HP Limit MS 880 bottom end

[Rusty-880]

I'm building an 880 as a high powered work saw, felling and milling. Based on the info I've read in a two stroke tuning book on port time/area ratios and expansion chambers and different formulas I think I should be able to get around 25hp if ports tuned for 10,000rpm and up to 30hp if built for 12,000rpm. This is with reasonably high comp, current comp is 135psi with .020 piston-head clearance, 98 or higher fuel will be used.
Does anyone have an idea if the bottom end will be reliable with this much power? If not, any ideas what a safe hp would be?
Thanks, Steve

 

[husq2100]

???????? are you joking??????25hp and work milling saw.....forget that

 

[mdavlee]

I would think about 12 hp would be the limit for using one as a work saw. That expensive of a saw to rebuild I would probably go a simple widen intake/exhaust and get the timing where you want it and let it go. Anything over that is probably pushing it too hard for a work saw. Don't they have a real high failure rate over in Australia anyway?

 

[redoakneck]

I don't know how to port a saw, but I do some milling and I would think you would want some good mid-range torque to lug the chain through the cut, not some high reving, finger numbing beast at 10k.

Seems I get the best results for milling when the saw is around 8-9 thousand RPM on a 7900, so I would guess an 880 would be lower?


Not sure, just my 2 cents

 

[komatsuvarna]

:hmm3grin2orange::hmm3grin2orange::hmm3grin2orange::hmm3grin2orange:










































































:help:

 

[Rusty-880]

???????? are you joking??????25hp and work milling saw.....forget that

Why is that?

Factory 125cc dirtbikes pull over 30hp and are quite reliable, 880 is basicly the same size, same revs, just if the Stihl crank will like 20+hp as Stihl only claims 8.5hp @8500. Raise the compression on a stock 880 to .020" piston to head clearance and corrected port timing should give you 13-14hp. This setup with expansion chamber which allows the motor to compress 122cc instead of just approx 75cc and should give around 20hp, all at 8500rpm.

If you don't think it will work please tell me why, thats why I posted the question instead of just doing it.

 

[Rusty-880]

I don't know how to port a saw, but I do some milling and I would think you would want some good mid-range torque to lug the chain through the cut, not some high reving, finger numbing beast at 10k.

Seems I get the best results for milling when the saw is around 8-9 thousand RPM on a 7900, so I would guess an 880 would be lower?


Not sure, just my 2 cents

Stock 880 is about the same revs, thats where peak hp is for both saws by the looks of it. The only reason I said about higher revs is if ports are matched and peak power is at higher rpm hp will also be higher. My main question I guess is if anyone had experience with how much the bottom end will handle, which is why i posted this in the Hotsaw section instead of the Chainsaw forum.

As regards to being expensive saw, I use it to slab timber for outdoor furniture, so slabbing for a full week it should pay for itself almost twice over. Basicly its a tool and if I can get more power and make it more efficient, I spend less time slabbing and more time making furniture.

 

[Deets066]

880 should be the same as a 125 dirtbike? I have asked this very same question before and the just that i got out of it was that it is such a compact engine with a shorter stroke and a box muffler as opposed to a pipe. Will never make as much power as a dirtbike, but with raised compression and a good port job I would think you would be more than happy with what it will do. JMHO

 

[mdavlee]

The bike has much better cooling when moving. I think the rod and crank would handle it in short bursts. I'm sure the piped hotsaws are probably getting a lot more power than a woods port but not sure how long they'll live.

 

[Rusty-880]

Makes sense. I might try and keep porting close to stock revs with higher comp. I believe I can build a pipe that will suitable for every day use apart from flush cutting, which I don't do. That in itself should make a lot of difference. Just wasn't sure how far I should try and go with it.
Thanks

 

[mdavlee]

I would definitely mod the muffler of make a pipe exit to direct the exhaust away from you when milling. Some light port work probably will increase your cutting speed a decent amount.

 

[Rusty-880]

I should be able to make an expansion chamber that finishes near level with back of rear handle, run it down bar side of the saw. Higher the saw is reving the shorter I could make the exhaust.

 

[Deets066]

I have never looked in to it on an 880 but maybe you check into a shorter piston and longer stroke? More stroke should be more torque. Just an idea, depends on how involved you want to get

 

[Rusty-880]

Stock it wasn't bad but I followed the advice on chainsaw section on porting and it turned it into a dog. I'm redoing it now with better info and working out how far to go.

With the stock stroke I should be able to safely rev it to 14,000rpm no load keeping just under 4000ft/sec piston speed. With this if I tuned the ports to 12,000rpm for peak power with high comp should give about 30hp with an expansion chamber and around 18hp without.

From whats been said I'll keep porting near 9-9,500rpm with higher comp and expansion chamber

 

[husq2100]

why wont it make the power of a dirt bike or kart..................

bore to stroke ratio

expansion pipe

power valve on ex

port area and port types.....have a look at the other 2 and compare to a saw.....

carb type

crankcase size

cooling

combustion chamber shape and size


they are completely different beasts.....


like I said in your other thread, Jennings is good but is bike focused....its not 100% apples to apples for a saw

If the saw is 8 or so hp stock and you want to use it for work/mill I think 10-11hp would be max

 

[Rusty-880]

As far as bore and stroke goes bikes usually have longer stroke and while I have not played with two strokes much I have played with V8 Chevs, at the moment have a mild 454 in my work truck. Testing by chev before they built the 454, with longer and shorter stroke engines of similar displacement, the short stroke made more power at same revs but was dirtier with emissions.
Short stroke engines usually need to be revved harder to make big power but because of the shorter stroke safely can be.

Mac kart engines WERE chainsaw engines with different carbies pipes ect.

I take you arnt reading my posts. I said about reporting and building expansion chamber. Later I would like to try a 35mm Tillitson, if it dosn't suit it I'll use it on a racing mower.

I will give you the point on cooling. For this I plan on running 98+ octane fuel to help prevent detonation and piston melting with higher comp.

To get .020" squish clearance I need to take a good .030" off cyl which is quite a jump in comp on a small head space. gave me 135psi cranking comp with stock ports. Re-ported last night and to get ports tuned to 8500 rpm duration needed to be made longer and area virtually all available space.

 

[husq2100]

As far as bore and stroke goes bikes usually have longer stroke and while I have not played with two strokes much I have played with V8 Chevs, at the moment have a mild 454 in my work truck. Testing by chev before they built the 454, with longer and shorter stroke engines of similar displacement, the short stroke made more power at same revs but was dirtier with emissions.
Short stroke engines usually need to be revved harder to make big power but because of the shorter stroke safely can be.

Mac kart engines WERE chainsaw engines with different carbies pipes ect.

I take you arnt reading my posts. I said about reporting and building expansion chamber. Later I would like to try a 35mm Tillitson, if it dosn't suit it I'll use it on a racing mower.

I will give you the point on cooling. For this I plan on running 98+ octane fuel to help prevent detonation and piston melting with higher comp.

To get .020" squish clearance I need to take a good .030" off cyl which is quite a jump in comp on a small head space. gave me 135psi cranking comp with stock ports. Re-ported last night and to get ports tuned to 8500 rpm duration needed to be made longer and area virtually all available space.

ok more power to you....when you do get over 20hp from this engine you will be winning races world wide, let alone using it as a worksaw.....show us you work as you go, sounds like it could be good

 

[rupedoggy]

If you want the bottom end to last add more oil. It is also a good idea for milling because it helps run cooler.

 

[mtngun]

Based on the info I've read in a two stroke tuning book on port time/area ratios and expansion chambers and different formulas I think I should be able to get around 25hp if ports tuned for 10,000rpm and up to 30hp if built for 12,000rpm. This is with reasonably high comp, current comp is 135psi with .020 piston-head clearance, 98 or higher fuel will be used.

Your bike/cart formulas will not work well for saws. Saws have lower compression ratios, bigger bore/stroke ratio, and a box muffler.

No need for hi octane fuel since the compression ratio is low.

Stihl only claims 8.5hp @8500. Raise the compression on a stock 880 to .020" piston to head clearance and corrected port timing should give you 13-14hp.

The Stihl power claim for the 120cc saws is wildly optimistic, IMHO. And no, you don't gain 5 hp just by setting squish. Maybe 1 hp.

I posted several MOTA charts for my 084 build (dimensionally similar to the 880).

Violet is a stock 084 with stock muffler.

Blue is stock 084 with muffler mod.

The red curve is my final build, with a pop-up piston.

My 084 thread. http://www.arboristsite.com/chainsaw/133334-9.htm

 

[imagineero]

peak HP vs continous HP is probably the bigger issue. It's like comparing PMPO watt ratings on amplifiers to RMS ratings. Saws are generally rated for continuous load, while things like bikes and go karts are rated for HP at a low duty cycle. Look at it another way - take an engine out of a car and marinize it and put it in a boat, and it loses a lot of HP! Cars are revving up and down all the time, but they have a low duty cycle. Boats are the equivalent of taking a car and running it up a steep steep hill in 3rd or 4th gear at full revs, and then doing that continuously all day long. If you had them tuned for max HP, they wouldn't last very long.

'industrial' type engines are generally rated for continuous full load usage, which is an entirely different beast from most engines. You can get the exact same block from cat in a range of HP ratings, with accompanying reduced duty cycles and rated engine hours until failure as the HP goes up on the same block. Step up to a bigger block and you can have higher duty cycles and longer engine life. No real surprises there, something that pretty much everyone knows. A chainsaw is made to run long, and continuously loaded.

If you can actually make enough money on an 880 slabbing for a week to pay for itself almost twice over (really? $5k a week?) then put that toy on the shelf and get yourself a bandsaw mill. Or sub some of your work out to me. I'll gladly do a week of CSM'ing for a mere $2500. I'll even supply my own saws, chains, a 7 tonne truck, fuel and oil. You can keep the other $2500.

There are many many bandsaw mill options out there, all of them fast, efficient and with minimal wastage. You'll get a lot more slabs done in a day and can probably pickup more work too. There are portable options available, which can be setup in the woods, as well as trailerable options and massive workshop sized units. They will all give a better finish than you can achieve with a CSM and in less time.

Shaun