Short stroke v's long stroke saws

[sawfun]

A single ring piston will make it easier to design the piston with a higher pin height allowing for a longer rod. I've seen pictures of pistons with a ring just about into the pin button. Of course as wear occurs the piston ends up leveraged into the side of the cylinder it is cocked at increasing the amount of wear further. This exaggerated wear seems to be easier to see in V configured engines.

 

[drf256]

Great topic guys.

Always made me wonder why domed/hemi piston and chambers are used by manufacturers as well. There will be more surface area to cool and exert force downwards with a dome.

I think of stroke like the pedals of a bike. Longer ones are a longer lever arm, so they will make more torque at a lower rpm. Piston speed will be faster with a longer stroke, so parasitic losses will be higher-inertia and friction. Use a 4 ft pipe on the back of a monkey wrench (no offense Randy) and you're gonna move something less distance but with greater force than a 1 ft bar. It will just take you longer to do so.

Bigger bores allow for more port surface area. That's why I always want the bigger bore on a model. It's not always because of the displacement increase.

The 038 vs. 044 issue has always boggled my mind. The 038 is more of a torque Saw than an rpm Saw, but it's more oversquare. It's likely the porting that makes the difference than the geometry itself.

Bigger bores also cool less effectively than smaller bores. There's less surface area of fins in relation to bore. Less heat needs to be added for combustion efficiency, hence why smaller bores like more compression.

 

[drf256]

Here's one I've been pondering a bit.

024 vs. 024S/026

30 vs 32mm stroke.

The 026 is limited in some ways by its high factory exhaust roof. More leverage from the longer stroke, but the roof will vent pressure at less crank degrees than the shorter stroke. Here are the piston position calculations for the two strokes:



So will the smaller stroke make more power because the port geometry of an 026 jug will allow a 105 exhaust with less machine work, or will the longer stroke take advantage of a longer lever arm even though it's venting combustion pressure earlier in the cycle than is desirable?

 

[drf256]

I believe the answer is that a longer stroke engine of the same bore will like a lower numerical degree exhaust roof because it's the same swept volume of the same bore with a shorter stroke at a higher numerical degree value.

Longer stroke motors should need a bigger exhaust port that has less duration but more area. The piston passes it faster.

Then there's the next confusing issue, piston speed at crank angle. The uppers will be open for an even shorter amount of time on a long stroke motor than on a short because they are near the bore center where piston speed is the greatest.

Sorry for the rambling here. I may be more confused now than when I started typing.

How does @awol 562 run so well with a 110* exhaust roof and a long stroke? If it was a Stihl 036 of the same displacement, that would relate to a roof in the mid to high teens. Not sure any of us would run that, but it makes one think.

 

[paragonbuilder]

I believe the answer is that a longer stroke engine of the same bore will like a lower numerical degree exhaust roof because it's the same swept volume of the same bore with a shorter stroke at a higher numerical degree value.

Longer stroke motors should need a bigger exhaust port that has less duration but more area. The piston passes it faster.

Then there's the next confusing issue, piston speed at crank angle. The uppers will be open for an even shorter amount of time on a long stroke motor than on a short because they are near the bore center where piston speed is the greatest.

Sorry for the rambling here. I may be more confused now than when I started typing.

How does @awol 562 run so well with a 110* exhaust roof and a long stroke? If it was a Stihl 036 of the same displacement, that would relate to a roof in the mid to high teens. Not sure any of us would run that, but it makes one think.

Those are some good thoughts Doc. I need to re read this a few times...

 

[Stump Shot]

I did find it interesting that some of the new breed of saws are longer stroke, smaller bore than the saws they replaced. Then I look at how much faster they spool up then their older counterparts and can't help but wonder how this goes on as it goes against everything I ever knew about the relationship between bore and stroke. Then again a lot of what goes on with this new breed of saw does not follow older rules. Will just have either get on board with it, or get the heck out of the way, cause it's obvious to see these new saws aren't just a flash in the pan.

 

[sawfun]

I did find it interesting that some of the new breed of saws are longer stroke, smaller bore than the saws they replaced. Then I look at how much faster they spool up then their older counterparts and can't help but wonder how this goes on as it goes against everything I ever knew about the relationship between bore and stroke. Then again a lot of what goes on with this new breed of saw does not follow older rules. Will just have either get on board with it, or get the heck out of the way, cause it's obvious to see these new saws aren't just a flash in the pan.

Don't feel too bad about the old school rules as a tunnel ram in the 70's and early 80's was for high rpm only and for the last 30 years or so it develops low rpm torque.

with the longer stroke trend in saws I think engineers have learned to get the horsepower up along with the torque rather than it being one or the other as in the past.

 

[LBracing]

DRF , here is something to think about , compare two 50cc engine

First engine :
40mm bore x 40mm stroke = 50.27cc
80mm conrod ( 2.0 ratio )
83° exhaust opening = 20mm high port
@50% of bore = 20mm wide
Total area = 400mm

Second engine :
80mm bore x 10mm stroke = 50.27cc
20mm conrod ( same 2.0 ratio )
Same 83° exhaust = 5mm high port
@50% of bore = 40mm wide
Total area = 200mm

So , the ports on the second engine are 4 times lower but only twice as wide ! Wich means two times less port area everything being equal other than the bore and stroke ratio !

Of course its exagerated but you get the idea. Of course piston speed also plays a role

 

[Hedgerow]

 

[drf256]

DRF , here is something to think about , compare two 50cc engine

First engine :
40mm bore x 40mm stroke = 50.27cc
80mm conrod ( 2.0 ratio )
83° exhaust opening = 20mm high port
@50% of bore = 20mm wide
Total area = 400mm

Second engine :
80mm bore x 10mm stroke = 50.27cc
20mm conrod ( same 2.0 ratio )
Same 83° exhaust = 5mm high port
@50% of bore = 40mm wide
Total area = 200mm

So , the ports on the second engine are 4 times lower but only twice as wide ! Wich means two times less port area everything being equal other than the bore and stroke ratio !

Of course its exagerated but you get the idea. Of course piston speed also plays a role

I like it.

Piston speed shouldn't play a role if port is 100% open. Both should be open for 194* of crank rotation as I see it. The longer stroke will open and close faster and there will be more frictional and inertia losses with the longer stroke, but the ports should both be open for the same amount of time.

The way I see it, your formulas show that a longer stroke motor should consistently have more port time x area than a shorter stroke motor of the same displacement. Backwards from what I thought, but numbers don't lie.

This may be the answer for why 562's like less exhaust duration. There should simply be more exhaust port area to expel exhaust gasses and not lose as much power stroke as needs to be tolerated in shorter stroke motors to gain time x area.

Very nice sir.

 

[Al Smith]

Great topic guys.

Always made me wonder why domed/hemi piston and chambers are used by manufacturers as well. There will be more surface area to cool and exert force downwards with a dome.
n
.

compression[/QUOTE] Hemi domed is nothing new .I had two motorcycles,Indian 249 single and 498 cc twin that had domed pistons and domed chambers vintage 1949 and 1950 .

The hemi has a different and maybe better flow across the piston face .On a 2 cycle it causes the flame front to propagate over the piston and apply more force on the outer sides sealing the rings better .A turned pop up has some what the same response or effect .It's just one method of increased performance design or redesign depending on how you look at it .

To turn a pop up on a flat piston is not hard to do .To cut a hemi it would take a radius cutter or a shaped tool which is not so easy .Either one once again is a preference not a hard fact that one works better than the other .

 

[jmssaws]

People call a 661 a tourqe saw but it's a bigger bore and a shorter stroke than a 660 but in general a longer stroke means less rpm and more grunt but I'm not sure that matters in a saw.

There has to be a good combination of both,a saw with no rpm and loads of tourqe is slow but the opposite of that is a pooch

The stroke in a 2 stroke is limited because it has to much have rpm to make power.
There can't be a small bore long stroke saw that turns 13.5k plus you can put a big bore short stroke in a smaller package.

We use to change the 5.7" rods to 6" rods in chevy motors to decrease rod angle to help longevity at high rpm

 

[LBracing]

On 660 vs 661
A difference of 2mm in stroke change the bore and stroke ratio from :
660 : 54/40 = 1.35 ratio
661 : 56/38 = 1.474

From my exemple above we determined the best bore to stroke ratio is 1:1 .

Knowing these numbers it is evident that porting design and other factors impact much more final comportement of the saw than a variation of .124 in stroke ratio making it go from very very oversquare to very oversquare.

Low rpm ?
I would like to hear why you can't have a long stroke small bore saw to rev 13.5k . I have successfully built a saw with a 38mm stroke and 42mm piston holding 15000rpm in the cut with a 9 pin sprocket . Plans for this year are 16000 rpm 10 pin sprocket with a custom built CNC cylinder .

Also note that 125cc gp engines are 54mm bore and 54mm stroke and make close to 54hp at 13000rpm .

Why do they do chainsaws manufacturers do it ?
A bigger bore and smaller stroke makes for a more compact and light engine wich is why chainsaws are designed this way not for performance reasons.

On rod length
Rod length reduces piston friction that's one point but not the most important . There are two other aspects :

-Long rods increase ''dwell time'' at both ends of the stroke wich means more time for the mixture to burn .

-Transfers , due to their overall height and BDC position , are the most impacted by the increased dwell time . Meaning that for the same 120° of opening they will be opened for a longer period of time on a long rod motor than a short rod .
Now , we should also know that any well designed race engine is limited by the transfers . Makes sense now why long rods is superior everythingelse being equal ?

 

[Red97]

I Really like small bore long stroke saws.

 

[Hedgerow]

I Really like small bore long stroke saws.

Yeah...
Me too..

They work bigger than what they are..

Then there's the 9010..
It's big...
And works bigger.

 

[Red97]

Yeah...
Me too..

They work bigger than what they are..

Then there's the 9010..
It's big...
And works bigger.

And they can spin too..

 

[Locust Cutter]

Yes they can. Though ported 390xp or 395xp (or both) will eventually be in the barn along with the 9010, just because.

 

[Iron.and.bark]

On 660 vs 661
A difference of 2mm in stroke change the bore and stroke ratio from :
660 : 54/40 = 1.35 ratio
661 : 56/38 = 1.474

From my exemple above we determined the best bore to stroke ratio is 1:1 .

Knowing these numbers it is evident that porting design and other factors impact much more final comportement of the saw than a variation of .124 in stroke ratio making it go from very very oversquare to very oversquare.

Low rpm ?
I would like to hear why you can't have a long stroke small bore saw to rev 13.5k . I have successfully built a saw with a 38mm stroke and 42mm piston holding 15000rpm in the cut with a 9 pin sprocket . Plans for this year are 16000 rpm 10 pin sprocket with a custom built CNC cylinder .

Also note that 125cc gp engines are 54mm bore and 54mm stroke and make close to 54hp at 13000rpm .

Why do they do chainsaws manufacturers do it ?
A bigger bore and smaller stroke makes for a more compact and light engine wich is why chainsaws are designed this way not for performance reasons.

On rod length
Rod length reduces piston friction that's one point but not the most important . There are two other aspects :

-Long rods increase ''dwell time'' at both ends of the stroke wich means more time for the mixture to burn .

-Transfers , due to their overall height and BDC position , are the most impacted by the increased dwell time . Meaning that for the same 120° of opening they will be opened for a longer period of time on a long rod motor than a short rod .
Now , we should also know that any well designed race engine is limited by the transfers . Makes sense now why long rods is superior everythingelse being equal ?

Nice post!

You designing the cylinder head / milling yourself?

On stroke to bore on big saws. Which is what I love.

3120xp 60/42 = 1.4285
084/088/800 60/43 = 1.3953
Echo cs 1201 58/44 = 1.3181
.....
Have to add this one, cos I am biased..
Dolmar 166 56/48 = 1.1666

Since I am underway
Mac 125 57.9/46.6 = 1.2424
Stilh 090 66/40 = 1.65

 

[LBracing]

Yes , designing everything from the ground up by myself , Solidworks skills come in handy !

 

[Redfin]

Yes , designing everything from the ground up by myself , Solidworks skills come in handy !

Would like to see some pics when you can.