Short stroke v's long stroke saws

[TJ the Chainsaw Mechanic]

Coming from the truck engine world....

A Long stroke tends to add more torque.

Whereas a Big Bore tends to add more power.

NOW...that is Super general. Air flow, cam, carb, bottom end weight, and compression tend to screw that theory up really bad.

I've noticed engines seem to make the best power when the bore was larger than the stroke. Mainly in my GMC's V6. The Huge bore and small stroke makes for low power, but decent torque for its size. Yet we have to consider.....this engine has a crankshaft that weighs as much as the bare block...low cam lift...and at the time..a small 2 barrel. Naturally it's gonna make a ton of torque at a low RPM. Lighten the bottom end...add cam lift and more carb...we make more power.


Oh hell....we're talking about chainsaw motors.

 

[Magic_Man]

How can rod length change stroke? 4" or 4', it still moves the same distance from bdc to tdc.
Or am I seeing this wrong?

Stroke is simply the distance from BDC to TDC. The length of the rod has no bearing on the stroke... It will be the same whether the rod is 1" or 1' in length.

That's what I thought. Thanks for clarifying. Thought I was losing my marbles...

Sorry gentleman, I was having a complete boneheaded moment. That's what I get for trying to get engaged in a conversation here while trying to work at the same time. You are correct in that rod length does not affect stroke.

 

[Hinerman]

@awol to the rescue.

 

[Magic_Man]

What I was trying to get at and got myself crossed up is what Shawn mentioned, time-area.

 

[Magnumitis]

smaller bore/longer stroke will be a better performer than the larger bore/shorter stroke of the same displacement but will wear out rings, cylinder, piston sooner because it is traveling faster and further up and down the bore than the larger bore/shorter stroke at any RPM

 

[paragonbuilder]

Sorry gentleman, I was having a complete boneheaded moment. That's what I get for trying to get engaged in a conversation here while trying to work at the same time. You are correct in that rod length does not affect stroke.

No worries. You made me think for a while. That's worth something! [emoji12]

 

[awol]

@awol to the rescue.

No worries Thomas, these guys got it covered and corrected just fine!


For my saw building, I'll take an extra mm of stroke over an extra mm of bore anytime. It seems like in the saw world, less oversqare engines have more of a 'hotrod' feel and like to rev, while more oversquare engines have a wider, more forgiving powerband and like to lug; the Husky 385/390 and 372xp/372xpw are good examples.

 

[mdavlee]

I like some of the longer stroke saws better for some things. Milling the 395 seems to do as good or better than the 9010. 4mm bore difference.

 

[Barneyrb]

Not only stroke length but rod length has a serious affect on torque output. A longer rod will allow the piston to have more dwell time at TDC which will build more energy in the combustion chamber and it will provide more leverage on the crankshaft when the energy forces the piston downward.

 

[Nitehawk55]

Big stroke = lots of torque at lower RPM (thinking Harley here)
Short stroke = lots of RPM (screaming Jap bike)

I'll take the Harley thanks

 

[Brewz]

For my saw building, I'll take an extra mm of stroke over an extra mm of bore anytime. It seems like in the saw world, less oversqare engines have more of a 'hotrod' feel and like to rev, while more oversquare engines have a wider, more forgiving powerband and like to lug; the Husky 385/390 and 372xp/372xpw are good examples.

Good description!

I am looking at this keeping the timber we Aussies have to cut in mind. Its hard stuff and more chain speed is not king.
More chain speed just seems to lead to excessive chain stretch, clutch wear and sharp chain that seems to want to skip over the wood because its traveling too fast to get a good bite due to the timber hardness.

After porting a few longer stroke saws like 026, 044, 064 etc, I am really happy with how they cut but they seem to have a narrow power band high up.
I am interested in playing with these shorter stroke saws to see if I can get the same cutting performance with a wider power band to handle the timber.

About to make a phone call and see if I can get myself a nice 028 super to mess with.

 

[exSW]

Just how much constitutes a long stroke saw. My 757 is 36mm. I believe my Alpina 700 is also 36mm. What's a Dolmar 6400,an 038 and so on.

 

[Brewz]

Big stroke = lots of torque at lower RPM (thinking Harley here)
Short stroke = lots of RPM (screaming Jap bike)

I'll take the Harley thanks

That's fine if you want a motorbike to cruise along on all day, but we are talking chainsaws.

 

[Brewz]

Just how much constitutes a long stroke saw. My 757 is 36mm. I believe my Alpina 700 is also 36mm. What's a Dolmar 6400,an 038 and so on.

Here is a good page for Stihl.

http://caseywise.com/?p=3790

There used to be one for Huskies bit I cant find it?

 

[huskihl]

Just how much constitutes a long stroke saw. My 757 is 36mm. I believe my Alpina 700 is also 36mm. What's a Dolmar 6400,an 038 and so on.

The 6400 is 37mm, 038 is 34mm stroke

 

[Magic_Man]

No worries Thomas, these guys got it covered and corrected just fine!


For my saw building, I'll take an extra mm of stroke over an extra mm of bore anytime. It seems like in the saw world, less oversqare engines have more of a 'hotrod' feel and like to rev, while more oversquare engines have a wider, more forgiving powerband and like to lug; the Husky 385/390 and 372xp/372xpw are good examples.

Big stroke = lots of torque at lower RPM (thinking Harley here)
Short stroke = lots of RPM (screaming Jap bike)

I'll take the Harley thanks

So I just had a uricka moment and it's all making sense now. In the 4 stroke world, bikes, drag cars, etc. we strive for bigger bores. Reason for this is the larger the bore the bigger the overhead valves we can fit and less they are shrouded by the cylinder walls. In 2 strokes we don't have valves, we have ports, and those ports are located on the cylinder walls. So the longer the stroke we have the more available port area we have with less constraint from the piston.

 

[sawfun]

I'm guessing here but the 661 has a longer stroke due to the EPA as it will burn more of the fuel thus likely run hotter further aiding in more burn? Stihl is trending to a wider torque band as well so a longer stroke aids in that as well. Another guess would be the reshaping of the ports or fuel direction to get the speed up giving a better supercharging effect thereby keeping the horsepower up rather than sacrificing it to the torque part of the equation. Maybe they will have some variable ignition timing in the future as well aiding in starting, engine snap, and preventing detonation or preignition.

 

[Iron.and.bark]

I like some of the longer stroke saws better for some things. Milling the 395 seems to do as good or better than the 9010. 4mm bore difference.

395 is murdered with a bigger bar milling by a 144 dolmar (early 70's). 90cc saw with 52mm bore and 42mm stroke (3120 has same stroke).

Also you have to look at transfer designs. Way tighter like the old Villiers 2 strokes. I took some pictures comparing an 084 cylinder to a 166 cylinder a while ago. Interesting to say the least.

 

[mdavlee]

I'm guessing here but the 661 has a longer stroke due to the EPA as it will burn more of the fuel thus likely run hotter further aiding in more burn? Stihl is trending to a wider torque band as well so a longer stroke aids in that as well. Another guess would be the reshaping of the ports or fuel direction to get the speed up giving a better supercharging effect thereby keeping the horsepower up rather than sacrificing it to the torque part of the equation. Maybe they will have some variable ignition timing in the future as well aiding in starting, engine snap, and preventing detonation or preignition.

661 is 38 mm of stroke vs 40 in the 1122 series.

 

[mdavlee]

395 is murdered with a bigger bar milling by a 144 dolmar (early 70's). 90cc saw with 52mm bore and 42mm stroke (3120 has same stroke).

Also you have to look at transfer designs. Way tighter like the old Villiers 2 strokes. I took some pictures comparing an 084 cylinder to a 166 cylinder a while ago. Interesting to say the least.

No doubt. Those old saws have plenty of torque.