Bore And Stroke Being Squared

[crawl9]

What are the Advantages and Disadvantages of Have a motor that is termed Squared and How does this effect power??
Say you built a 125 and was wanting to make it like a 147 cc.
how would makeing it a 54mm bore with a 64.5 stroke(makes it a 147) Differ from making it a 57 Bore and a 57.5 Stroke (makes it a 147) Differ from making it a 59 bore and a 54. stroke.making it a 147

They all would have the Same Displacment. Would one have more low end or top end as compared as the other? Or could they all be set up to make the same power with porting and head mods?

 

[Fark]

Typically a longer stroke lowers the RPM ceiling. And I can't base this statement on any hard data but I think a longer stroke will also produce more torque.

 

[bclapham]

do a search on "stroking" there was a couple of great threads on this

 

[MikeT]

I think Fark is correct. From what I understand, an oversquare configuration say 64mm bore and 54mm stroke will produce more top end power and an higher RPM ceiling. An undersquare config. will yeild more low end power and a lower RPM ceiling.

 

[jmics19067]

everything is a compromise. Although the shorter stroke doesn't have to throw the piston as far from the center line you are throwing a bigger piston.
And even though you have more mechanical leverage with the longer stroke you dont have as much btus of energy being produced on top of the smaller piston.

It appears that in the 70's they used large bore short stroke engines that went square in the 80''s to 90's and now are favoring a longer stroke smaller bore . The 1970's YZ400 actually have less stroke than a new YZ 250. yet I am sure the 250 will rev higher. Apples and Oranges but I know but you can kinda cathc my exaggerated feeling. I am not sure how one would design something from scratch but if modifying an existing engine you definately have to find the limitations of existing design before you try and add.
Besides the obvious of boring it out to the water jackets or stroking it so the crank saws the cases in half you have to look at what would complement or at the very least not hinder the stock location of what was left. Probably a four year degree and definately lots of math to try and get even close to giving you a decent answer on this.

 

[motometal]

there is a difference between high RPM power and high RPM safety.

A short stroke needs bigger ports to fill a larger cylinder (all else being equal). A short stroke reduces piston speeds, allowing the engine to rev to a higher max RPM safely.

Of course, this is an oversimplification. We haven't even addressed rod to stroke ratio yet...

 

[cujet]

Since I assume you are talking 2 strokes, there is also a simple geometric difference in port area available in your examples. That is one reason why a 500CC single 2 stroke even one that is a direct enlargement of a smaller engine won't rev as high. Think about it this way, the surface area of the cylinder (therefore avail port area) does not increase at the same rate as the displacement. Pi*r2*H=displacement, Pi*d*H=cylinder surface area.

In addition, the size and shape of the ports changes slightly with the various examples. This also has an affect on the power output.

Chris

 

[bwalker]

One more trade off-large piston surface areas are more prone to detonation.

 

[crawl9]

Hey motometa,

So your Saying if You stroke a motor Say with a 54mm bore and a 54.5 stroke and make it a 54mm bore with a 64.5 stroke. That Your Ports will Have to Be Smaller on the 64.5 stoked motor than the 54.5 Stroke motor? Also, What's Rod to Stroke ratio and how does it affect the engine?

Also, Does anyone Know what the Safe Limit Piston Speed is on a modern bike Is? Also, Does anyone know how to figure out The Blow down duration of a engine at a certain RPM?

 

[cujet]

crawl, you example of a stroked engine allows the use of taller ports, again simple geometry.

The long rod engine has certain benefits, among them is more piston dwell time at TDC/BDC. This in itself changes port timing slightly. It also allows combustion to occur more completly.

Chris

 

[Brianc]

Originally posted by crawl9
What are the Advantages and Disadvantages of Have a motor that is termed Squared and How does this effect power??
Say you built a 125 and was wanting to make it like a 147 cc.
how would makeing it a 54mm bore with a 64.5 stroke(makes it a 147) Differ from making it a 57 Bore and a 57.5 Stroke (makes it a 147) Differ from making it a 59 bore and a 54. stroke.making it a 147

They all would have the Same Displacment. Would one have more low end or top end as compared as the other? Or could they all be set up to make the same power with porting and head mods?

The short answer is the longer stroke engine ( under square ) will produce more torque and a more broad power range ( less peak hit ). The oversquare engine allows the use of larger valves which could improve the volumetric efficiency. But you probably don't have the facility or resources to change your valve(s) to take advantage of the bigger bore. If you are road racing ( superbikes; etc. ) you will want the higher revs from the oversquare design. For us mere mortals in the dirt, you will probably reap more benefit from the long stroke - under square design. An engine with a stroke of 64 mm will safely spin 13,000 r.p.m. Another important factor is all other parts being equal, your long stroke version will have a higher compression ration which will produce more torque and horsepower. If you increase the stroke from 54 to 64.5 your compress will increase almost 20% ( using the same piston, etc. ). I think most people go to the big bore piston kits because it is a lot less work and a lot cheaper. But you get more fun usable power from stroking your crank.

 

[Brianc]

Originally posted by crawl9


Also, Does anyone Know what the Safe Limit Piston Speed is on a modern bike Is?

[/B]

Drag engines go as high as 7000 fpm, for dirt bike I remember reading 5000 fpm is a safe piston speed for long engine life.

 

[motometal]

piston speed is commonly referred to, but I would think PISTON ACCELERATION is what really matters. The rod length and stroke would affect that.