# Horsepower ratings (125cc motorcycle 33 hp!!! 123 cc chainsaw 8 hp)



## hardhat (Jan 17, 2012)

My partner today was talking about his 125cc motorcycle made 36hp at the rear wheel. I said no way and looked it up and the yz125 yamaha was rated at 33hp at the rear wheels. How is the possible when the 3120xp only makes 8.4 hp IIRC?

Any thoughts on this. Perhaps water cooling allows you to dissipate heat faster and thus make more power for longer, but I can't see it quadrupling power producing ability. I just don't get it. Can anyone explain this?


Larry


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## 7hpjim (Jan 17, 2012)

Longer stroke me thinks:eek2:


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## funky sawman (Jan 17, 2012)

Tune pipe, better carbs, and usually higher compression on bike engines, thats why:msp_thumbup:


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## 7hpjim (Jan 17, 2012)

You got Cape Buffalo in Idaho?:hmm3grin2orange:


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## hardhat (Jan 17, 2012)

funky sawman said:


> Tune pipe, better carbs, and usually higher compression on bike engines, thats why:msp_thumbup:



Cr 8.6:1 on yz 125. I think this is in line with the chainsaws. 54mm x 54 mm bore and stroke 124 cc

3120 xp has a 60mm bore and 42mm stroke. 118cc.

These numbers are different, but I cant see this counting for 4 fold increase in horsepower.

Any how, why don't they do a little tweaking on the saw motor and double its output if indeed this is all that is different?


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## BloodOnTheIce (Jan 17, 2012)

125cc modern MX motor has the advantage of. 
-Boost ports
-Case induction
-Reed valves
-Tuned pipes
-34 to 38mm carbs
-Power Valves
-Full circle cranks
-Thin ring pistons


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## mowoodchopper (Jan 17, 2012)

totaly diff engine, water cooled, reed valve, variable exhaust valve size, pipe, carb etc chainsaws are very weight limited on what the can do,and size, you dont want a pipe and a big carb with a huge air box on a work saw


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## blsnelling (Jan 17, 2012)

Yup, all of the above. A lot of compromises are made to fit the package of a chainsaw.


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## 7hpjim (Jan 17, 2012)

BloodOnTheIce said:


> 125cc modern MX motor has the advantage of.
> -Boost ports
> -Case induction
> -Reed valves
> ...



WOW he stayed @ a Holiday Inn Express last night!!:hmm3grin2orange:


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## Hank Chinaski (Jan 17, 2012)

besides, who the hell want's knobbies on their chainsaw??


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## funky sawman (Jan 17, 2012)

7hpjim said:


> You got Cape Buffalo in Idaho?:hmm3grin2orange:



Aint got no buffler up here but we do have griz...


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## 7hpjim (Jan 17, 2012)

funky sawman said:


> Aint got no buffler up here but we do have griz...



My BIL had a Winchester 70 in 375HH and the only thing we shot with it was a pine tree (pre CAD) and a groundhog, more of a novelty around here, I've shot a quite few big bores up to the 50BMG but never got to see the terminal results in anything bigger than prairie poodles, I'll bet that 500 will anchor them thar bares!!:msp_thumbup:


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## funky sawman (Jan 17, 2012)

7hpjim said:


> My BIL had a Winchester 70 in 375HH and the only thing we shot with it was a pine tree (pre CAD) and a groundhog, more of a novelty around here, I've shot a quite few big bores up to the 50BMG but never got to see the terminal results in anything bigger than prairie poodles, I'll bet that 500 will anchor them thar bares!!:msp_thumbup:



HAA, it anchors the bears and myself very well:hmm3grin2orange:


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## 7hpjim (Jan 17, 2012)

Sorry to the OP but when I saw the smoke poles in the sawmans sig I couldn't help myself!.... 2 2 stroke smoke and smokeless powder for fun, only in America!!!!!


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## lone wolf (Jan 17, 2012)

And dont forget more rotating mass!


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## mp5n (Jan 17, 2012)

*It's all about heat.*

It shouldn't be too difficult to get way more power from a saw motor, but you'll never be able to get rid of the heat without going to water cooling or modifying a cylinder and head with 4-5 times the fin area. Then how will you shroud everything and not have it be huge. You can probably get similar horsepower numbers from a race saw, but it'll never be able to run for any extended period of time. With a modern bike motor you have much more advanced porting, reed valves, carburation, ignition timing, exhaust systems and of course cooling systems. In my younger days I raced professionally and then built motors when I ran out of unbroken bones. I was running RM125s which I ran the compression at 225 psi with 36mm carbs. Way more than you'll find in any work saw.

MP5N


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## ridecaptain (Jan 17, 2012)

i wonder why they use motorcycle engines to build hot saws?


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## weimedog (Jan 17, 2012)

BloodOnTheIce said:


> 125cc modern MX motor has the advantage of.
> -Boost ports
> -Case induction
> -Reed valves
> ...



All this is possible because the designers of motorcycle competition motors have....more space and weight in the formula to work with. Not going to expand into a weimebook, but all things listed so far in this thread are possible in the MX world because they have the SPACE to work in and WEIGHT constraints allowing freedom in design solutions. Starting with cooling...if more power is to be had..more cooling is required. You have to have some foundations to build on to have more power...bore & stroke and the freedom based on casting size to have more port area, larger transfers, fancy intake and exhaust devices such as power valves & case reed induction systems...etc. Just goes on and on...expansion chambers.. I just can't see a chainsaw solution practical with water cooling & fans to get the air flow as they are static in operations, expansion chambers. 20plus pound motors making 30 plus hp can you just imagine what that would look like? Almost like blood's bike saw! With water cooling...


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## rwoods (Jan 17, 2012)

I haven't seen any hp numbers posted but I'm sure a 101B equipped MAC with a 35 mm carb is putting out way more than 8 hp even with a stock muffler. Not exactly a work saw but 101B's with chainsaw carbs were. I would bet they were putting out far more than 8 hp as well. I understand that heat was a real factor with these "hot" work saws. Ron


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## z71mike (Jan 17, 2012)

Just to add to the cooling topic, as brought up a few times already.... Keep in mind, with a saw, you move your own air with the flywheel and it's usually warm air surrounding the saw. On a bike, you have multiple-mph airflow of fresh air that's relatively cool blowing by your heat sinks. Like the air-cooled Honda 400EX...... don't sit in one spot with the engine running too long or you'll easily overheat it. Just simple physics there. As weimedog said, it starts with the cooling and the ball rolls downhill from there.


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## cbolze (Jan 17, 2012)

*Apples to walnuts*

Chain saw is rated at the crank shaft and the bike is rated at the rear tire, through some gear train. If the bike HP was rated at the engine, I'm sure it would still out rate the saw due to all the listed advantages but not by 24 horsepower.


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## weimedog (Jan 17, 2012)

cbolze said:


> Chain saw is rated at the crank shaft and the bike is rated at the rear tire, through some gear train. If the bike HP was rated at the engine, I'm sure it would still out rate the saw due to all the listed advantages but not by 24 horsepower.



Yes it would. Probably by more than the hypothetical 24 as there are so many losses in the gear box, chain drive, wheel bearings even the aerodynamics of the spokes & tires!


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## z71mike (Jan 17, 2012)

weimedog said:


> Yes it would. Probably by more than the hypothetical 24 as there are so many losses in the gear box, chain drive, wheel bearings even the aerodynamics of the spokes & tires!



Yup. Just like the old automatic versus manual trans discussions at the track. The bigger the resistive load between powerplant and business end, the more loss.


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## bcorradi (Jan 18, 2012)

Don't forget also.. it hauls your ass instead of you hauling it's ass. So weight compromises don't need to be factored in during design


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## hmmach1 (Jan 18, 2012)

weimedog said:


> Yes it would. Probably by more than the hypothetical 24 as there are so many losses in the gear box, chain drive, wheel bearings even the aerodynamics of the spokes & tires!



Thats exactly right ,if the bike was dyno'd at the crank it would put out even more power with out all the parasitic losses.


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## openloop (Jan 18, 2012)

Dirt bike engines are about the pinnacle of 2 stroke engine performance. Chainsaws and other lawncare tools are actually bottom tier... I cant think of any other 2 stroke engine where the output is the least important of the design criteria as it really is with chainsaws. As important as output is TO US, it falls far behind cost, emissions, packaging, and noise levels to the companies making them. 

Reed valves, power valves, tuned pipes, and greater available space for port layout all have much to do with their output. All of those things are difficult to integrate into a lightweight dependable chainsaw.

As much as it may burst may peoples bubble...chainsaw 2 strokes are garbage in the output department in comparison to just about every other modern 2 stroke engine, hell even a 50cc model airplane engine develops 30% more power at only 9000 rpm and thats little more than a toy lol.


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## Nitroman (Jan 18, 2012)

Come-on people...it's the supercharger. That's right, that big ungainly lookin' thang out front of the bike...the PIPE. This many people on a 2-smoker forum and you guys don't run snogo's? Sad. Very sad.

Hey Funky Sawman, you have a .500 A-Square? So do I.


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## cpr (Jan 18, 2012)

Now you did it Ron. "He" is going to attack this thread, you watch. :taped:


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## CJ1 (Jan 18, 2012)

cbolze said:


> Chain saw is rated at the crank shaft and the bike is rated at the rear tire, through some gear train. If the bike HP was rated at the engine, I'm sure it would still out rate the saw due to all the listed advantages but not by 24 horsepower.



You got it backwards and I have never saw a dirtbike rated at the rear tire. All of them have been either crank or countershaft. CJ


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## cbolze (Jan 18, 2012)

*Boy was I wrong!*



cbolze said:


> Chain saw is rated at the crank shaft and the bike is rated at the rear tire, through some gear train. If the bike HP was rated at the engine, I'm sure it would still out rate the saw due to all the listed advantages but not by 24 horsepower.



Easy to admit, been married for 22 years. Too late to reply and the Keyboard was faster than the brain. 
What I meant to type was engine dynos are way different than wheel/tire dynos and correction factors make results more complicated. The hand held saw engine by design is way constrained than a bike engine (weight and packaging). Taken at the crank, the bike is close to 40 peak HP! Now if I can just tote all that weight, radiator etc and not burn myself on that pipe....


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## jropo (Jan 18, 2012)

I personally think its because its simply not needed.
I'm sure on the West Coast where they grow BIG trees Fast its a different story.
This is Arborist Site "The Largest Tree Care Forum on the Web". Out of the massive global population, the most users on line was 1,646 (Forum wide). Average active "Members" on Chainsaw is about 350. Out of that 350 how many people think the 3120 is lacking power.
Around here Hard wood timber fallers cutting grade in the winter use 385/390's w/ a 20'' bar, and the only reason they use that big a saw is so they don't pull any heart wood. And most mills won't take the big stuff.

Most tree services that work 8/10hrs. a day 5-6 days a week have the 395/ 660 ect. for the occasional big tree/ stump and most everyone hates using it when and if it has to come out, from what I hear is under ten times a year.

I don't know much about dirt bikes but I think Full Circle cranks is a very expensive high preformance up grade.

Snowmobiles on the other hand, a liquid 340 will hang right w/ a fan 500. Which is a good thing because the liquid will need the ol' slow fanner to pull it home when it starts pukeing coolant all over and is in need of a few hundred dollars in parts
Now I'm not trying to start a Liquid vs Fan war, I'm just saying simplicity in design and most people have problems w/ that, and I would assume that is how most of us are even here on this site to begin with. Including my self. I had a problem w/ my XL-2 Homelite, now I have more in to saws than my truck.
Now I'm not saying that a Liquid saw is not cool, simply not needed. After all the biggest trees on this planet were felled w/ a Axe and hand saw, and were skidded out w/ ox.
Good Thread!:msp_thumbup:


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## brages (Jan 18, 2012)

125cc chainsaw is closer in performance to an early 70's trail bike (suzuki TS125?). Broad power curve, very durable, simple, light.

Power/lb is more important for a working saw than power/cc. Power/cc is important if you are racing displacement limited classes (motocross).


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## timmcat (Jan 18, 2012)

A more fair comparison for hp sake would be a 3120 vs an early 70's air cooled sled engine, where most of them were derived from long stroke industrial applications. A 295 single on an old skidoo was rated around 13 hp, a little more reasonable than the racing dirtbike engine. I have a Husqvarna 110 cc sled engine on one of my mini sleds, a Somovex Chimo, and its only rated at 8 or 10 hp.


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## trappermike (Jan 18, 2012)

I agree with comments like openloop,motorcycles and snowmobile motors are in a far higher league than saw motors for all the reasons stated,especially with only 2(Now sometimes 4)miserable little transfer ports and a ridiculously tiny carb that saws have. A lot of the deficiencies can be remedied somewhat,but the lack of large multiple transfer ports can't be. However on the plus side saws are fan cooled and have a really light crank assembly and piston assembly,and a short stroke for rpms,but this will never be enough to allow saws to achieve power levels of motorcycle or snowmobile motors.


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## Woodomaker (Jan 18, 2012)

mowoodchopper said:


> totaly diff engine, water cooled, reed valve, variable exhaust valve size, pipe, carb etc chainsaws are very weight limited on what the can do,and size, you dont want a pipe and a big carb with a huge air box on a work saw



I do too.....:msp_smile:


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## weimedog (Jan 18, 2012)

brages said:


> 125cc chainsaw is closer in performance to an early 70's trail bike (suzuki TS125?). Broad power curve, very durable, simple, light.
> 
> Power/lb is more important for a working saw than power/cc. Power/cc is important if you are racing displacement limited classes (motocross).



Actually most saw motors are way over square, with very simplistic intake & exhaust designs, and therefore quite peaky, dead at low RPM's relative to the much longer stroke with heavy flywheels two strokes dirt bikes even of that period..SINCE saw motors are so simple, they end up targeting a narrow rpm range to be efficient. Saw motors run better WIDE open while the motorcycle world, especially trail bikes are a part throttle world, lots of throttle controls and changes are the way of life and therefore the design reflect that reality.

I actually had a TC-125 with the "hi-low" range transmission. All my old Bultaco race bikes of the period had relatively peaky powerbands as compared to current two strokes...even my 1989 Suzuki RM-250 is peaky relative to the last generation race bikes. Even so..it made way more power over a much wider range than any saw motors. My Solo 603 (100cc) is the closest I've run so far to an old dirt bike..it has that heavy rotational mass feel and solid power at part throttle...

Definitely and grapes to oranges comparison...(Saw are grapes..almost a fruit! as compared to dirt bike motor designs)


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## tolman_paul (Jan 18, 2012)

I used to race karts for years, 100cc and 135cc air cooled, and 80cc and 125cc water cooled dirt bike motors. One of the best ways for a two stroke nut to get their fill of the smell of castor oil.

Anyhow, as to the ways and whys of making power in a two stroke, or not, here is what I've learned and seen over the years.

The pinnacle in single cylinder gasoline burning two strokes can be argued between the camps whether it was the Intercontinental A class racing kart engine, a class that was sadly discontinued. The engines were 100cc, and the final derivation was liquid cooled pumping out 30+hp with a peak rpm of 20,000. The air cooled engines weren't far behind, but there is plenty of cooling on a kart that is running 50mph. The pinnacle in gearbox is either the GP bike or the International C kart, liquid cooled 125's that are putting out about 45 hp and revving to ~14,000 rpm. MX 125's can be tuned to put out 40hp by bumping the compression ratio and playing with the porting, and typically the power valves are plugged. Power valves aid low to mid range power, they do nothing for peak power. Also all these race engines run 110 octane fuel.

The keys to making power are the ability to flow the air/fuel charge into the engine, the ability to flow the exhaust out, the compressio ratio, and the ability to deal with the heat of combustion.

In simple terms, saw engines are designed to maximize power to weight ratio, kart and bike engines are designed for max power. An air cooled 100c kart engine weighs 30#'s, and that is just the bare engine, no fuel tank, no oil tank, no bar, chain, clutch or exhaust.

In the desire to get max hp per pound, saw designers use what can be termed a very over bore engine, i.e. short stroke and large bore. This reduces the length and hence weight of the con rod, and also reduces the dia and hence weight of the crank. Whereas with a kart or bike engine, they run pretty close to a square engine i.e. equal length bore and stroke. The short stroke large bore saw engine does not flow as well as a square bore, and the larger dia piston can't take the same amount of heat nor the same piston speeds.

And finally the tuned pipe. Tuned pipes make a tremendous amount of power, nearly doubling the power of a muffled engine. But it comes back to packaging. Pipes are large and heavy and simply aren't an option on a working saw. It's easier and lighter to gain power on a saw by adding cc's vs. adding a pipe.

It would be interesting to see if an engine was designed using a square bore/stroke and tuned pipe what cc you could drop it to and get 8hp on pump gas. Then compare the weight and size to a saw engine, my guess is the saw engine would be lighter, and definitely less expensive. Speaking of which, when I got out of kart racing in the early 90’s, a new 100cc air cooled engine was running ~$1200, and that was w/o the tuned pipe or header which added another $200. So I’m having a hard time seeing a saw for under $2000 using a kart/bike type engine, not to mention it being double the weight. I doubt there would be many takers for a 40+# saw, even if it put out 20-30hp


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## Little Al (Jan 19, 2012)

cbolze said:


> Chain saw is rated at the crank shaft and the bike is rated at the rear tire, through some gear train. If the bike HP was rated at the engine, I'm sure it would still out rate the saw due to all the listed advantages but not by 24 horsepower.



Surely if the bike was rated at the crankshaft it would give more power as it would not lose the drag/friction of the tranny /chain, rear wheel etc the last Honda RS125 gp bike I owned developed 46.3 BHP at the crank & 43.75 at the rear wheel. The last of the Aprilia 125 gp bikes [2011season] were making over 50bhp at the rear wheel [very cloak & dagger stuff so only the teams knew the exact numbers] but the Honda`s were not competitive[ due to no development for a number of years] interesting to note the Honda was reed & power valve motor. Aprilia is rotary induction some with/without power valve


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## 24d (Jan 19, 2012)

22 horse power? What RPM? Friend? McCulloch sp125/101 better than Husky Stihl.:msp_confused:


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## troutfisher (Jan 19, 2012)

that saw motor has to last 2000+ hours too....... a MX bike needs a piston kit every 50 hours.


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## oaktree (Jan 19, 2012)

*re*

Heres a rc aircraft engine thats about 5 lbs at 9.2 hp .... Not bad but pricey !
3W-85xi 


rt


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## oaktree (Jan 19, 2012)

*re*

These are some of the highest quality model aircraft engines .... wrong for application ,but interesting engineering .. Lightest weight to power is what there after .

pic


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## Guido Salvage (Jan 19, 2012)

Hank Chinaski said:


> besides, who the hell want's knobbies on their chainsaw??



Good point, but they work pretty well on the wood hauling wagon...


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## trappermike (Jan 19, 2012)

A lot of the Suzuki's were "square",54x54mm. Weimedog I had a TC125 too,my dad bought 3 new ones,I bought the Suzuki race kit for it,basically a TM125 motor parts,it had piston,cylinder,head,bigger carb and intake and air filter assy,pipe,front sprocket and small oil tank. All for $100 new from the dealer!
Saws use a really "oversquare" motor with a short stroke to reduce piston speed,allowing higher max. revs and allowing them to run at high speeds for a long time.
As has been said here saws gotta be very light so have to be a pretty simple design.


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## rwoods (Feb 2, 2012)

FWIW there is a 1974 Alsport TS-101 (A little McCulloch101D powered tricycle) with some manufacturer's literature for sale on eBay. The literature rates the mufflered 101D at 14 hp which pretty much squares with the experimental aircraft rating of 15hp for the MAC 101B. Ron


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## Team FAST (Feb 23, 2012)

I echo the comments on kart engines. Former kart racer here too. On my roadracing 125cc shifter kart, we were seeing 43hp on the dyno at 13,000

On my 250cc superkart with a highly modified CR-250, we were over 60hp and quickly approaching 65hp on the dyno at about 11,000 rpm

Both engines are watercooled. The other significant benefit that my kart engines had over a saw engine is a digital fully programmable ignition which is HUGE. And one other thing kart engines get is a real carburetor with a slide valve....massive amounts of air at WOT.


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