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 we know Power = Torque X Rpm. Rpm is the speed at which the power and torque readings are considered. 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 bike to higher revs. This type of engine has to sacrifice lower end torque. Why? The engine has to be made oversquare to make it rev higher. Lesser stroke results in lesser torque. As the bore is large, this type of engine can use larger valves. So while the engine has fantastic power, it needs to be revved higher. This affect low speed operation - these engines are not relaxed at low speed - in other words, insufficient torque to pull effortlessly from lower rpm. But they are the more powerful engines.

Eg - R15 engine to some extent. Apache RTR 160 to some extent. More examples still to come.

2. Horse power = lower rpm X enormous torque - This type of engine does not rev higher but has enormous torque at lower revs. This type of engine has to sacrifice higher rev power. Why? The engine has to be made undersquare to make it generate higher torque. Greater stroke results in greater torque. It also limits max rpm - due to vibrations and piston speed limits. At higher rpms, this engine looses efficiency. Why? This engine has lower bore, so it must use smaller valves. This affects breathability of the engine. Also, this type of engine has strenghened lower end (like diesel engines). So, more weight too. But, the low end torque enables the engine to pull effortlessly at lower rpm- the engine feels relaxed.

Eg, - Zma engine to some extent. Bullet engine is a perfect example!

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 affect us. Hence, we prefer a poweful oversquare engine for flat out riding. 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 undersquare engine for city riding.

Gearing, Carb, Head Design and Cam profile helps to some extent to modify the characteristics of the engine. But, try to make an oversquare engine behave like an undersquare engine - you loose engine effieciency.

The Internal Combustion engine has lots of improvement to make - only if petrol does not runs out sooner.

An one thing, any engine runs most efficiently at peak torque - so drive a bullet engine at lower rpms and a R15 engine at higher rpms for peak mileage! Don't be surprised if a R15 or Apache redlined through out the day returns decent fuel efficiency! If your bike is not giving you decent mileage, it means the engine is not suited to your 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.

Sirjee, kitna marks milega?

You forget another thing. The stress a bloody engine transmits to you (think of a p150 DTSiv1 cruising all day at 100 vs the same on a Uni) trumps alot of other things.

And yes, gear changes/gearing is also king. Bullet can thump its way around town in fourth gear. Others require hajaar gear changes.

That is because of poor refinement from Bajaj! A short stroke engine does not necessarily has to be poor in refinement. But yes, it does takes more work to make a refined short stroke single cylinder engine. Look at the litre class bikes - they are all short stroke but still refined! Even Uni is only slightly undersquare but still refined!

Superb explanation bro!! Extremely informative and cleared a lot of my doubts!

P.S. - In our mechanical class, our lecturer showed us a relatively huge 4 stroke engine which supposedly generates 15bhp at around 800rpm, and told us that our bike engines require around 8000rpm to generate the same amount of power because of its smaller size. He didn't explain why though. After reading your post, I know exactly why!

when u bring it to halt and then try the scene wil be different...even the incline which i am talking it would do easily wen itis in motion..if u stop it in midway then try to pull it would give up...jus u can feel that motor has no juice and will shutdown...my bullet had an all new engine internals and even my friends bullet didnt succeed...

i dont know about other bullets...

Thanks for the compliment!

i had a p220 and i was very much unsatisfied with its mileage and performance(ok now to my main question)

you were saying about two main category

1.Horse power = higher rpm X respectable torque

you said about r15 and apch 160

2.Horse power = lower rpm X enormous torque

you said perfect example is bullet.

i found that the p220 which i owned has a somewhat lower punch and a established midrange power but it tails out at high revs.
i cannot group this engine under any category, either manufacturer had made the balance of the both two category u said above

further at any revs u maintain ur ride , either at low revs or mid rev or high rev it never gives u a moderate mielage. if ur aware of facts about this kindly enlighten.

P220 is a example of the first category. It is an oversquare engine. Top end power depends on other factors too. Also for an engine to rev higher, valve trains need to be designed to operate at higher rpms. Fueling has to be proper. Ignition timing has to be proper. Also, as I had said - the camshaft profile can modify the basic characteristics of the engine to some extent. One of these factors limit the high rev performance of the P220. Either, it is intentional or due to R&D limitations of Bajaj. But the basic engine layout has the potential to rev high.

Best mileage of any engine is at the point where it attains peak torque. For P220 it is at 6500 rpm. It translates to around 80kmph (not sure). If you drive the bike at this constant rpm, it should give you maximum fuel efficiency. What usually happens is that we accelerate and deaccelerate constantly. And if we try to maintain this rpm in city, we will need to accelerate and deaccelerate even more frequently. Hence, we get poor fuel efficiency.

So for highway, aim for 6500 rpm constant speed riding. For city, aim a little lower rpm such that constant acceleration and deacceleration is avoided. Even though we are not riding at peak engine efficiency, avoiding acceleration and deacceleration will impprove efficiency.

Also other factors such as aerodynamics affects efficiency when the engine is placed in the bike.

yup , very true . @6500 rpm , its 88-89 kmph

basically the more heavy the engine is(longer stroke,heavy crank) the more torque it generates which is = lesser gear shifts,easier to ride,lesser stress on the engine,longer engine life,but on the downside these bikes lack top end prormance

eg bullets,yezdi's,the karzma etc

on the other hand light weight engines(short stroke,light internals) rev up better = more power which = more pickup(if the powerband is mantained),
but on the downside gears have to be changed often,and engine life is comparitivly less..

eg r15,rtr160,rx100 etc

I don't get the engine life part. Why will the engine life be less? Just because the engine revs higher?

exactly!
people who rev their engines alot have to face an engine rebuild sooner than low rpm riders..btw - there are a few exceptions

heres a short answer to the question

more cc = bigger cylinders = more torque.

but bigger cylinders means that they cant rev higher as they have further to travel up and down.

small cylinders means they can rev higher and produce more power.


hence take 1000 cc twin and inline 4 cylinders

twins have 2 500 cc cylinders and have more torque and will rev lower than the inline 4's

inline 4's have 4 250 cc cylinders and rev higher and hence produce more power

Without going into the intricacies of Eng language ....... I think what you mean is longer cylinders!! Bigger as in fatter cylinders will mean bigger bore not stroke!!

i thought that was obvious. but thanks for pointing out anyway.

so yea when the cylinder size doubles , it will mean bigger bore and stroke. but longer stroke means it cant rev as fast coz it need to travel further.