effects torque is intake quality. In general small ports (intake tunnels) increase fuel mixture speed, increasing atomization quality of the fuel mixture (gas with the air). Motor companies know that, and some have come up with useful ideas:

  • Honda V-tech uses 4 valves per cylinder (2 intake, 2 exhaust). At a low TO mid range RPM, Only 2 valves open (1 intake, 1 exhaust) . From a certain RPM (approx 6000 rpm)  the system opens 2 more valves.
  • the old (and better in my opinion) Vmax v-boost system, used relatively small 4 carburetors. At low RPM, each cylinder used one carb, but at about 6000 rpm , a valve opened to allow the cylinders to breathe from two carbs each, instead of one.

A good analogy to understand why small opening enhances fuel flow is a water hose. If you try to partially block a running hose with your finger, the flow speed will dramatically increase, making the water reach way farther. The same happens in an engine.

Can we enjoy both worlds – High torque and high output (horse power) ?
The more extreme the engine is, the harder it is to produce torque that is available all around the RPM range.
Hyper sport engines produce their peak torque way up in the RPM, which gives them the ability to use every GEAR up most. So they will be probably not too strong at the bottom RPM as in the HIGHER RPM range.
On the other hand, an average tourer will produces a linear torque all around the RPM range but will have a lesser top speed.
There are technical limitations as well. In order to produce a high revving engine, it’s piston must have a relatively short stroke which means it has a small diameter crank with short strokes. As you remember before, a stroke equals leverage. The shorter the stroke is , the shorter the leverage is, meaning the piston produces less torque. You can feel it especially in small to mid size sport engines 250-600 cc, they can be super fast, but don’t have much torque in low RPM. To get them moving, you need to be heavy on the wrist 😛

Do you need lots of torque for a high top speed?

Not necessarily. A Harley Davidson Electra Glide has about 13 KGM (92 lb ft)  torque , about twice as the common 600 cc hyper sport motorcycle has, yet the later will be much faster. There is more emphasis on the RPM , which the peak torque is produced. And I’ll give a classic example – the two stroke 125 cc Cagiva Mito. This sport bike had a tiny 125 cc engine, with little torque produced way up in the RPM range. Even so, it had a top speed of aprox 180 kph, similar and maybe higher to that of 400-500 cc bikes. So, even though this engine had a torque similar to this of a Vespa PX scooter, its peak torque was located way up in 8500-9000 rpm, allowed it to fully use its gears, and reach these high speeds.

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