Really nice info here. Thanks to Payeng/Joel/Technician & everyone who is contributing to this thread.

Thanks for the info guys...

so short stroke engines have high rpm range on their rpm meter compared to the long stroke bikes????

so all superbikes have ratio mare than 1.2 right????

what do u mean by the highlighted point??? dwell means wat??? forced induction????
high piston speeds???? but i thought that the square and the short stroke engines have high piston sppeds for producing more power at high ends!!!

Whats the bore /stroke of SHOGUN??

@rennycornellius: Shogun's BorexStroke : 52.5mm x 50mm
AX 100: 50mm vs 50 mm
Shaolin: 57.6mm vs 53.0mm

So I guess AX100 is a perfect square unit.

Am confused, could you please explain the above statements.

I thought higher RPM in short strokes would result in faster piston movement though the length of piston travel would be lesser.
Whereas lower RMP in long strokes would result in slower piston movement though the length of piston travel would be greater.

@Kyanmantis: I'll try and put it simply here. Remember we are talking about piston speed.
Speed is distance covered in a certain amount of time.

Consider one specific RPM..say 5000 RPM. Means the piston goes up and down 5000 times a minute. Now consider two hypothetical stroke-lengths..50 mm and 100 mm.

In 1 minute, both pistons have to travel up and down 5000 times each. But the one that needs to travel 100mm during each stroke has to travel faster (than the one having 50mm stroke length) as it has a longer distance to cover in the same time. Which means higher piston speeds and so relatively more wear and tear as Joel says.

@ Old Fox: Simple and couldn't be clearer. Thanks! Yes, statistically speaking, at the same RMP, longer stroke will be moving at greater speed. However if you look at it from KMPH, for example, I normally cruise at 60 KMPH at approx 4k RPM on my long stroke ZMA. Now short stroke engine will be spinning faster to maintain the same kmph. Meaning the movement of piston for short stroke engine will be faster than long stroke at same KMPH. Hope am making sense.

BTW, I've noticed you have a way of explaining things in simple and clear language .

Don't worry..you are making sense. Only that the confusion arises when we talk of identical issues but related to different 'system boundaries'.

We were discussing piston speeds as a reason for wear and tear and there's no confusion here about long stroke engine pistons traveling at higher speeds than those of short stroke engines and so wearing out quicker if pushed at high RPM's. When you start thinking in terms of road speed related to RPM, the confusion crops up. A long-stroke engine usually has better low-RPM torque and so can be geared taller (resulting in higher road-speed at lower RPM) than a short stroke engine which makes most of its torque and power higher up in the RPM band.

Think and you'll probably sense a natural balance here. The long stroke wears out as much at 4000 RPM as a short stroke does at say 7000 RPM. Its about distance travelled inside the bore. We can safely assume that it is primarily sliding friction that induces wear and is dependent on both relative speed of travel between the surfaces and in the piston's case, the number of ups and down that add up to a total equivalent distance covered while sliding against each other. The long-stroke piston covers as much distance inside its bore at 4000 RPM as the short stroke one does at 7000 RPM.

And BTW, the RPM figures are of course just for explaination purposes and not to be taken seriously as real .

Joel, Payeng, OF Sir - Appreciate some inputs w.r.t these 2 points pls...

1.
2.
Source - Honda Worldwide | CBR250R

So which one is right? 50mm deserves to be called as short stroke or 55mm deserves to be called as short stroke?

Pls help understand. Tks.

my guess is Joel is referring to the standard reference of calling engines with 50mm or less as short stroke, whereas the Honda is referring to a comparative short stroke, of their CBR.

Thanks to all of them who contributed to this to add some knowledge to our brains.

But Ji, isn't that 50mm universal accepted stroke size? That's what I understood from the posts. Did I get something incorrect? Pls guide. Tks.

Wrong. For any particular rpm, a shorter stroke has lesser piston speed.

I mean, rpm's are directly related to crankshaft speed. BUT, Piston Speed is different! Piston speed is the distance the piston travels per second. Piston speeds, at a particular rpm can be calculated by the following formula, which I created myself using my understanding:

Piston speed in metres/ second = (rpm's X 2 / 60) X (Stroke length in mm/ 1000)

Around 24 metres/ second is considered a near-danger limit. Most road bikes stay below it only.

Let's take R15's example, which is screaming on its rev limiter (10,000 rpm):

=(10,000 x 2 / 60) X (58.7 / 1000) = 19.56 metres/ second. Safe enough.

While, a short stroke motor will have less stroke length and hence, less piston speed. The End Result?? You can make the rpm higher... right? Tune it, make more power, whatever...

As far as I think, YZF R6 is the highest revving road bike ever. I'm not sure though. Let's find its piston speeds @ 16,000 rpm! R6 has a 67 x 42.5 mm. Very short stroke!

(16,000 x 2 / 60) X (42.5 / 1000)

= 22.67 metres/ second.

See the short stroke trick? 16000 screaming rpm's and you're still under piston danger limit! That's why short strokes can rev higher safely!

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And Joel is very much correct that Bore x Stroke is not the only way to determine power output. It can be the cams, crankshaft, fuel injectors, tuning, etc.

@ aargee: That's not necessarily universal. The term "Short stroke" is quite loosely defined.
What Joel probably wants to convey is that having lesser stroke than bore is too common nowadays, but the result machine is not necessarily high revving.
For Example, KTM Duke has shorter stroke than bore (102 x 84.5mm), would you call it high revving? No, because 84.5mm of stroke itself is a big value that restricts high revving.
Joel wants to say that even if you have a 180 X 80 mm Bore-Stroke, it can't be termed short stroke, because 80 is a very high value, irrelevant of the Bore.

Stroke value should be below 50mm for some serious high revving. Otherwise, piston speeds would be high and revs would have to be kept low.

a very informative thread guys... thanks to all!!