Thread: Rpm, Torque, Displacement ???

I'm about half asleep but I've been contemplating a question (multi faceted) for those in the know about saw performance. Here goes-
We often make comparisons between saws and talk about rpms and chain speed for "high performance" saws. Somewhere in there the subject of torque or "stump pulling" power comes up.
There IS definitely a difference in lugging power between saws but are there any "rules" for determining a saw's 'torqueyness' short of actually running it? ( I know that some manufacturors publish torque output specs but I'm looking for general rules).
If 80cc saw A turns 13,500 rpm (no load) and perhaps 9000 rpm in the cut. And 80cc saw B has an "operating rpm" of 7000--- obviously saw A makes more Horsepower. What about lugging power? The "rules" I remember say that long stroke engines usually have greater low end torque than short stroke engines but Short strokes can rev higher and are usually more throttle responsive. Also, big heavy flywheels are harder to bog out that little light ones. Okay.
Suppose that Saws A and B have similar bore and stroke figures. Is the higher speed of A then purely a function of Port Timing? (Or rather mostly Port timing --I'm sure that crank and piston weights and flywheel weight all have some effects.) Will the slower saw be more torquey or is it simply a lower performer in all respects. If greater torque comes with lower peak rpm presumably we can change gearing with higher pin count sprockets and catch up some with the screamer but if performance is simply lower then a 50 cc 13,500 rpm saw will likely equal our 80cc slowpoke-Right?

Come on guys 'splain it all to me.

P.S. Everytime I contemplate RPMs above 12000 it simply amazes me!

hey stumper... now i know ,im the one u had in mind for getting your answer.
but one thing i dont get,, is why long stroke would be more torq,, than short stroke. having limited experience ,i have to go back,, to what a fellow taught me as a kid. namely that the chevies of the day had shorter stroke motors and were stronger in drag racing stock because of it. he said the fords longer stroke had a higher top end ,because of their longer stroke. to me it seems that more torq would be neccessary for drag racing .also possibly for creating a greater force on the chain in heavy cut
and higher rpm for top end also which in sawswould be chain speed[.uh ohforgot cars got gears huh]. things not being anywhere near,, the way they were in the late 50s. mabe most of these guys dont even know what im talklin about. or mabe ye ole memory faileth me [again]. as to lugging power and really your whole question.ill be hoping to read some good stuff on it ,as it interest me also.

Well guys I thought somebody would jump in and help with some enlightenment. The simplified question is:
Between saws of equal displacement and similar bore and stroke figures will the lower peak rpm saw be relatively torquey/harder to bog out than the high speed saw or is it just a relative dog in every respect?

Don't know much about port timing and all that hocus-pocus, but I'll offer an opinion on bore and stroke. Somebody once showed me a graph showing average engine bore vs. stroke over time. In the past stroke was always higher than bore. Sometime in the fifties or sixties, bore overtook stroke and never looked back.

Now, from experience I can tell you while my friend's IH Super M does't have the RPMs it will out-lug a newer tractor like there's no tomorrow. Seems to me that the same feature that let's this tractor lug all day is also responsible for the slow RPMs. A longer stroke is like a longer lever--apply the same force further out on the crank and you'll generate higher torque. Unfortunately, longer stroke means that the piston, rod, etc. have a far greater distance to travel.

Still, some people will turn these engines way up and it makes you want to cry when you know a 100 lb crankshaft and 4 12 inch rods are being thrown around at 3500 rpm and the crank is only supported by two main ball bearings.

Chris B.

not to offend . but stumper,, it may be that the honeymoon is over, and these
guys in the real know,,,have decided the knowledge they worked so hard to gain,, ought not be givin away so freely.
if thats the case ,,i could understand...
but it sure would drop the level of learning that the university of aborsite.com,has offerd up to now.
if so i ll try not to forget,, what ive picked already. lets see now, wheres my tooth.. , oh i done put it in.

Great question, Stumper. I have been pondering that one myself lately. I did ask one of the saw builders here and didnt get an answer that made me believe they had given it much thought. Before I slowed down to log with horses, I used to get my adrenaline rushes from raceing dirt bikes ( motocross and enduros) in certain track surfaces, the little 125cc bikes could turn in the fastest lap times of the day. But.. when the traction was tougher, like sand , deep mud or even real loose deep loam, the bigger 250s and open size bikes would be best. Bigger displacement, lower total rpm to make peak hp, and that equaled a bettert transfer of power to the task at hand. High rpm =lots of wheel spin, lower rpm= wheel hooked up and driving forward. How does this transfer over to chain saws???? I log with an 046 and a 2095 now, but have had a 372. The 372 was a screamer, but on big white oak or hickory stump cuts, it seemed like I had to rock out of the cut a lot to bring the saw back up to speed. Maybe it was my imagination, but I have since sold the saw. The 2095 gets the job done, but its like packin a cruise ship through the timber., so the 046 is my weapon of choice. I am looking for a new falling saw, and this rpm / hp / torque thing is on my mind. I see the Shindaiwa 757 makes peak hp at 9000 rpm and I can get one new for 575 bucks, but I have not ever spoken with anyone that has logged with one. I searched the site here and found one guy refer to using it in a tree service environment, but that aint the same as luggin er all day. Anybody else use one?

Lots of variables to consider Stumper.Obviously if engine A and B have dissimilar volumetric efficiency , the one that can fill and exhaust that cylinder with the most air/fuel will provide more power at any given RPM.

The rule for torque? The longer the distance between journal centerlines the more multiplication of twisting force you get at the main centerline. Deeper bores also are capable of using more energy per stroke thus giving you more power per degree of crank revolution. Still torque is torque at any given rpm, you can build a high torque engine to operate at 1000 rpm and still gear the final drive and outperform a high revving engine.

But don’t think that long stroke engines can’t rev as high as short strokers, they can although they may not be as reliable due to stresses of rotating weight and higher piston speed. It all comes down to V.E.

Do Stihl's have more low range torque than Husqvarnas?HORSELOGGER,sir my power equipment dealer(Husqvarna and Shindaiwa) while on vacation in Maine last spring dirt bike riding said the Shindaiwas were very popular with loggers there.I have a 488 and am happy with it,though I'd like to get something bigger also.The only two downsides with Shidaiwa is that dealer support may not be to good in some areas and that they might vibrate more than a Husky,Ron K.

When people talk about saws with stump power what they are reffering to is a broad flat power curve. The width of the power curve is determined by the port timing. In general for a given displacement the higher the hp the narrower the curve will be. Think of it this way. A torque curve is basicaly a graph of cylinder filling ability. A port that flow optinal for 13,500 rpm cannot flow as well at 7,500 rpm. Now the trade of is trying to comprimise and get good high end hp AND a flat torque curve. things like quad transfers, variable exhaust valves and intake reed vlaves all help toward this end. Bore and stroke size do have a influence, but less so in a two cycle. For example all of the japanese 250cc mx bikes have the same under square bore and stroke hile my 500 has a over square b/s. The 250cc obviously would make less power than the 500, but you would also think it would have lower popeak rpm than the 500 based on b/s numbers. It clearly doesnt. The 500 in reality peaks around 4,000 rpm sooner. If you want a saw with "stump power" you need to step up to a big saw as all of the sub 80cc saws today are screamers.

As Ben said, the new saws have very peaky torque curves. For pulling a bar in long cuts what you need is a saw with a long flat torque curve. The Husky 3120XP sticks out in my mind, because while all the new saws have peak rpms of 13000 and 14000 rpms (including the 395, 066 and 088), the 3120 peaks at 10500. This means it has a less peaky powerband. This is why older saws like the 056, 090, 288XP and 181 are so fondly remembered, is because they had flatter curves and less rpms to run up to.

Some of it is port timing and some is the new lightweight materials that the pistons, rings, rods, cranks and flywheels are made of.

HORSELOGGER, it sounds like you need lots of cubic inches to be satisfied. I agree that a 372 is more "peaky" than the 046, which is why you prefer it. I am surprised you are not happy with that 2095, as it should have more "nuts" than yours does. Anyways I think you should consider getting one of the NOS 288XP powerheads that Madsens is selling for 550 bucks. It is a torquey, durable saw that can swing big bars and take a beating. I dont think a 757 will answer your problems, it will run about the same as your 046. Otherwise it looks like you are an 088 or 3120 candidate

Some of it is port timing and some is the new lightweight materials that the pistons, rings, rods, cranks and flywheels are made of.

Those "light weight" materials I am affraid are for the bottom line only. Saw engines are built to 60's technology standards. If the mfgs would ever install reed valves and VH exhaust valves on saws you could have grunt and peak hp in one saw.

Originally posted by HORSELOGGER
I see the Shindaiwa 757 makes peak hp at 9000 rpm and I can get one new for 575 bucks, but I have not ever spoken with anyone that has logged with one. I searched the site here and found one guy refer to using it in a tree service environment, but that aint the same as luggin er all day. Anybody else use one?

I have two 757's that I run. If you are that interested we could chat via the phone!

Originally posted by bwalker
Those "light weight" materials I am affraid are for the bottom line only. Saw engines are built to 60's technology standards. If the mfgs would ever install reed valves and VH exhaust valves on saws you could have grunt and peak hp in one saw.

The technology is the same, yes, piston porting and no reed valves on most. Variable exhaust ports would be nice too, eh?

But I am referring to materials themselves. Pistons in saws today are thinner, lighter, and have different metallic contents. Also cranks and rods are lighter to survive at higher rpms. Thats what I was referring to

Have either of those 2 things ( reed valves / variable exhaust port) ever been tried by any manufacturers? what about by any hot saw guys?

here is 125cc's of pure vintage mcculloch power, reed valves, third boost port, and all, circa 1977. no, it don't crank the rpms like my 088, but it will out pull it with the bar buried, and if you ever get a chance to run one in some big hardwood, you will come away with respect for it, guaranteed.