2 Stroke theory in relation to chainsaws

[thomas1]

Plus much longer intake tracts, larger mixing areas in the crankcase, longer rods on the crank, larger and longer transfer tunnels, etc. I've cut down a couple of older motorcycle pistons to use in saws but it was much more work than it was worth.

Is that why you switched to tuna cans?

 

[Nitroman]

Try running 87 octane in a saw that has 200 psi comp and then a sled that has 200 psi comp and tell me how they compare.

Okay, I ran into this form of stupidity on the AS forum. GO BACK and READ my replies to the initial poster IN CONTEXT.

 

[Deets066]

Okay, I ran into this form of stupidity on the AS forum. GO BACK and READ my replies to the initial poster IN CONTEXT.

I can read, I understood your posts the first time I read them. I still disagree, that's my opinion. No need to imply stupidity. You could rephrase my last post and change sled to dirt bike (air cooled) with the same end result. A 60cc saw will never make the power of a 60 cc bike. Regardless of of carb or pipe.

 

[metallic]

There is a lot of info on 2 strokes out there ,but most relates to bikes and sleds ,how much is transferable to saws when doing mods ?

I think the biggest difference for "chainsaw engineers" is designing the engine without a tuned exhaust (aka expansion chamber). When the engineers at the factory know the desired operating rpm, then they can work backwards to case volume, exhaust duration, blowdown, etc. When they hit the physical limits due to sound wave resonance, then they throw in a power valve or two to broaden the torque curve. I can tell you when the power valve stuck on my dirt bike it felt like I was riding a scooter, so the sh$t works!

So what are we left with for our simple air-cooled, non-expansion-chambered dinosaurs? Well, the 80cc water-cooled motocross bikes are pumping out 20hp and a good 80cc saw is probably around 7hp. That 80cc bike will need new rings after each race weekend (maybe 2 hours of running time). Would you want to have to put new rings in your saw each night when you got home from work?

One thing I can say for sure is that, these days, I'm done working long before my saws are ready to quit!

 

[Terry Syd]

I used to have a link to a free copy of Blair's work on two-stroke design, unfortunately it was lost in a computer crash. Here's a blurb on the book - http://books.sae.org/r-161/ The book deals with all sorts of two-stroke design from top end race bikes, outboard engines and down to the lowly chainsaw engine. It is VERY intensive with mathematics, but the concepts are there along with the graphs and charts.

When you look at the design of the saw engine, it is a completely different design than other two-strokes. Much of the difference lies in that it must make good power over a broad RPM range (no transmission). The biggest difference I found was the reduced time/area of the transfers. If you ever looked at Gordon Jennings figures for transfers, a saw is well below the minimum acceptable for a bike. A couple of other things popped up, the crankcase compression was kept much lower to allow a broader powerband. The carburettor size wouldn't come close to the minimum necessary for another two-stroke.

The saw engine has it's own requirements and trying to use some of the tuning techniques from other two-stroke engines can make you go backwards in your tuning.

 

[Ozarker]

Google to the rescue! Design and Simulation of Two-Stroke Engines, by Gordon P. Blair

Be warned, it's a large download.

 

[Terry Syd]

The first time I looked at an open port chainsaw jug I wondered how it could run as well as it did. I knew it would never work on a race bike engine, but it works on a chainsaw fairly well. After reading Blair, I understood why an open port cylinder on a chainsaw could work as well as it did. The BMEP was much lower and the loop scavenging wasn't as critical as the scavenging efficiency would still be fairly high.

 

[Marshy]

Modern 2 strokes that have exhaust power valves, reed valves and are water cooled are not a good comparison to chainsaw engines. They are a completely different technology. If anyone cares to dig up some info, we should be comparing 60s and 70s air cooled 2 stroke engines of comparable displacement. That would level the field and be more applicable.

 

[Mastermind]

We run into this every so often......

It is possible to get a saw engine to make a lot of power. But at what point have you turned it into a unless pile of parts?

There's a reason bike saws were invented.....

 

[Wolverine]

stupidity on the AS forum.

Which is where implying stupidity belongs IMO. Keep it over there...

 

[Jughead]

Modern 2 strokes that have exhaust power valves, reed valves and are water cooled are not a good comparison to chainsaw engines. They are a completely different technology. If anyone cares to dig up some info, we should be comparing 60s and 70s air cooled 2 stroke engines of comparable displacement. That would level the field and be more applicable.

i was gettin ready to tap out the same thing.
60's and 70's air cooled suzis and yamahaha's were down on power vs displacement.say 40-50hpish to 400-492cc even bigger carbs and chambers only added a few hp.porting a little more.factory race bikes were a slightly different animal.still low on hp compared to what came out with reeds and watercooling and factory chambers.

right now this one is probably only pushing 50hp max up from 46hp.thats with the chambers,milled heads and bigger mikunis.no port work other than some polishing.
kaw h1 triples were much different with 60hp from the factory.with only 6cc more displacement.

 

[Marshy]

i was gettin ready to tap out the same thing.
60's and 70's air cooled suzis and yamahaha's were down on power vs displacement.say 40-50hpish to 400-492cc even bigger carbs and chambers only added a few hp.porting a little more.factory race bikes were a slightly different animal.still low on hp compared to what came out with reeds and watercooling and factory chambers.
View attachment 8643
right now this one is probably only pushing 50hp max up from 46hp.thats with the chambers,milled heads and bigger mikunis.no port work other than some polishing.
kaw h1 triples were much different with 60hp from the factory.with only 6cc more displacement.

Yamaha RD60 had like 4 HP. That's right in line with a chainsaw. Of course its not easy coming up with hp number for that vintage of a bike but its the same exact technology used in saws today. Today's saws might be more refined today though.

 

[Jughead]

the rd60.those were cool little bikes.i parted one of those out several years ago.
what numbers posted back in the day are pretty close to numbers on the dynos these days.
what i would really like to see would be the strato charging come back in some really sick and twisted two stroke twins and triples.

 

[exSW]

I found this one. Any other reccommdations?