^^ adding to that wikipedia & some car related sites Also says that multi-cylinders engines have low friction ('cause of smaller bore & stroke, given the same displacement) compared to single cylinder engine (or 4 cylinder v/s 6 cylinder).
And different way of combustion; spark flame takes longer for full combustion of gases in a long stroke cylinder compared to short one.
@Samarth sir err.. I mean bro,
at what range max efficiency of an engine lies.. Max torque or max bhp?? (I'm not talking about fuel efficiency)

@Samarth That info gave some fantastic insight. Its true that an engine cannot explode like a bomb or something

@men_in_jean
I think that the safe limit that Samarth stated is actually dependent on the mass and quality of the components. A piston has to absolute reverse its direction of motion every cycle, the more mass it has the more is the inertia. So if a piston is made of iron, its gonna have a larger mass as compared to aluminium or better still titanium.

So I think a 250cc engine can be revved upto 12000 or above with a titanium (lighter) piston but if the piston is heavy like made of iron then the sheer momentum of it will bend the crankshaft and send the piston flying out of the engine which is what happens during blowouts. Ofcourse blowouts also occur when the heat and pressure inside the cylinder deform the shape of the cylinder walls itself rendering the engine useless. They also occur when components are not engineered perfectly, for eg a piston which has slightly off center center-of-mass.

This is also the reason why F1 engines can be revved up to 18000!!! Cause every component of the engine is simply perfect! The engines are still V8s but the US muscle cars with V8s redline at 6-7000 rpms whereas F1 go skyhigh!!

also the valves in an F1 car are pneumatic actuated whereas most use springs to close the valves (except Ducati which uses the cam to do the job), springs develop a 'fatigue' at high revs typically above 15000 RPM's, thus limiting their use in high revving engines.

Yes, quality of components determine the potential of the engine. There are chinese bikes costing around 40k which you can rev upto 11,500 rpm. But, you know how much quality they have and how frequently can they be high revved.
These Indian bikes (Bajaj, TVS, Hero, etc.) are much much better in quality than the chinese ones.

Piston travel (stroke) matters more than weight, because more piston speed= more friction.
Yes, Weight slows down revving but it doesn't limit it. For example, if you rev a heavier piston upto 15,000 rpm, its NOT dangerous within a certain limits.

Pistons generally are such light components (600-1000 grams), that it can't be made any lighter or heavier than a range. A heavier piston is however slightly "in-efficient", "resource consuming" and "less fruitful" than a lighter piston.
A typical YBR 125's piston I saw in my Yamaha Training:



Blowouts happen mostly for other reasons than a heavier piston because today's world we have high class components available off the shelf.


@ honda CBF: Didn't get you. Efficient in what way exactly? Efficient means to do a work without wastages, with full utilization. So, what else does an engine waste except fuel?


@ Abhijeet: Yes, but the normal spring type valves can sustain upto 17,000 rpm or so safely.
Many street bikes in the past have used spring system, a good example being Yamaha FZR250 (red line on 17000 rpm). Yes, it was a 4 cylinder inline...

Its only when one touches 18,000 and above that pneumatic valves (valves using air based suspension for the valves) are used.

nice piece of info there mate

with efficient I meant to ask at what rpm an engine is not strained but producing max power (torque or bhp)?

Huge thanks
I'll keep bugging you in future as I read more about engines.

At Max bhp point, most engines are NOT stressed, unless they're modified or running lean.

They can sustain long times of riding on the maximum power spot. Only on the redlines do engines have a long duration riding problem. Redlines are purposefully made for indicating an "avoidable if possible" spot.

Then why do tourers choose the Max torque spot? You might ask. Tourers use Max Torque spot due to the following reasons:

1. On Max bhp spot, cruising is not so comfortable. You feel "busy" at this spot. Any headwind, tailwind, climb up slope, etc. affects your rpm's as they rise & fall more often.
The Result? You have to manage your throttle accordingly all the time, thus not enjoying your long ride or shifting gears too.

This is relatively lesser on max torque spot. You can keep constant throttle and expect constant rpm to be carried for longer times, thus it is much more relaxing.

2. Yes, Engines are reliable but they still make more heat at Max bhp spot.
So, max torque spot is ideal as it heats up a bit lesser for long rides.

3. Even mileage-wise, max torque spot is the better deal.

On most modern engines, staying below the redline will NOT cause any sort of problem, if the vehicle has completed its running-in for the initial 2000 kms. So, for a short dash of 130-200 kms, feel free to be on max power spot, but only if you're ready for the less mileage, heat and cruise-ability.
Avoid the redline for longer times though...

--
@ sunny: yeah sure, anytime...

OMG!! how on earth you looked into my mind.. Looking for the very same (satisfying) answers and reasons.
As a tourer I always loved the max torque band, unknowingly why.. Which is now I known of, thanks to you..

Max power is my fav. zone, my fav. riding style is staying on the power band which for my bike happens to be 5k rpm.

(finally some net connection after 2 weeks :P)

when i posted this thread and when i didnt get a reply close to 3 days i was under the assumption that, man....it was a stupid thing to ask.

but now, thanks a lot guys, simply awesome read. i can literally feel the piston pumping and crankshaft rotating on the twist of the throttle

^^ Thanks... a great read!

A similar doubt - When to change gears?

This thread has certainly clarified many doubts about RPMs and Torque.
Can anyone suggest me the best speed(s) to change gears on a ZMR.

I read that it makes max power at 7000 RPM.
So, do I have to wait till the engine revs at 7K, or something else.
I usually drive in 50-55 kmph speed and I barely cross 4500 on the odo.

For me, the gear shifts are something like:-
1st gear - 0 to 10 kmph
2nd gear - 10 to 20 kmph
3rd gear - 20 to 30 kmph
4th gear - 30 to 40 kmph
5th gear - above 40 kmph.

But some friends of mine ride at 55 kmph in 4th gear.
Can you please help. Do I need to improve my gear shifting?

@Divya Sharan

I think that your gear changes are just fine for a city ride.

Ofcourse if you are on a race track then you want to select your gear in such a way so that the engine always remains in the band where you will get the best acceleration on turning the throttle. And that is near about the max torque point of your engine.

Ofcourse the above way is an EXTREMELY crude way of saying this and there are a WHOLE lot of subtleties involved about which I myself am not too familiar with, having never ridden on a track unfortunately.

But as far as riding in traffic inside a city is concerned, do as you already are doing...

p.s. : There are already quite a few excellent threads regarding when to change gears and other useful riding mantras, reading them will clear up your doubts immensely.

Thanks a lot for the thumbs up! I'll read those threads too, but I think I won't get as much mileage as now. Now I get a mileage of around 44-45 kmpl.