Why do the old saws make more torque?

[Stihl 041S]

Any Ideas on Posts 10 & 12?

 

[Rookie1]

Are you holding the cylinder horizontal or vertical?

Ummm I was holding it crossways.

 

[Poley4]

Are you holding the cylinder horizontal or vertical?

LOL!
Honda did some testing a few years ago and discovered that a longer rod produced more torque without increasing the length of the stroke. They claimed that by moving the piston away from the crank with a longer rod put it at a lesser angle, which helped their torque numbers.

 

[timberwolf]

It's simple, the old farts that use old saws got to have something to crow about, and if the old saw aint as fast as the new saws, they boy ya better bet they got more torque. Snow was always deeper back in the day too, five miles to school uphill bothways into a head wind...

HP = (torque x RPM) / 5252

So to produce the same HP a saw turning 6,000 RPM is going to need to produce twice the torque of a saw turning 12,000 RPM.

 

[ShoerFast]

It's simple, the old farts that use old saws got to have something to crow about, and if the old saw aint as fast as the new saws, they boy ya better bet they got more torque. Snow was always deeper back in the day too, five miles to school uphill bothways into a head wind...

HP = (torque x RPM) / 5252

So to produce the same HP a saw turning 6,000 RPM is going to need to produce twice the torque of a saw turning 12,000 RPM.

Only 4 miles for me, but the same snow, hill and wind!

But had to fight Indians for my lard sandwiches.

 

[kevin j]

Honda did some testing a few years ago and discovered that a longer rod produced more torque without increasing the length of the stroke. They claimed that by moving the piston away from the crank with a longer rod put it at a lesser angle, which helped their torque numbers



yes, the longer the rod, the straighter it is against the crank through the uper part of expansion stroke, at the 3 oclock positon . Imagine a 4 inch rod, there is a lot of sideways angle, so the side thrust on piston is more, and the effective crank radius is less because of the angle. Imagine a 24 inch rod, it moves more striaght up and down, very little side force on piston

my 'not' meant, the dwell time is related to rpm only. one rev in the same time, regardless of geometry. There is difference in piston accelerations and motions due to stroke, and the rod to crank angle is affected near the top of the stroke, but the torque is the sum of many factors. Ultimately, the expansion of hot gasses through the stroke of the piston is what defines the power and torque, and that is determined by those many factors. mixture, burn rate, type of fuel, ignition timing, swirl of the charge in the cylinder, combustion chamber shape, heat losses to piston and chamber, cam timing/port timing, compression ratio in cylinder and in crankcase, transfer port angles, shapes, etc etc.

Sure, stroking an engine increases torque, because it is a bigger displacement... and again, the bore is smaller, so valve ara and breathing dynamics are different, so

basic physics, torque is the actual measure twisting forse on the crankshaft out. hp is a rate of doing work, calculated by torque times rpm / 5252 if t is foot lbs, or divided by 63025 if T in inch lbs. (5252 x 12 = 63025)

So any engine with more power has more torque IF both are measured at the same rpm. jsut that we are comparing apples and oranges. xx hp at 14000 is some torque number. for the same xx hp at 7000 rpm takes twice the torque number.
For a given engine, the torque rises as speed rises, then peaks, and drops off as rpm keep increasing. However, since hp is a product of speed and torque, the hp can increase even if torque drops off, as long as the torque drops less per centage wise than the rpm keeps increasing.

The shape of that curve, and where it occurs, high or low rpm, is a product of MANY factors. We just associate the differences with the most obvious factor we can see, in this case 'long sroke'.

Compare to 2cycle road racing motorcycles: Reed valves are fairly restrictive at high rpm, but very effective at low rpm, so they were great for low rpm engines. As speeds increased, piston ports, expansion chambers, exhaust cones, intake lengths all worked great for waymore power, but the torque curve became more peaky. The efficiency of the intake and exhaust depends on all the pressure waves going back and forth like organ pipes. When those waves hit right, the engine is 'on the pipe' and very strong. when the interact wrong, the power and efficiency drop dramatically.

4 cycle engines are mechanically complex, but simpler to engineer to run pretty well.
2 cycle engines are deceivingly simple looking mechanically, but more much complicated to engineer to run strong across a wide rpm range.

Saws, string trimmers, blowers are about perfect applciations: narrow well defined loads at one speed range, not really that important to run cleanly in acceleration or trailing throttle, idling is fairly easy to do.

getting power across wide rpm cleanly, (like observed trials motorcycle engines), is tougher to do.

The more I learn, the more I am amazed how much power these little die cast wonders put out.

All of the poijnts people are making are true in various applicaton and to greater or lessor extents. I am not critiquing any persons posts, just pointing out there are many factors, hidden or not, and to make blanket statements of this design feature does this or that is quite often over simplifying, or perpetuating some 'old wives tales' information.....


k

 

[stevethekiwi]

long stroke 2/ engines are no better than short stroke 2/ for torque or power as such. with a long stroke engine you are effectively reducing the bore as a ratio, and therefore piston area is not as large as it could be so you are limited to how much force can be exerted on the piston. you also lose crank speed on a long stroke due to the amount of inertia generated. so, long stroke = lower rpm = lower potential power. long strokers are however more efficient due to the smaller combustion chamber, and their ports operate more effectively, so they are not as bad as they may seem, just not better.


also, torque is not exactly power * rpm as someone previously mentioned either...

torque is a function of displacement and bmep (brake mean effective pressure).

as power is also funtcion of bmep, rpm * power = torque is a bit of a myth for people who cant measure bmep.

stk

 

[Brmorgan]

my 'not' meant, the dwell time is related to rpm only. one rev in the same time, regardless of geometry. There is difference in piston accelerations and motions due to stroke, and the rod to crank angle is affected near the top of the stroke, but the torque is the sum of many factors. Ultimately, the expansion of hot gasses through the stroke of the piston is what defines the power and torque, and that is determined by those many factors. mixture, burn rate, type of fuel, ignition timing, swirl of the charge in the cylinder, combustion chamber shape, heat losses to piston and chamber, cam timing/port timing, compression ratio in cylinder and in crankcase, transfer port angles, shapes, etc etc.

k

I think he was saying, and it's my opinion too, that GENERALLY in a longer-stroke cylinder, the expanding gases have more time to push the piston before compression is lost when the piston passes the ports. Of course your observations about the impact of cylinder design is right on though.

Regarding flywheels being "stored power", that's a good way of putting it. Flywheels are models of the law of physics which states that "an object in motion tends to stay in motion" unless acted upon by other forces. So the heavier the flywheel, the less power required to keep the rotation going. Think of the old hit-and-miss engines, where the engine would fire once, and the flywheel could keep doing the work of the engine for perhaps dozens of turns before the engine would have to fire again. Our chipper at work needs at least 100hp to start up, but I bet you could run it with less than 10 once it's going (not under load, of course).

 

[Tzed250]

uh oh..... Deja vue...

A couple of years ago there was a long and heated thread debating whether a heavier flywheel "increases" torque.... I'm with the "Not so" crowd....

Here is a link to the thread Andy mentioned.

Continue at your own risk.....


http://www.arboristsite.com/showthread.php?t=41214&highlight=torque+horsepower




.

 

[Bruce Hopf]

I noticed that one Important thing here was not mentioned, and that is Lugging Power. A lower RPM Engine will have more Lugging Power than a Higher RMP Engine.
I have a Fordson Super Major, and is around 60 to 65 HP, that is rated to rev out to 1600 to 1800 Max RPM'S. My Uncle has a 674, International that is also around 60 to 65 HP, that is rated to rev out to 2100 to 2400 MAX RPM'S. Both have Diesel Engines.
While pulling the same Identical Cultivators in 12' width, same Ground Speed and hitting a slight wet spot, the International would almost always stall out, and you would gear down, and lift the Cultivator a little, while the Major would drop down to 1400 RPM, and hold at the RPM, and without gearing down, or lifting the Cultivator until it was through the wet spot, and after would regain the previous RPMS.
Sure these new Chain Saws have a lot more RPMS than the older Chain Saws, but when it comes down to the Lugging Power, there is no comparison.
20 some years ago, I did some Custom cutting of fire wood, and I had one customer who had a 61 CC Stihl, that was almost brand new. He thought it would be faster to cut with than my Pro Mac
60, and both Saws had a 20" Bar.
He wanted to save Money, and pay me less for cutting his fire wood. In the cut, the Stihl would bog way down, but the Pro Mac 60, didn't drop to many RPMS. After I made a few cuts with both Chain Saws, he took his Stihl, and left me to keep cutting with my Chain Saw. Bruce.

 

[Tzed250]

Bruce, you are describing "torque rise", a charachteristic of engines that normally operate beyond their torque peak.

 

[Bruce Hopf]

Bruce, you are describing "torque rise", a characteristic of engines that normally operate beyond their torque peak.

OK. Thanks for helping me get this cleared up. I knew it had something to do with Torque, but was not sure exactly what. Bruce.

 

[DanMan1]

my 'not' meant, the dwell time is related to rpm only. one rev in the same time, regardless of geometry. There is difference in piston accelerations and motions due to stroke, and the rod to crank angle is affected near the top of the stroke, but the torque is the sum of many factors.
k

Kevin, You seem pretty knowledgeable on engine theory, except on this point
you are off base.
Rod ratios do effect torque curves. Rod ratios do effect the "dwell" time of the TDC phase. It is theoretically documented, and has been test proven by many.

 

[maico490]

Getting boring now. Possibly should be lumped in with oil threads.



Gary?

 

[DanMan1]

Getting boring now. Possibly should be lumped in with oil threads.



Gary?

Don't wanna play?

 

[stevethekiwi]

what oil gives the best torque? lol

 

[rms61moparman]

what oil gives the best torque? lol

DIESEL!!!!

 

[Crofter]

Bruce, you are describing "torque rise", a charachteristic of engines that normally operate beyond their torque peak.

+1

I have an Oliver Brown tractor that has max HP somewhere around 2100 RPM yet max torque is around 1100 I think it has something like 50% torque rise as you lug it down; certainly a totally different experience from operating an engine that has a high rpm torque peak and little torque rise as rpm drops.

Output per pound is better with higher rpm and lower torque.

Where weight is an asset and the output is at a low rpm anyway like on a tractor or ship engine, heavy torque engines shine. I have my eye on a 6 horse 650 RPM Lister diesel that weighs 750 lbs and needs a thousand pound base to soak up the power pulses from that single 4.5 X 5.5 inch piston. I dont think it would make a topping saw!

 

[heimannm]

O.K. I think I understand better...

I followed the link and read through the entire torque thread referenced by Tzed250 and stand corrected.

It seems that the older saws that operate(d) at lower RPM have a wider power band, where the torque does not fall off as readily as with the modern high RPM saws. So, the old saws don't actually have more torque, just more available when you start lugging it and dropping the RPM.

Guess I saw this in action yesterday running my old McCulloch 550 and Jonsered 621. The Mac with 24" bar and .404 chain was just about unstoppable even when the chain was dull (hit a rock in the stump). Push as hard as you can and it just keeps chugging. The Jonsered (O.K. not exactly a modern job) with 16" bar and .375 chain in the same stump (might even have been the same rock) would just stop turning the chain uless you just barely touch the wood. Not exactly a fair comparison (99 cc vs 56 cc) but demonstrated pretty clearly to me.

BTW, I got to run 5 saws altogether during the day, 2050 Jonsered, 621 Jonsered, 036 Stihl, 066 Stihl, and 550 McCulloch. The 066 only has a couple of tanks of fuel on the new piston/rings and keep getting harder and harder to start (I mean pull the starter, compression is coming on). What is this thing going to be like when the rings are seated?

It was great fun for me anyway.

Mark

 

[motosierra]

I guess we're giving up torque for chain speed these days, but what's the difference in the engine configuration. I assume that it has much to do with the porting, but I'd like to hear more.

I believe that displacement is a large part of the equation.