(courtesy of beaudaniels.com)Firstly, some definitions -
Cubic Capacity - It defines the size of the engine. We, consumers should not even think about this. The engineers who design the engine decide its cubic capacity such that the engine meets required power delivery characteristics. So important for engineers, not for us - unless we decide to modify an engine in which case higher cubic capacity engines generally have more potential.
Torque - It is the pulling power of an engine. More the torque, easier it is for the engine to pull load. So, there is lesser need to change gears in a high torque engine.
Power - It is basically the amount of work that can be done by the engine. Power is basically a function of torque - Power = Torque X RPM.
An analogy. In case of electricity, electrons move through an conductor under the influence of a force (potential difference). We know, Power = Voltage X Current. In this case - Torque (pulling power) = Voltage (force applied to the electrons), RPM (engine speed) = Current (electron speed) and power(Hp) = well, power (Watt).
Now, we all want torque. After all, torque spins the wheel! The question is where do you want your torque - at the lower end of the tacho and at the higher end of the tacho?
As can be seen, there are theoretically two ways to achieve engine power. Let us discuss both the ways in a simplified manner -
1. Horse power = higher rpm X respectable torque - This type of engine revs higher and has sufficient torque to pull the vehicle to higher revs. This type of engine has to sacrifice lower end torque. Why? The engine has to be made over square (cylinder bore is greater than stroke) to make it rev higher. Lesser stroke results in lesser torque. So, the torque curve is not flat - It increases as engine speed increases, peaks at some engine speed and then again decreases. As the bore is large, this type of engine can use larger valves. So, breath ability of the engine at higher engine speed is excellent - this results in higher top end power. So while the engine has fantastic power, it needs to be revved higher. This affects low speed operation - these engines are not relaxed at lower engine speeds - in other words, they have insufficient torque to pull effortlessly from lower rpm. But they are the more powerful engines of the two.
2. Horse power = lower rpm X enormous torque - This type of engine does not rev higher but has enormous torque at lower revs. The engine has to be made under square (cylinder stroke is greater than the bore) to make it generate higher torque. Greater stroke results in greater torque. So, the torque curve is flatter across the engine speed range. This type of engine has to sacrifice higher rev power and hence also has lower top engine speed - due to vibrations and piston speed limits. At higher engine speeds, this engine looses efficiency. Why? This engine has lower bore, so it must use smaller valves. This in turn affects the breath ability of the engine. Also, this type of engine has strengthened lower end (like diesel engines) to handle the strong torque pulses. As a result, it weights more too. But, the low end torque enables the engine to pull effortlessly at lower rpm - the engine feels relaxed.
Most manufacturers strike a balance between the two extremes.
What do we prefer? While riding flat out, we always stay in the upper revs. So lower low rev torque does not matter to us. Hence, we prefer a powerful over square engine for flat out riding. Eg- On the racetrack. If we cruise or ride in the city, we hardly get the chance to rev the engine to higher revs, so low end torque matters. Hence, we prefer a torquey under square engine for city riding.
Gearing, fuelling characteristics, cylinder head design and cam profile helps to some extent to modify the characteristics of the engine. But, try to make an over square engine behave like an under square engine - we loose engine efficiency.
Another thing, any engine runs most efficiently at peak torque - so we should drive a high torque engine at lower rpm and a high power engine at higher rpm for peak mileage as well as power! We should not surprised if a high revving engine red lined through out the day returns decent fuel efficiency! After that is how it was designed to run. If the vehicle is not giving decent mileage, it means the engine is not suited to the riding style!
As with most things in life, what we want to ride is preferably the best compromise of all these factors - power, torque, efficiency, engine life etc. There is no perfect engine suitable for everyone. Each engine offers it's own mix of character and style. What we buy should be suitable for the way we drive.
Bottom line - if you like to rev the nuts out of an engine and do not mind frequent gear shifts due to lack of low end torque - go for a over square powerful engine, else if you like to drive in a relaxed manner without excessive gear shifts and do not really want to squeeze the last bit of power from the engine - go for a torquey engine. It is as simple as that!
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