Bore x Stroke

[Big Dave]

Would someone care to explain the differences in characteristics between a square bore and a long stroke motor?

And why it has an effect?

[FJRider]

The square bore (short stroke...bore width is the same as bore stroke) means(generally) more firing strokes per revolution(faster reving multi cylinders) as in contrast to the long stroke (usually [V] TWINS) which fire less often (slow reving)
I think thats how it goes...
The firing pattern on the twins is a whole different kettle of fish...Ask a Ducati-ite...

[popelli]

large bore = large valves = ability to move more air and make more power

shorter stroke means lower piston speed = ability to rev higher = more horsepower

longer stroke = greater leevrage on crank = more torque

[Owl]

Wouldn't a square bore be considered a long stroke motor in this day and age?

[JMemonic]

Popelli has it basically right

Long stroke, or under square is generally (in automotive terms) an older design now and rarely used, it will be low revving with high torque, one of it disadvantages is the need for a heavier flywheel to carry it through its cycle. In this engine the diameter of the bore of the cylinder is less than the distance the piston travels through a cycle. I am not sure if I remember right but this design could possibly hark back to steam.

Square bore. The diameter of the bore of the cylinder is equal to the distance the piston travels through a cycle. Revs higher than a long stoke, and has a reduced flywheel requirement. Still a lot of torque at lower revs.

Short stroke or over square, the diameter of the bore of the cylinder is greater than the distance the piston travels through a cycle. These engines will rev and fast, quick to come up to speed due to having basically no flywheel requirement due to their higher speeds, think Ferrari vs a Massey Ferguson both are good examples of the extremes of these engines.

Modern motorcycles have been square or under for ever, well most brands some have until recently been stuck in their agricultural heritage.

[JMemonic]

Wouldn't a square bore be considered a long stroke motor in this day and age?

No it does not have to have a long stroke to be square, small piston small stroke square motor, none the less thinking :).

[davejenknz]

Popelli has it basically right

Long stroke, or over square is generally (in automotive terms) an older design now and rarely used, it will be low revving with high torque, one of it disadvantages is the need for a heavier flywheel to carry it through its cycle. In this engine the diameter of the bore of the cylinder is less than the distance the piston travels through a cycle. I am not sure if I remember right but this design could possibly hark back to steam.

Square bore. The diameter of the bore of the cylinder is equal to the distance the piston travels through a cycle. Revs higher than a long stoke, and has a reduced flywheel requirement. Still a lot of torque at lower revs.

Short stroke or under square, the diameter of the bore of the cylinder is greater than the distance the piston travels through a cycle. These engines will rev and fast, quick to come up to speed due to having basically no flywheel requirement due to their higher speeds, think Ferrari vs a Massey Ferguson both are good examples of the extremes of these engines.

Modern motorcycles have been square or under for ever, well most brands some have until recently been stuck in their agricultural heritage.

Think you've got your squares mixed up.

Short Stoke = over square
Long Stoke = under square

http://en.wikipedia.org/wiki/Stroke_ratio

[sels1]

One wonders why BD is up at 1:30am pondering these things!
But thanks guys, I have added a bit to my knowledge now too

[merv]

The square bore (short stroke...bore width is the same as bore stroke) means(generally) more firing strokes per revolution(faster reving multi cylinders) as in contrast to the long stroke (usually [V] TWINS) which fire less often (slow reving)
I think thats how it goes...


Hey best you modify your thinking slightly because the number of firing strokes per revolution is exactly the same - either its a two stroke and fires every revolution or its a four stroke and fires every second revolution - that's in each cylinder. Now sure, add more cylinders (I think that's what you were trying to say) and you get more firing strokes per revolution.

However, that's not the point of Dave's question re square versus long stroke as some older four cylinder bikes weren't as short stroke as they are these days.

Much as has been said above, traditionally long stroke motors had good torque at low revs due to the longer crank throw and greater lever arm, but couldn't rev hard because of piston speed limitations. Square or short stroke motors in comparison had less bottom end torque but could rev harder and make more power.

These days the fantastic world of electronics and some design changes have improved a lot of this as I have raved on many a time about the bottom end torque of my WR250F. Its is a very short stroke engine but pulls like the bejeesus at low revs due to having a very short piston which allows for a very short con rod which changes the angle of the dangle to the crank and then the electronics, using throttle position sensors and the like, takes care of the rest. The modern 4 cylinder bikes are similarly as short in the stroke with all the whizz bang stuff including fuel injection.

This has opened up a whole new world in terms of engine wear and need for maintenance which is one of the trade offs but to a degree is being overcome by better materials and oils with less friction.

So I'm saying these days the engine characteristics are altered greatly over previous experiences but generally what still applies is the long stroke motor can't rev as hard and make as much power but the old adage that short stroke engines couldn't pull at low revs has somewhat gone.

[avgas]

Yep what others have said has been my understanding.
Bore = capacity = big bang = power
Stroke = waving either the short or long end of the stick (conrod) = engine momentum = torque.

Easiest way i thought of it was if you wanted to throw a bucket and the bucket got bigger everytime - you get slower and require more effort in each throw (bore)

If you sweep the floor, and shorten you stroke (cut broom handle in half) then you can do more strokes for less effort over a shorter period of time.

The fun starts when you have big bore, short stroke V8's....... good for drifting as the rpm curve states that hp kicks in nice and low, where the engine accelerates quickly

[Ixion]

It gets more complicated on proper bikes what don't have all those cams and valves and other unnecessary stuff.

Because the valves are holes in the cylinder walls, the length of cylinder wall available acts as a control on the port timing.

[Big Dave]

One wonders why BD is up at 1:30am pondering these things!
But thanks guys, I have added a bit to my knowledge now too

Nights are for work - days are for play.

[Blackbird]

Just to add to the erudite comments, "longer stroke" motors have higher piston speeds and put more stress on componentry like con rods and pistons. Most of this stress occurs as the piston/rod decelerates and accelerates around top and bottom dead centre. The limiting mean piston speed from memory in the days when the British twins reigned supreme was about 4000 ft/min. This will have improved a little with forged components, better alloys etc, but not hugely so; hence the move to shorter strokes to reduce piston speed whilst increasing power.

A really good example was when I drag raced a supercharged 350cc Triumph twin. The supercharger allowed it to breathe more freely so it would rev more easily. The higher revs added significant extra stress and I was forever having the piston crowns detach at the oil control ring. I'd reached the strength limits of the standard stroke motor, so I machined a short stroke crank and married it to a modified 500cc (bigger bore) barrel to give a very over-square 350. This dropped the piston speed but I also lost torque - the essential ingredient of drag racing. Months followed of testing different flywheel weights, cam timing, boost and upping the percentage of nitro in the methanol until it was competitive again.

So there you are.... a practical demo of engine design 101:done:

[cooneyr]

......flywheel weight.....

Thesis time . . .

While fly wheel weight is a factor in this - more relevant is the length of the stoke and what that does re accelerations of the piston. If the stoke is long (as a result of a large big end crank offset) the larger the velocity of the piston at mid stroke compared to a short stroke engine (for the same given rpm). Now as rpm is involves a unit of time and velocity over time is acceleration, as the velocities increase (i.e. long stroke) the accelerations increase.

Imagine this - at TDC or BDC the piston is stationary and then has max velocity at mid stroke hence it must accelerate from stationary to max velocity to stationary for one stroke. As mentioned above the higher the max velocity the higher the accelerations.

We all know that to accelerate fast takes more "effort" than accelerating slowly. This "effort" is lost power in an engine. Hence a short stroke engine will have more power than a long stroke engine all other things being equal. For the same reason a short stroke engine will increase in revs much faster and can pull higher revs (as the mid stroke velocity is less at certain rpm than a long strong motor). As I understand it the max piston velocity is one of the parameters that influences red line rpm (along with valve characteristics etc etc).

As the other fullas have mentioned that long stroke = more torque. This is because torque = force (fuel explosion) x distance (crank to big end off set). Unfortunately due to high piston velocity this also means less rpm (or less usable rpm as the effort to accelerate the piston becomes to significant at higher rpm).

It is also worth noting that power = torque x rpm.

Traditionally long stroke = torque but not very high rpm hence using the above relationship not much power, short stroke = high rpm and lower torque but lots of power and square = is a compromise between the two. The net result (think of formula 1 cars here or a 250 4 cylinder pulling 17000 rpm) is the more rpm the engine will pull the more power, however we must keep piston velocities down to achieve more rpm hence we must have short stroke.

Modern short stroke engines somehow managed to develop reasonable torque but I dont understand this much. It might be more to do with more efficient fuel mixtures and ignition timing as much as anything.

Cheers R

Disclaimer - I'm a civil eng not a mech eng so I could be taking a complete load of rubbish :wacko: but I dont think so.

Doh - Blackbird bet me too it.

[Big Dave]

Cool - I had a pretty good handle on it - that Ixion is on another planet - not actually a planet - that is.

I was reconciling/writing the Goldwing.

-----------------------

More impressive is how it goes.

Its Liquid-cooled 4-stroke, 12-valve, SOHC flat – 6 cylinder engine has a
Displacement of 1,832cc with a rather square 74 x 71 Bore and Stroke and a compression ratio of 9.8:1.

It’s got some ‘schtonk’. It puts out 87kw (117hp) at 5,500rpm, but that isn’t the important figure. The 167nm of torque at 4,000rpm, is.

--------------------------

'Rather square' is too subjective?

[avgas]

I was reconciling/writing the Goldwing.
What you need to study is momentum by the sounds of things lol

[Big Dave]

What you need to study is momentum by the sounds of things lol

The next paragraph mentions inertia. 405kg wet.

edit - I just re-read it - momentum is a better angle. ;-)

[Mikkel]

It's hip to be square.

[nudemetalz]

A lot of the 2-stroke performance road-bikes have square bore and stroke.
eg RG-250 54mm x 54mm and RGV-250 56mm x 50.6mm (which is barely oversquare).
These engines don't really rev high (10-12,000 rpm) compared to the performance multis but have the peak hp and torque relatively close together and are peaky.
So it's a whole different kettle of fish when doing the 2-stroke vs 4-stroke bore and stroke talk.

Just thought I'd throw that meaningless bit of info in.

[merv]

Just to add a light bit of further comment to this thread is I think its funny we refer to square when a piston is normally round, so even though many of the good old 2 strokes had a 54x54 engine and we called them square, how square is it anyway?

Now we know Dave is talking about the Gold Wing which is slightly short stroked and doesn't rev that high we know he is talking about an engine built for its task - stacks of torque and the ability to cruise forever at reasonably low revs with multi-cylinders and horizontally opposed to keep it smooth.

[avgas]

The next paragraph mentions inertia. 405kg wet.

edit - I just re-read it - momentum is a better angle. ;-)
like an anchor to water

[Ixion]

Cool - I had a pretty good handle on it - that Ixion is on another planet - not actually a planet - that is.

I was reconciling/writing the Goldwing.

-----------------------

More impressive is how it goes.

Its Liquid-cooled 4-stroke, 12-valve, SOHC flat – 6 cylinder engine has a
Displacement of 1,832cc with a rather square 74 x 71 Bore and Stroke and a compression ratio of 9.8:1.

It’s got some ‘schtonk’. It puts out 87kw (117hp) at 5,500rpm, but that isn’t the important figure. The 167nm of torque at 4,000rpm, is.

--------------------------

'Rather square' is too subjective?

One's enough. Two's plenty. Three's luxury. Four's a car. Six is a truck.

[JMemonic]

Think you've got your squares mixed up.

Short Stoke = over square
Long Stoke = under square

http://en.wikipedia.org/wiki/Stroke_ratio

Thanks for that was thinking I was wrong but that section of the brain had already fallen asleep. Edited my original post.

[nadroj]

While we are on this subject what is the accepted maximum piston speed now days?
As previously mentioned around 4,000fpm used to be max and the first Honda 750 fours were 4,100fpm at redline if memory serves me correct.
I did calculations on a Ford Flathead V8 powered Bonneville roadster a few years ago & came up with 5,200fpm so it obviously had a rather strong girdle to stop it farting south!

[Big Dave]

like an anchor to water

Quite remarkably and I admit - surprisingly - not so.

It's testament to the importance of the height of the centre of gravity as much as the mass.

They skids along.

[Forest]

While we are on this subject what is the accepted maximum piston speed now days?
As previously mentioned around 4,000fpm used to be max and the first Honda 750 fours were 4,100fpm at redline if memory serves me correct.
I did calculations on a Ford Flathead V8 powered Bonneville roadster a few years ago & came up with 5,200fpm so it obviously had a rather strong girdle to stop it farting south!

As a datapoint, the Ducati superbike engines max out at around 5,400 fpm.

[Motu]

[QUOTE=JMemonic;
Long stroke, or under square is generally (in automotive terms) an older design now and rarely used, it will be low revving with high torque, .[/QUOTE]

I suggest you check out bore/stroke specifications of modern car engines,they are most often long stroke.More to do with emissions really,a short stroke engine is prone to producing Co and HC because of the large combustion chamber.

Some preconcieved assumptions about short/long stroke....and Dave is right to be puzzled.A short stroke engine produces more low speed torque than a long stroke - think hydraulics....the large surface area of the piston applying a lot of pressure,a short sharp shock giving more torque.Trials bikes have the highest low speed torque of all motorcycle engines....and are short stroke.

It was something I noticed over 35 years ago when riding British bikes - why did the short stroke BSA A65 (and Yamaha XS1 with the same 75x74mm) have more bottom end grunt than the revvy long stroke Triumph engine .... it should have been the other way around.

[Big Dave]

>>More to do with emissions really,a short stroke engine is prone to producing Co and HC because of the large combustion chamber.<<<

Because it is less efficient?

[Motu]

Too much surface area....and quench areas that force the fuel back into a liquid state.

[popelli]

- why did the short stroke BSA A65 have more bottom end grunt than the revvy long stroke Triumph engine .... it should have been the other way around.


and the bsa had more power on the top end as well

nothing to do with bore / stroke but everything to do with head design and the amount of air you can flow through it

[Ixion]

What somewhat confuses this argument is that bore and stroke can seldom be considered in isolation.

The old long stroke singles, f'instance, ALSO had very small valve areas. mainly for the sake of fuel economy. And because a small bore means a small head surface area (as noted, good for emissions) , but hard to fit big valves in. that was one reason for the move to four valve heads.And becuse noone cared so much about max power figures. If you wanted more power, you got a bike with a bigger motor. Sort of like crusiers nowdays.

Which is one reaosn why the Beezer had more grunt. It could pump more air.

But I don't think the hydraulic analogy is valid. Sure, there is a bigger poiston area. But the VOLUME of gas is the same (all other things being equal, which they never are), so the bigger piston area implies a lower pressure per square inch .

[TLDV8]

Did i miss the bit about conrod ratio's ?

<img src=http://i30.photobucket.com/albums/c345/manurewa/CR2.jpg>

[Motu]

Not a lot of air flow at low speeds,rather the opposite.One of the problems with the Triumph was the valves and ports were too large - you will notice that by the '60's the exhausts were stepped down to a smaller size to increase the speed of gas flow.The A65 head was a later design,and had better low speed gas flow....faster,not slower.The XS650 had the opposite problem in that the head couldn't flow at high speed and that always held them back.Kenny Roberts had some special heads made by Yamaha to get his 750's as fast as a Triumph.

[Big Dave]

Some of the high torque attributes I favour about the Harley thunderstorm engine I'm told are due to the shape of the combustion chamber.

It can be more steeply domed because the cams are still down below.

[Motu]

Did i miss the bit about altered conrod ratio's ?

Short rod - high torque.

[Titanium]

square = (piston) bore and (distance between TDC and BDC) stroke are the same measurement.

I am thinking that one of the most famous "square" engines was the Z900 kawasaki with a 66 mm bore and 66 mm stroke. This was one of the reasons for its superior performance in its day.

Check out wiki.

http://en.wikipedia.org/wiki/Oversquare

[Big Dave]

So Give list of engine parameters to maximise torque?

(and I assume it will be the opposite for power)

[Titanium]

Short rod - high torque.

short rod = high revs. Develops power and torque at revs.

oh, and there is no replacement for displacement! when it comes to torque!

[Ixion]

Power IS torque. More torque, more power. You mean to flatten the torque curve. Move the peak torque down the rev scale.

[Titanium]

So Give list of engine parameters to maximise torque?

(and I assume it will be the opposite for power)

displacement and long stroke.

look at big ships, trucks, diesel engines .... all big displacment and low revs, big torque and high compression (diesel engines that is)

Power is high compresion, perfect afr's, bang on fuel / timing / spark ratio and moving a whole lot of fuel / air at the right mixture through an engine (intake, combustion and exhaust)

Torque = http://en.wikipedia.org/wiki/Torque

Stroker kit = http://http://en.wikipedia.org/wiki/Stroker_Kit

[Big Dave]

Power IS torque. More torque, more power. You mean to flatten the torque curve. Move the peak torque down the rev scale.

Design an engine optimised for low end torque.

The Theorixion 1000.

[Big Dave]

displacement and long stroke.

look at big ships, trucks, diesel engines .... all big displacment and low revs, big torque and high compression (diesel engines that is)

Power is high compresion, perfect afr's, bang on fuel / timing / spark ratio and moving a whole lot of fuel / air at the right mixture through an engine (intake, combustion and exhaust)

Do I want heavy pistons and a short conrod? Or a heavy flywheel.

edit - oops I see long stroke.

[Ixion]

Long stroke often sort of presupposes a heavy flywheel , cos you need some inertial momentum to spin the motor all the way down and all the way up again.

[Edbear]

Just to add to the erudite comments, "longer stroke" motors have higher piston speeds and put more stress on componentry like con rods and pistons. Most of this stress occurs as the piston/rod decelerates and accelerates around top and bottom dead centre. The limiting mean piston speed from memory in the days when the British twins reigned supreme was about 4000 ft/min. This will have improved a little with forged components, better alloys etc, but not hugely so; hence the move to shorter strokes to reduce piston speed whilst increasing power.

A really good example was when I drag raced a supercharged 350cc Triumph twin. The supercharger allowed it to breathe more freely so it would rev more easily. The higher revs added significant extra stress and I was forever having the piston crowns detach at the oil control ring. I'd reached the strength limits of the standard stroke motor, so I machined a short stroke crank and married it to a modified 500cc (bigger bore) barrel to give a very over-square 350. This dropped the piston speed but I also lost torque - the essential ingredient of drag racing. Months followed of testing different flywheel weights, cam timing, boost and upping the percentage of nitro in the methanol until it was competitive again.

So there you are.... a practical demo of engine design 101:done:

Nice story! Bet you had some fun! Unfortunately I can't bling you again yet!


It's hip to be square.

It used to be square to be hip... Has it come back around again?

[Titanium]

Do I want heavy pistons and a short conrod? Or a heavy flywheel.

edit - oops I see long stroke.


Flywheels ..... that is another thread!

Typically a light small dia flywheel will offer up a very fast, free, high reving engine. But have a low inertia contribution. A light flywheel also helps in cornering due to a reduced gyroscopic effect.

Heavy flywheels can still be spun very quickly and will assist with momentum but will take longer to "spool" up.

Flywheels also act as a dampner in the drive line to protect the runnig gear from the mechanical "noise" created during the combustion process.

[Motu]

Design an engine optimised for low end torque.


Now....what sort of torque do you want? A loooong stroke engine will give you ''hang on'' ability....it''ll tow start a Pajero up hill without strain.A short stroke engine will give you neck snapping grunt.Both engines give low speed torque and not high power and revs....so what sort of torque turns you on?

Do I have to mention trials bikes again? One clue is high intake gas speed....small ports,small valves,small carbs,small exhausts.Like Ixion says,move the torque curve down.

[Big Dave]

Now....what sort of torque do you want?

So when I whap the throttle at 1500rpm - it immediately wheelstands.

[Ixion]

Torque plays only a part in that. Flywheel weight, fueling, wheelbase, as important, probably more so. Think trials bikes again. But you wouldn't want to ride one very far on the road.

[Motu]

I've owned both long and short stroke high torque engines.The one in my avatar is an M20 BSA,a 500cc sidevalve with 13 hp to pull it's 400lb weight.But it's not about hp - it had so much torque that it could pull a child/adult sidecar with that 13hp.It would drop in speed on a long hill until it hit max torque,and then just hang on forever at that speed.And I've had a 50cc Zundapp run away from me at the lights.That is the traditional long stroke engine everyone is talking about.

I also had a 350cc Bultaco Sherpa T,and I think they were 19hp.These engines started out as a 175,and were just bored out eventually to 325cc,so a very short stroke,and short rod.A motor that would never stop so long as the throttle was open,no matter what the revs or load.So much torque it would pull the front wheel up in 2nd from idle and hold the wheel wherever I wanted it,like the front wheel was controlled by a wire from the throttle.This is the short stroke torque that is not quite understood.

[Big Dave]

Bloody engineers.

If I were to design an engine that would be described as torquey.

The bore and stroke would be described as.........

The conrods world be as ..........I could make them

The flywheel would be as.............I could make it.

The ideal compression ratio would be..............

It would be a ............ cylinder.

Blardy engineers.:girlfight:

[Movistar]

Bloody engineers.

If I were to design an engine that would be described as torquey.

The bore and stroke would be described as.........

The conrods world be as ..........I could make them

The flywheel would be as.............I could make it.

The ideal compression ratio would be..............

It would be a ............ cylinder.

Blardy engineers.:girlfight:


Fill in all blanks with.......BIG!

[Big Dave]

Fill in all blanks with.......BIG!

Big dave doesn't fire blanks....oh, I see.....

[Ocean1]

Fill in all blanks with.......BIG!

Yup. Single most important design consideration. Everything else is just tweaking the curves.

Oh, and BD, you know yer TT is well undersquare?

[Ixion]

Bloody engineers.

If I were to design an engine that would be described as torquey.

The bore and stroke would be described as.........

The conrods world be as ..........I could make them

The flywheel would be as.............I could make it.

The ideal compression ratio would be..............

It would be a ............ cylinder.

Blardy engineers.:girlfight:

Well, you could fill those in with almost anything. You need to be asking about camshafts. Valve angles. Inlet port diameters. Exhaust resonance (want more torque ? Stuff a spud up the exhaust). Valve overlap. Ignition advance. Not to mention more fundamental stuff. Before we move on to th etechnical stuff. In other words, it ain't as simple as that.

But the short one line answer to your question is

"If I were to design an engine that would be described as torquey."

"It would be a side valve"

Enjoy.

[Motu]

"If I were to design an engine that would be described as torquey."

"It would be a side valve"



Or a two stroke - with a firing impulse every revolution,it is a longer sustained push.2 strokes are not always a high revving screamer....they are just as often a low speed grunter.No accident those locos pulling freight are 2 strokes....or those ships hauling containers.

We won't even mention electricity....that would be unfair....they don't even make a noise when they torque....

[cooneyr]

.....the large surface area of the piston applying a lot of pressure,a short sharp shock giving more torque.....


But I don't think the hydraulic analogy is valid. Sure, there is a bigger poiston area. But the VOLUME of gas is the same (all other things being equal, which they never are), so the bigger piston area implies a lower pressure per square inch .

I tend to agree with Ixion - pressure if force / area i.e. more surface area (bigger piston) = less pressure


What somewhat confuses this argument is that bore and stroke can seldom be considered in isolation.....

......The old long stroke singles, f'instance, ALSO had very small valve areas. mainly for the sake of fuel economy.


Do I have to mention trials bikes again? One clue is high intake gas speed....small ports,small valves,small carbs,small exhausts.Like Ixion says,move the torque curve down.

Your both make a good point - stroke and bore are only part of part of the equation. As I understand it high gas speeds (due to small carbies and or small valves) results in better air/fuel mixing hence better combustion pressures and less emissions. The 'restrictions' mean that the engines cannot flow enough air for high rpms though.

So we now have (warning this may be complete shite!)


long stroke motors with small valves, due to bore dimensions, relative to capacity with high torque. They have to have high fly wheel inertia to get though the stroke which means they dont spool up very fast though are good luggers. Low power as cant pull high revs.
short stroke motors with small valves, relative to capacity with high torque. They have low fly wheel inertia (don't need it to get through the stroke) so the spool up well. Low power as cant pull high revs as cannot flow enough air to pull the rpm.
short stroke motor with large valves, relative to capacity with low torque due to low gas speeds hence poor combustion at low rpm, and yet high power as can pull high rpm.


So what we want is a short stroke with low inertial fly wheel and with a variable 'restriction' of sorts to keep gas flow speeds up at low rpms and yet can open up at high rpm to flow enough gas?

Am I about 1/2 right????

Cheers R

P.S. fly wheel inertia is the key not weight.

Edit - Damn it - course it they had to add more stuff - stop being so bloody technical I and M :Pokey:

[Ocean1]

I Am I about 1/2 right????

Perfect.

Sounds simple when you say it quick don't it.

Been a few variable intake devices over the years.

[Ixion]

Or a two stroke - with a firing impulse every revolution,it is a longer sustained push.2 strokes are not always a high revving screamer....they are just as often a low speed grunter.No accident those locos pulling freight are 2 strokes....or those ships hauling containers.

We won't even mention electricity....that would be unfair....they don't even make a noise when they torque....

Of course. I did mention right at the beginning that PROPER bikes etc etc.
And one of the torquiest bikes I ridden is the GT750. The Titan is pretty grunty, too.

[Big Dave]

Am I about 1/2 right????


I'm asking the qvestions, Schtarker!

Nice Job. :-)

I'll communicate with ixion when he lands.

[Ixion]

Landing? Ah, now we are talking about thrust. There is a relationship can be derived between torque and thrust . That introduces some new concepts into our discussion. Coefficient of friction, hysteresis, lots of nice interesting stuff.And quite a bit of pure physics. We'll need some mathematicians as well as engineers. This could be a worthwhile discussion.

[scumdog]

Hi-tech talk.

Long stroke = more torque but less revs

Bigger bore = more rev capability.


I think.

[Forest]

So when I whap the throttle at 1500rpm - it immediately wheelstands.

Sounds like you want an electric bike.

An electric motor develops its maximum torque at zero rpm.

[Forest]

As a real-world example of an under-square engine, look at the 1,800cc twin used in the Yamaha MT-01.

Bore 97mm and stroke 113mm.

[scumdog]

Short rod - high torque.

Excessive side-loads on cylinders too...

[Motu]

So now after 5 pages of conflicting theories,we can conclude that engine characteristics has nothing whatsoever to do with bore/stroke relationships,2 or 4 stroke,valve or camshaft configuration,or cylinder layout or numbers.What really matters is what the designer has in mind.....and that means all the little things are more important than saying ''my bike has more torque because it's a V twin.'' Personally I still consider that the flywheel is the most influential component of an engine.

[Titanium]

Ah stuff it ...... go forced induction! chuck on a turbo or supercharger! or even both!

[cooneyr]

Perfect.

Sounds simple when you say it quick don't it.

Been a few variable intake devices over the years.

After that post was standing in the shower (good place for thinking that) and realised that Vtec is a perfect example of variable intake 'restrictions' using cam timing (opening durations).


Landing? Ah, now we are talking about thrust. There is a relationship can be derived between torque and thrust . That introduces some new concepts into our discussion. Coefficient of friction, hysteresis, lots of nice interesting stuff.And quite a bit of pure physics. We'll need some mathematicians as well as engineers. This could be a worthwhile discussion.

Keep it going - I'm interested. Where is there hysteresis in an engine system? :scratch:


Ah stuff it ...... go forced induction! chuck on a turbo or supercharger! or even both!

That just cheating :laugh: Just read the Press article about the VW 1.4TSi engine this morning - super charger and turbo (yes I know they are both just different types of super chargers).

Cheers R

[Ocean1]

So now after 5 pages of conflicting theories,we can conclude that engine characteristics has nothing whatsoever to do with bore/stroke relationships,2 or 4 stroke,valve or camshaft configuration,or cylinder layout or numbers.What really matters is what the designer has in mind.....and that means all the little things are more important than saying ''my bike has more torque because it's a V twin.'' Personally I still consider that the flywheel is the most influential component of an engine.

I gave up playing with engines way back when you could no longer take a typical standard offering and easily double it's performance without spending a hell of a lot. Things have got more sophisticated. Even so, the short list of simple design variables in those 5 pages add up to a huge range of possible engine designs, each one with quite different power delivery characteristics.

Murphy usually gets a say in any build too, some engines just seem to work better than they should, some never seem to quite work as you expect, in spite of numerous tweaks or rebuilds. Even with the vastly improved design and analysis software used now it's still more art than science.


After that post was standing in the shower (good place for thinking that) and realised that Vtec is a perfect example of variable intake 'restrictions' using cam timing (opening durations).


Yeah. That's more to do with fill rates than port velocity though.

There's an ideal air speed for fuel mixing, the only way to maintain that over any change in rev's is to change the cross sectional area of the port.

There's been variable volume intake tract systems that do work, but to get the best effect you want the csa to change smoothly, over most of the port length. Difficult. If you manage the mechanical bit ideally you'd use another channel on the ecu to control it...

[rwh]

I would suggest making the conrods more than half the stroke.

Richard

[Trouser]

What effect does differing rod/stroke length relationships have?

[Ocean1]

What effect does differing rod/stroke length relationships have?

It changes the velocity profile of the piston.

That means you can optomise the rod angle to the crank to best suit the duration of the pressure profile for the rev's you want and the fuel you're using.

[Ocean1]

Here y'go, best description I can find...

Effects of Long Rods
Pro:
Provides longer piston dwell time at & near TDC, which maintains a longer state of compression by keeping the chamber volume small. This has obvious benefits: better combustion, higher cylinder pressure after the first few degrees of rotation past TDC, and higher temperatures within the combustion chamber. This type of rod will produce very good mid to upper RPM torque.
The longer rod will reduce friction within the engine, due to the reduced angle which will place less stress at the thrust surface of the piston during combustion. These rods work well with numerically high gear ratios and lighter vehicles.
For the same total deck height, a longer rod will use a shorter (and therefore lighter) piston, and generally have a safer maximum RPM.

Con:
They do not promote good cylinder filling (volumetric efficiency) at low to moderate engine speeds due to reduced air flow velocity. After the first few degrees beyond TDC piston speed will increase in proportion to crank rotation, but will be biased by the connecting rod length. The piston will descend at a reduced rate and gain its maximum speed at a later point in the crankshaft’s rotation.
Longer rods have greater interference with the cylinder bottom & water jacket area, pan rails, pan, and camshaft - some combinations of stroke length & rod choice are not practical.
To take advantage of the energy that occurs within the movement of a column of air, it is important to select manifold and port dimensions that will promote high velocity within both the intake and exhaust passages. Long runners and reduced inside diameter air passages work well with long rods.
Camshaft selection must be carefully considered. Long duration cams will reduce the cylinder pressure dramatically during the closing period of the intake cycle.

Effects of Short Rods
Pro:
Provides very good intake and exhaust velocities at low to moderate engine speeds causing the engine to produce good low end torque, mostly due to the higher vacuum at the beginning of the intake cycle. The faster piston movement away from TDC of the intake stroke provides more displacement under the valve at every point of crank rotation, increasing vacuum. High intake velocities also create a more homogenous (uniform) air/fuel mixture within the combustion chamber. This will produce greater power output due to this effect.
The increase in piston speed away from TDC on the power stroke causes the chamber volume to increase more rapidly than in a long-rod motor - this delays the point of maximum cylinder pressure for best effect with supercharger or turbo boost and/or nitrous oxide.
Cam timing (especially intake valve closing) can be more radical than in a long-rod motor.

Con:
Causes an increase in piston speed away from TDC which, at very high RPM, will out-run the flame front, causing a decrease in total cylinder pressure (Brake Mean Effective Pressure) at the end of the combustion cycle.
Due to the reduced dwell time of the piston at TDC the piston will descend at a faster rate with a reduction in cylinder pressure and temperature as compared to a long-rod motor. This will reduce total combustion.

[Big Dave]

Conversely, pros connoting the pros and cons of conrods has pros and cons.

[Zealot]

Ah, engineering. Where everything is a comprise between opposing factors to achieve a desired result.

[Ocean1]

Ah, engineering. Where everything is a comprise between opposing factors to achieve a desired result.

Except for size.

There ain't no substitute for cubic inches.

[cooneyr]

Except for size.

There ain't no substitute for cubic inches.

Lighter vehicle weight?

[Motu]

Lighter vehicle weight?

I've always found a big advantage in light rider weight.....lard arses kill the performance of their bikes.

[Big Dave]

I've always found a big advantage in light rider weight.....lard arses kill the performance of their bikes.

My mate Trip was bitching because Wendy forgot to bring the ramp on the trailer when his Norton broke down - again.

So I picked up said Norton and placed it gently on the trailer.

:-P

[Blackbird]

My mate Trip was bitching because Wendy forgot to bring the ramp on the trailer when his Norton broke down - again.

So I picked up said Norton and placed it gently on the trailer.

:-P

Hahaha - good one:Punk:. A mate of mine in the UK has one of the early Triumph Tridents with Lucas Prince of Darkness electrics. He reckoned that his wife having to turn out with a trailer to surrounding counties most weekends rather strained their relationship. He also said that he was on first name terms with every AA patrol officer within 100 miles!

[Motu]

So I picked up said Norton and placed it gently on the trailer.


Isn't human nature such a wonderful thing? Tough guys are so keen to show off their strength - I just step back and watch them do it....job done! Brains beats brawn every time....so do fast legs.

[Big Dave]

Isn't human nature such a wonderful thing? Tough guys are so keen to show off their strength - I just step back and watch them do it....job done! Brains beats brawn every time....so do fast legs.

Like I EVER show off.......:chase:

[Ixion]

Except for size.

There ain't no substitute for cubic inches.

Aerodynamics. And weight.

[Ocean1]

Lighter vehicle weight?


Aerodynamics. And weight.

For sure. But we were discussing engines...

[Motu]

I've got a few engines under the bench - but they don't do a damn thing until I fit them into something.....

[Mikkel]

I've got a few engines under the bench - but they don't do a damn thing until I fit them into something.....

They can still make noise can't they?

I mean - with speed limits being what they are noise seems to be the most important aspect of any engine these days. :yes:

[Mr.Pete]

Except for size.

There ain't no substitute for cubic inches.
In a way,you're right.An increase in cubes will always make more torque at low rpm.If you change nothing else,don't expect to make more power;but expect to make the same power at lower rpm.

A long stroke 490cc cylinder could be 79mm bore X 100mm stroke.It might make max power at 6000 rpm.
A square 490 cc cylinder would be 85.54 bore and stroke.It could make max power at 6491 rpm with approx the same inertia stress level.It would take more intake /exhaust flow to achieve that,but less cam duration because of the lower piston speed. 8.18% more power.
A short stroke 490cc cylinder could be 111.7mm bore X 50mm stroke.It could make max power at 8485 rpm with about the same inertia stress as the longer stroke cylinders.It would require even more intake/exhaust flow and slightly less cam duration.It could make 41.4% more power than the long stroke engine and 30.7% more than the square cylinder.It's piston speed would be 70.7% of the long stroke engine.

You don't need much more flywheel inertia for a long stroke.If you still push 490cc to 8.5:1 compression,it takes the same amount of energy for a long or short stroke engine.The conrod will have slightly more weight,because it needs to be longer.The smaller piston will have much less weight.Piston weight is about 3 times the weight of the rod small-end.There's not much kinetic energy in a piston and conrod at idle,even with the slightly higher piston speed of a long stroke.
As the piston approaches TDC the piston's kinetic energy is absorbed by the crank,and is available to accelerate the piston on the following down-stroke.You don't need any extra flywheel inertia from added flywheel mass to achieve this.The kinetic energy is not lost or wasted;it's merely transfered from piston to flywheel,then back to the piston.You might lose a very small amount in friction at the conrod bearings.
Flywheel inertia is mostly a function of your intended idle speed.If you're happy to double the idle rpm you could reduce flywheel inertia to 25%.

A given cam duration will suit a certain piston speed (as measured in feet/minute or metres/minute).The long stroke engine will reach that piston speed at lower rpm and,if all else is correctly proportioned,you get maximum torque.This leads to the wives' tale "long stroke engines make better torque at low rpm".
A short stroke engine can make the same torque at the same low rpm,it just requires less cam duration than the long stroke engine.Give it the same intake and exhaust flow,and it will make the same power curve as the long stroke engine.

The long stroke engine does have a small advantage in fuel efficiency and economy.There is less piston and cylinder head area absorbing precious heat and energy from the combustion process.There is also less piston ring circumference to allow blowby past the rings.

[DEATH_INC.]

So when I whap the throttle at 1500rpm - it immediately wheelstands.
You want a big arsed compound turbo two stroke diesel then. Possibly with nitrous to help it spool up quicker. :D