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Consider cylinder bore size and stroke size...

its seems most (bikes included) engines have a larger diameter bore than the stroke is long ("under square"). why?

when you put a 400 crank in a chevy 350ci you get a longer stroke (called a "stroker") and 33 extra cubes. its torque and horsepower are also much improved.

as i understand...under square engine will rev quicker and develop power at lower rpm's.

over square (stroked engines) will have their power hit on the top of the rpm.

what do you guys think?

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I think you have it reversed.

An over square engine will generally rev higher and make more power at the top end whereas an under square engine will generally make more torque.

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I think you have it reversed.

An over square engine will generally rev higher and make more power at the top end whereas an under square engine will generally make more torque.

You got it. The undersquare motor allows the engine to turn higher rpm's without destroying itself. This is due to piston travel in fps. I forget what the exact numbers were but a piston cant exceed a certain number without becoming incredibly unreliable. With a stroker motor, the piston moves further up and down therfor it is moving a farther distance, and as rpm's rise it approaches the fps margin at a lower rpm. Hence the power and torque at lower rpm's. Where as a over square motor is just the oppositte. allowing high revs without risking piston and other such problems allowing for greater power in upper rpm's. :cry:

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You guys are on the right track. This has allways been and interesting topic. To take it to extremes though, in a four stroke engine you can only make the bore so big until the size of the valves and their weight can't keep up with the high RPM. Therefore, more smaller valves. That's why there are 4 (most) and 5 (Yamaha) valve motors. Honda took it to an extreme with a street bike motor (I think the NCR?) that had oval pistons and 8 valves. This was to get around a rule in GP racing that the engine couldn't exceed 4 cylinders. What they did was essentially make an 8 cylinder but ovaled out the bores and pistons so there were only 4.

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well let me clarify a little bit. i am really interested in how two identical engines would perform. same displacement, same cam, same valves, same comression ratio etc...the only different being the bore and stroke.

it seems to me that the undersquare would have less torque because the crank shaft geometry, but would allow for high revs as already pointed out by Jester220.

an oversquare crank radius would place the crank pin farther away from the center of the crank...giving the piston a better mechanical advantage and thus more torque, but would prevent high revs, or reving too fast.

***************************************************

this is also under the assumption that bore > stroke = under square *

and stroke > bore = over square...............................................*

***************************************************

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I'm not sure but I doubt you could get optimum performance from both configurations using the same cam. Unless you are going to race your car go with the stroker for the same reason a 2-stroke mx bike would suck on the street.

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I think I read somewhere that theoretical optimum piston speed is around 4000fps. The more mass there is, bigger bore, the more the tendancy is to keep going instead of coming to a total stop at tdc and bdc and reverse directions.

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************************************************** *

this is also under the assumption that bore > stroke = under square *

and stroke > bore = over square............................................ ...*

************************************************** *

You are correct in your understanding of the relationship between stroke and power characteristics and rev limits. I think your definition of oversquare/undersquare are backwards, though

I believe oversquare traditionally means bore>stroke (high HP, high rpm limit, think grand prix racer). Undersquare traditionally has meant bore<stroke (good low end torque, lower maximum revs and HP potential, think tractor or BSA 441 Victor).

Obviously compression ratio, valve size, cam lift, and other design characteristics of any specific motor will have large influences on power characteristics.

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5000 FPS is the practical limit for most piston engines.

A longer stroke will have greater port velocity at a given RPM, more torque due to more leverage on the crank, will achieve it's greatest efficiency at a lower RPM, and have less peak potential than a shorter stroke motor. Smaller combustion chambers are also more efficient, with the flame front having a shorter distance to travel- this leads to being more detonation resistant, and having an advantage for emissions.

Bigger bores with shorter strokes have the potential to turn higher RPM's, and larger/more valves will fit into bigger combustion chambers. Since the HP race involves turning ever higher RPM's to make more power, the oversquare motors have increased in popularity particularly when it comes to motorcycles.

Due to the emissions requirements, the lack of widespread consumer interest in 10,000 RPM cars, and the better reliability from lower RPM's, most car/truck engines are nowhere near as oversquare as motorcycle engines.

While it would be interesting to check how one bore/stroke combo would compare to another, it's important to remember that any results you find would be different with other engine combo's. Port size/shape, camshaft profile, exhaust backpressure, intake shape/length, piston style, compression ratio, and a thousand other items would affect the results. Each change would favor one bore/stroke ratio over the other. It's the overall package that counts, not one specific item.

Huge ports, aggressive camshaft profiles, short intake (high RPM type motor) would likely show a preference for an oversquare motor. If your test motor were designed more like a John Deere, the results would show the advantage going toward the undersquare motor.

I remember a magazine that did a test just like what you're describing using a Ford 351 and a 347 (stroker 302 motor) in a street type application. The longer stroke 347 outperformed the 351. If they had tried it using Nascar components, the 351 would probably have done better.

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you can see this in action with the KTM's. The SX has the high reving motor--shorter stroke bigger pistion....the EXC has the longer stroke, mellower hit, more tractable power delivery. Both have the same CC's....and the same basic engine design....so one is great of motocross and the other off-road...although sometimes they over do it for the off road and the bikes become to mello--

ANother example: when Husky wanted to make the 450 a 510--they didnt just bore it out...but went with a longer stroke. SO the bigger Husky is actually a mellower power delivery over the smaller 450.

Dont forget too that there are square engines that have equal pistons and strokes...

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well let me clarify a little bit. i am really interested in how two identical engines would perform. same displacement, same cam, same valves, same comression ratio etc...the only different being the bore and stroke.

it seems to me that the undersquare would have less torque because the crank shaft geometry, but would allow for high revs as already pointed out by Jester220.

an oversquare crank radius would place the crank pin farther away from the center of the crank...giving the piston a better mechanical advantage and thus more torque, but would prevent high revs, or reving too fast.

***************************************************

this is also under the assumption that bore > stroke = under square *

and stroke > bore = over square...............................................*

***************************************************

you do have it backwards

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You can take the same built engine,per say a small block with a 4.125 bore,3.750 stroke,same indentical heads and valve train.....

Now in one engine have a 5.700 connecting rod length and the other a 6.00 rod and the one with the 6.00 rod will build better H.P. and torque figures,will breath better and have a better rod ratio witch means less piston friction..

And yet tou can still use a longer rod yet,say 6.125...

The longer rod makes the piston dwell better and makes the engine breath better and less side loading on the piston,inwitch makes better horsepower.

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frankstr is correct ------you can run a stroke and add a long rod to help slow the piston down and reduce the side load,------most all of my motors are long rod strokers motors,--you can start to get the best of both worlds------i take the athena 488 kit on a crf 450 and add a 1.75 mm stroke and keep it ama legal 502 and with the correct cam and cam timing and porting it is a killer,-----i have a good trx 250 motor i build with a 132 mm ktm rod and a 3mm stroke with a 72 mm bore witch i call a 310 and i Finlay developed a pipe to work with my strange porting and it is a banshee killer up the hill,---the thing torques hard at 3500 and revs all the way to 10,000 and the power doesn't drop off ------it is so kool to have a small giant killer,---it gets protested all the time------you dump the coolent ,pop the head and measure it out and it is only a 305--------developing a motor does take alot of time and energy and its funny sometimes what ends up working,--------and the oval piston honda was a nsr 750 ,---i raced that bike in the suzuka 8 hr event in 87 -----that thing was so fast and bitchen and it was way kool on how it all worked i helped developed the slipper clutch on it and worked for months to come up with a cam and the correct cam timing that worked--------you can make all the stuff work but most people stay with what is already out there and developed and some of it just barely worked so when you change something it screws the whole thing up-------this is what people don't under stand ------you have to be very carfull and work with it with patients!

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frankstr is correct ------you can run a stroke and add a long rod to help slow the piston down and reduce the side load,------most all of my motors are long rod strokers motors,--you can start to get the best of both worlds------i take the athena 488 kit on a crf 450 and add a 1.75 mm stroke and keep it ama legal 502 and with the correct cam and cam timing and porting it is a killer,-----i have a good trx 250 motor i build with a 132 mm ktm rod and a 3mm stroke with a 72 mm bore witch i call a 310 and i Finlay developed a pipe to work with my strange porting and it is a banshee killer up the hill,---the thing torques hard at 3500 and revs all the way to 10,000 and the power doesn't drop off ------it is so kool to have a small giant killer,---it gets protested all the time------you dump the coolent ,pop the head and measure it out and it is only a 305--------developing a motor does take alot of time and energy and its funny sometimes what ends up working,--------and the oval piston honda was a nsr 750 ,---i raced that bike in the suzuka 8 hr event in 87 -----that thing was so fast and bitchen and it was way kool on how it all worked i helped developed the slipper clutch on it and worked for months to come up with a cam and the correct cam timing that worked--------you can make all the stuff work but most people stay with what is already out there and developed and some of it just barely worked so when you change something it screws the whole thing up-------this is what people don't under stand ------you have to be very carfull and work with it with patients!

the long rod stroker seems like the way to go. did you have to modify the head, cam, valves at all so the increased piston reach at TDC wouldn't smash into them?

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the long rod stroker seems like the way to go. did you have to modify the head, cam, valves at all so the increased piston reach at TDC wouldn't smash into them?

The long rod kits I've seen use a spacer under the cylinder. On a thumper you may need a longer cam chain depending on thickness, available slack, etc.

Other things to think about are clearance for the valve cover, carb/frame, oil line length (YZ's and WR's), etc. Look at your bike and determine what would be affected if you raised the cylinder.

:cry:

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Read this:

Engines : An Introduction

by John L. Lumley

Assuming you already know what all the bits do, this will tell you all that you want to know.

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5000 FPS is the practical limit for most piston engines.

5000 Feet Per Second? You surely must mean feet per minute?

WR450F is approx 4600 feet per MINUTE.

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