70cc class Dyno day

[MustangMike]

Some evidence of the stroke doesn’t dictate torque output conversation from the main dyno threads.

Port layout and general design will effect torque output more than Simple bore/stroke ratio for equal size two strokes, good volumetric efficiency makes good torque, long or short stroke.

If you are designing a high torque engine you most likely aren’t designing a lightweight high rpm unit, chances are it’s somewhat industrial, so service life, reliability and thermal efficiency are high on your list of needs.

Long stroke can help achieve these requirements, long stroke engines have lower surface area to volume ratios, so less heat lost to cylinder walls (good fuel efficiency) and Lower comparative big and little end bearing loads, good for bearing life or to allow reasonable bearing loads at lower rpms than short stroke units of comparable output.

High torque engines are often long stroke due to general design requirements that just often go hand in hand with each other, they aren’t high torque as a direct result of having a long stroke, it just so happens that if you want high and reliable torque output you will probably be long stroke, a bit of chicken or the egg.

Another factor you did not mention: Generally high torque is desired at low RPMs and high Hp are higher RPMs. A longer stroke will result in a larger diameter of crank rotation. This is not a problem at lower RPMs, but the centrifugal force increases rapidly at higher RPMs, as does the piston speed, creating additional stress on the bearings.

So, for high RPM Hp a shorted crank is generally preferred, but for lower RPM torque a longer crank is generally preferred.

 

[Sawrain]

Another factor you did not mention: Generally high torque is desired at low RPMs and high Hp are higher RPMs. A longer stroke will result in a larger diameter of crank rotation. This is not a problem at lower RPMs, but the centrifugal force increases rapidly at higher RPMs, as does the piston speed, creating additional stress on the bearings.

So, for high RPM Hp a shorted crank is generally preferred, but for lower RPM torque a longer crank is generally preferred.

For sure a long stroke will limit your your maximum rpm due to high piston acceleration forces, a mechanical issue to overcome.

If you need to turn high rpms you might at some point need to limit/reduce stroke.

If you need to make large amounts of torque (forced induction amounts) with a short stroke you will have high bending loads across the piston, high con rod compressive loads and high bearing loads, all mechanical considerations, but no hard limits like you hit with high rpm long stroke.

I’m just pushing the idea that stroke is not that responsible for power delivery and torque output.

Another factor you did not mention: Generally high torque is desired at low RPMs and high Hp are higher RPMs.

Are we talking chainsaws or generally?

 

[MustangMike]

Are we talking chainsaws or generally?

Generally, most saws limit at a similar RPM range.

 

[00wyk]

A saw has to be small.
Small engines make less vibration with shorter strokes.
A shorter stroke makes the engine lighter.
The longer a stroke, the more counter balance(weight) and over-all size(weight) an engine needs, even if it is not laying in your hands(weight).

McCulloch knew this when they made their 125 a 58mmX46.6mm affair vs Yamaha making their YZ125 motocross engine a 54mmX54.4mm. Both engines have a buttload of ports and lovely reed valves, but one is made to take advantage of the fact it is strapped to a big motocross frame with a real gear box and intake/exhaust system allowing it to produce good power, while the other is designed to be hand held and not run out of fuel before you are finished with a face cut. One has a big rod and crank, big carb, and big counter balancers. The other, not so much. One is square, the other not. One makes double digit horse power from 5k RPM all the way up to 13K RPM(delivering 35hp in it's modern stock form at 12.5K), the other is the McCulloch.

McCulloch 125 crank(courtesy of BigMac):

Yamaha YZ125 crank:

TL;DR - Weight+Vibes

 

[Sawrain]

A saw has to be small.
Small engines make less vibration with shorter strokes.
A shorter stroke makes the engine lighter.
The longer a stroke, the more counter balance(weight) and over-all size(weight) an engine needs, even if it is not laying in your hands(weight).

McCulloch knew this when they made their 125 a 58mmX46.6mm affair vs Yamaha making their YZ125 motocross engine a 54mmX54.4mm. Both engines have a buttload of ports and lovely reed valves, but one is made to take advantage of the fact it is strapped to a big motocross frame with a real gear box and intake/exhaust system allowing it to produce good power, while the other is designed to be hand held and not run out of fuel before you are finished with a face cut. One has a big rod and crank, big carb, and big counter balancers. The other, not so much. One is square, the other not. One makes double digit horse power from 5k RPM all the way up to 13K RPM(delivering 35hp in it's modern stock form at 12.5K), the other is the McCulloch.

McCulloch 125 crank(courtesy of BigMac):

Yamaha YZ125 crank:

TL;DR - Weight+Vibes

If you are designing a high torque engine you most likely aren’t designing a lightweight high rpm unit

I.e. a handheld implement.

Great point about shorter strokes helping with vibrations, I had not considered that.

Thanks for the pictures, I had not seen a McCulloch 125 crank, I think we are agreeing that it is overall design that largely dictates an engines performance, and stroke is just one of many puzzle pieces of the picture?

This can be a hard topic to discuss online, if not only for the fact that we are all probably thinking of different scenarios when reading and replying, when I mention high torque engines, I am not really thinking about chainsaw two strokes, in terms of power and torque the reality is they are not that high performance, as pointed out it is just not their primary design goal, which is made obvious every time you handle a two stroke dirt bike cylinder.

Getting back to the original intention of my comments on torque and stroke, a good example is the 462 and 572, some have seen the 462 has a 34mm stoke, the 572 a 39mm stroke and decided on the operating characteristics then and there, can't do that.

The below quote from AnotherSite is one such that just makes too many assumptions and claims hinging on the 5mm stroke difference.


"The MS 462 has a 52mm Bore and 34mm Stroke
The MS 461 has a 52 mm Bore and a 36mm Stroke
The 572xp has a 48mm Bore and a 39mm Stroke

The longer stroke will mean more low end torque especially with heavier components. The 461 has a heavier flywheel than either of the new saws with the other two being very close to the same flywheel weight.

The longer stroke of the 572xp with the new style engine designs that force in outside air the 572xp will be very close in power with a 461 but have a lot of potential ported. It wont be a screamer like a 462 but it will do what a 70cc is supposed to do better."

Joes Dyno has found this not to be the case, and even if it was, which it very well could have been, the reasoning behind it would have still been flawed.

The McCulloch vs Yz125 was an interesting one, lets also compare some contemporary, similarly dimensioned (bore/stroke) two strokes with completely different performance criteria.

The KTM 65 SXS from a dyno chart (found online) shown further down, this being the factory hot rod version rated at 19.5hp (at 65cc!), and the 572xp, using Joes dyno results.

And I am not at all trying to be negative to the 572xp with this comparison, it just had the closest bore/stroke dimensions of those listed here.

KTM 65 SXS
bore and stroke 45.0 x 40.8 mm,
64.9cc,
17.4hp @ 11,700 rpm,
7.9ftlb @ 9,800rpm

Husqvarna 572xp
bore and stroke 48.0 x 39.0 mm,
70.6cc,
6.70hp @ 9,500 rpm,
3.8ftlb @ 7800rpm

Considering drive train losses reducing the dyno figure, this dirtbike engine is putting out 8.5ftlb!, same as the ported 3120 "practice saw" with 60x42mm bore and stroke.


KTM Crank Below, basically the same stroke as an ms660 or 2100, though a little more sturdy looking.



572 crank below, looks like they wanted the volume/compression benefits of a circle crank, but didn't want the weight or need the strength, so added crank stuffers.


Below a KTM 65 sx cylinder, check out those transfers!

KTM dyno 65 SXS dyno chart, 572xp roughly overlaid.


Ok, so I am not at all suggesting this could be a saw engine, it would take a large amount of airflow to cool it for one, I am just highlighting a few things, how much is theoreticality left on the table in terms of torque output (though the bike engine greatly benefits from a tuned pipe compared to OPE), that generally high horsepower also equals high torque (you just pay the price of power delivery being less than optimal) and that a modern saws torque output is not decided by stroke or crank and flywheel weight.

 

[lehman live edge slab]

Everyone should own an 090 for nothing more than to remind them all new saw's are featherwhights lol
I remember when Stihl saw's used to be heavier than Husky saw's and the Husky guy's would say Stihl saw's are heavy pigs but now it's the other way around the Husky guy's say it's because Husky saw's are build with more meat and build to last unlike that other rubbish saw manufacture lol
I can't keep up not sure if a heavy saw is better or a light saw anymore is to confusing for me

I have an 090 and it kicks my arse every time I run it on the mill or bucking big rounds for half the day

 

[lehman live edge slab]

that’s what husky guys do

Unless your the husky guy I work with, he tried a Stihl bar with adapters to compare to the husky/ Oregon ones he was using and decided he liked the Stihl bar way better. Only thing is he has to paint the bar so people don’t know he’s running a Stihl bar on his husky. I told him if you think the bars are good just imagine if all the parts had that unmentionable name on them.

 

[Stump Shot]

Torque in a two stroke is achieved in just completely the opposite way it is in a four stroke engine.

 

[Red97]

Torque in a two stroke is achieved in just completely the opposite way it is in a four stroke engine.

Which way? I'm curious.

 

[Deets066]

Which way? I'm curious.

When you wake up in the morn with a rod on and you bend it down to piss. When it picks you up on your tippy toes..... that’s torque.

 

[Stump Shot]

Which way? I'm curious.

Stroke

 

[Nutball]

Back to that KTM cylinder, do they inject fuel through the transfers next to the exhaust above the main transfers? With or without air coming from those special transfers?

 

[rogue60]

Industrial engines such as chainsaws have nothing in common with performance 2T engine's as found in mx bikes for example they are there own breed of engine.
Due to design restraints everything about an industrial chainsaw engine is a compromise.

 

[Red97]

I am very interested in this stroke thing.

Only 1 of 4 long stroke saws out performed the short stroke counterparts...

That was the 661 stroker.

The rest of the "long" stroke saws have little to no more tq than the short stroke.

Dolmar 37mm
572 39mm
Echo 620 37.6mm


Just wonder if it has something to do with the operating range? Maybe if one saw used a big heavy long stroke and a lower range? It may show greater tq?

That big echo had 7 ftlb and a 44mm stroke.

But all the other shorter stroke saws made even more tq than the big echo? Granted that was ported vs stock.

More questions than answers.

 

[Stump Shot]

I am very interested in this stroke thing.

Only 1 of 4 long stroke saws out performed the short stroke counterparts...

That was the 661 stroker.

The rest of the "long" stroke saws have little to no more tq than the short stroke.

Dolmar 37mm
572 39mm
Echo 620 37.6mm


Just wonder if it has something to do with the operating range? Maybe if one saw used a big heavy long stroke and a lower range? It may show greater tq?

That big echo had 7 ftlb and a 44mm stroke.

But all the other shorter stroke saws made even more tq than the big echo? Granted that was ported vs stock.

More questions than answers.

In a four stroke engine the torque will increase with the lengthening of stroke as the power stroke is a full 180 degrees of push on the piston(power). Where as the(piston ported) two stroke is limited by the exhaust port as soon as the piston reaches it, the power stroke(pushing down) is over. Also piston speed is increased mid cycle from the extra throw of the crank from the longer stroke which helps with speed(horse power).
We only have to look at the examples of yesterday's models of power saw's to prove this, as newer models replaced older versions piston diameter shrank while stroke was increased and the new saws ran faster.
Which is why I say it's the opposite of four strokes.

 

[Sawrain]

Back to that KTM cylinder, do they inject fuel through the transfers next to the exhaust above the main transfers? With or without air coming from those special transfers?

Standard carburettor case inducted two stroke, all
Intake supply is wet.

Industrial engines such as chainsaws have nothing in common with performance 2T engine's as found in mx bikes for example they are there own breed of engine.
Due to design restraints everything about an industrial chainsaw engine is a compromise.

My example had one perfectly good thing in common with a chainsaw, it’s stroke was close enough to make it relevant, for this conversation.

Of course nothing else is really comparable, but it’s still a good example, and interesting to see what you get with our design restraints removed, and I imagine some people didn’t expect such torque.

I Just wonder if it has something to do with the operating range? Maybe if one saw used a big heavy long stroke and a lower range? It may show greater tq?

That big echo had 7 ftlb and a 44mm stroke.

But all the other shorter stroke saws made even more tq than the big echo? Granted that was ported vs stock.

More questions than answers.

It has everything to do with operating ranges.

It we are talking saws of similar performance (power) but different operating ranges, torque is the variable.

The echo makes 7hp at only 6500rpm, therefore it has to make big torque figures to make good power, an inescapable fact, it needs to make good torque to make power at a low rpm.

If a dolmar 9010 (42mm stroke) made 7hp at 9500rpm it would only need ~70% of the torque.

Now run different sprockets that cancel out chain speed differences and the actual “pull” on the chain is the same.

Within the somewhat narrow spread of bore/stroke ratios we have in our (modern) saws it is all about the porting, not bore/stroke, does port shape dictated by bore/stroke play a roll? well yes I guess it might for high performance applications, but we know factory saws probably have a minimum of 20% Torque left on the table, so it makes it hard to predict a saws character from specs Alone when they are a manufactures choice, not limited by design. (Stock)

Food for thought, picture a 9010, oh yeah that long 42mm stroking leverage is going to make great torque, hammer time, 9010 is 52mm piston.

Now Picture a 661, hell yeah, the big old 56mm piston is gonna push down hard on that crank, all that area for combustion pressure to push on, torque city here we are, 661 is 37mm stroke.

What the 661 loses in Stroke it makes up for in the same proportions in piston area.

As for porting to take advantage of different strokes, (not rpm ranges) that is for other people to Answer, however I would think most chainsaw bore/stroke ratios close enough to each other for this to not be a huge factor?

 

[Red97]

Now Picture a 661, hell yeah, the big old 56mm piston is gonna push down hard on that crank, all that area for combustion pressure to push on, torque city here we are, 661 is 37mm stroke.

What the 661 loses in Stroke it makes up for in the same proportions in piston area.

As for porting to take advantage of different strokes, (not rpm ranges) that is for other people to Answer, however I would think most chainsaw bore/stroke ratios close enough to each other for this to not be a huge factor?

I think you may be on to something, I'll have to go back and look at the early tests, but the larger bore size seems to make more tq than the longer stroke.

I have to find the stock 066 Vs 395 graph

Being all but the largest saws make peak power around 9k the long stroke isn't really helping at those speeds.

I think it gets confusing, a broad power band, vs high tq.

 

[Rogee]

This is a very interesting subject . I was told that long stroke equaled torque but in driving tractors from a young age I found the big bore short stroke Ford tractors had a lot more in the paddock torque than the the longer stroke John deeres of the same displacement. I came to the conclusion that its all in how an engine breathes that determines its power . If it can get the air into the cylinders it can produce power.

 

[Sawrain]

I think it gets confusing, a broad power band, vs high tq.

Very much, I was going to say something similar.

I think some of our disagreements come from our terminology, and what we mean by torque.

When I say torque I generally mean torque the absolute figure, where you can easily say one saw has more torque than another.

Obviously I know that the term torque is used as a way of saying a saw has good low down power or torque, but this is more about torque curve than absolute torque.

Saws that have good torque gain when pulled down in rpm give a sense of being “grunty” and easy to use, also described as torquey, conversely this kind of saw is the one that disappoints on the dyno, as having a good torque gain when dragged down is the same as losing torque when rpm increases, the saw might be strong under heavier than normal load, but just can’t deliver between true peak torque and normal cutting speeds.

Edit, even the type of motor we are takings about changes the context of the word torque, for example saying that a Honda quad bike (I have one) has good torque can only mean it has good low down torque-power, you would never say a Honda quad bike has good torque with the absolute figure in mind.

 

[Red97]

Very much, I was going to say something similar.

I think some of our disagreements come from our terminology, and what we mean by torque.

When I say torque I generally mean torque the absolute figure, where you can easily say one saw has more torque than another.

Obviously I know that the term torque is used as a way of saying a saw has good low down power or torque, but this is more about torque curve than absolute torque.

Saws that have good torque gain when pulled down in rpm give a sense of being “grunty” and easy to use, also described as torquey, conversely this kind of saw is the one that disappoints on the dyno, as having a good torque gain when dragged down is the same as losing torque when rpm increases, the saw might be strong under heavier than normal load, but just can’t deliver between true peak torque and normal cutting speeds.

Edit, even the type of motor we are takings about changes the context of the word torque, for example saying that a Honda quad bike (I have one) has good torque can on your mean it has good low down torque-power, you would never say a Honda quad has good torque with the absolute figure in mind.

That is the way I like to refer to torque too. The actual number.


Seems some of the older tq saws don't have an abundance of low end tq, they just simply lack the horsepower.

8k unloaded rpm and 6k peak and the clutch stalls at 5k. Only a 3k rpm spread

Take a modern saw that spins 14k and still stalls at 5k. That is a 9k rpm drop, which feels like a lot more than the old saw, but in reality that new saw still is delivering more tq across the board.

Say homelite SXL at 58cc
Vs modern 60cc of whatever flavor