My sons KX60 (little 2 stroke dirt bike) makes 15 hp, stock. My 60 cc saws make about 1/3 of that. Ive taken both apart, the major differences I see are reed valve on the intake and a tuned expansion pipe on the exhaust.
Why do bike motors make so much more power?
- Thread starter Mike-M
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Nitroman
Addicted to ArboristSite
Case reeds and with the much larger carb that is your answer. Wwwaaaaayyyyy bigger than on a saw. My ancient CR125 had a 36mm Mukuni on it. My 120cc 3120 has an 18mm if I remember correctly. Poor motors are starving for O2 and gas.
Look at the size and weight of it compared to a saw.
fearofpavement
Trying them all
^^^^^^
What he said.
What he said.
nmurph
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Case volume, stroke, cylinder size and the ability to 4,5,6 transfers, water cooling, weight takes a on a relatively lesser import...
Everything. Carbs, reeds, much more advanced porting, especially transfers, and pipes.
Space.
Space to carry an expansion chamber.
Space to optimize bore & stroke
Space to have ports optimized... included as was mentioned before the intake arraignment.
Space to carry power valves & hardware.
Space to carry water cooling.... and cooling is a huge component of how much fuel a motor can burn....as a percentage ends up in heat.
And weight constraints....
Simply a different set of design parameters.
Space to carry an expansion chamber.
Space to optimize bore & stroke
Space to have ports optimized... included as was mentioned before the intake arraignment.
Space to carry power valves & hardware.
Space to carry water cooling.... and cooling is a huge component of how much fuel a motor can burn....as a percentage ends up in heat.
And weight constraints....
Simply a different set of design parameters.
nmurph
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Yes, and pipes!!
sunfish
Fish Head
What is the difference in stroke on these two?
cuttinties
ArboristSite Guru
Depending on the saw somewhere around 8mm. Piston speed is dramatically different between the two.
nyctreeman
ArboristSite Operative
Why does a .357 Magnum produce so much more power than a .38spl ? They're the same size bullet
Length of the case = more space for explosive stuff = bigger bang = mo powah!
jar944
ArboristSite Operative
Its the pipe mostly.
The aprilia factory engineers tested their RAS125 without the pipe and with re-optimized jetting. results 54hp with the pipe, 18 hp w/o
The short stroke and short rod also hurt power output, then the air cooling brings down the max power as well.
The aprilia factory engineers tested their RAS125 without the pipe and with re-optimized jetting. results 54hp with the pipe, 18 hp w/o
The short stroke and short rod also hurt power output, then the air cooling brings down the max power as well.
jar944
ArboristSite Operative
most 2t bikes are square to under square, typically only the large displacement (500cc) are over square.
ex
KTM 50sx 39.5mm bore x 40mm stroke,
KTM 65sx 45x40.8
KTM 85sx 47x48.95
KTM 125sx 54x54.5
KTM 250sx 66.4x72
KTM 300sx 72x72
cuttinties
ArboristSite Guru
The "pipe" helps but without the port timing the gains from the pipe aren't as high as you'd like to believe. The only way that you get the port timing is having room which requires a longer stroke. Longer stroke makes faster piston speed. The speed in which this all happens makes reed valves super effective. Trapped volume allows for a more complete charge transfer. So many variables and things that I don't really care about typing as they don't really need to be on a saw forum. But this gives you an idea.
jar944
ArboristSite Operative
Of course port timing on a non piped piston port engine is vastly different than a reed valve piped engine, however the outcome remains the same. A 2t engine without a tuned pipe will never come close in power output to one with a pipe.
Flywheel effect
I'm confused on the" longer stroke makes faster piston speed" I thought the opposite?
cuttinties
ArboristSite Guru
If the piston has to travel a farther distance and the crank is turning the same speed the piston will be moving fast as it has a longer distance to travel.
Well which engine would rev more short stroke.
cuttinties
ArboristSite Guru
You can pipe a saw as well as a bike. Tuned pipes won't gain you all of this mythical power. 15-20% gains is being pretty generous. Piston ported or not pipes aren't the deciding factor of power difference between short stroke and long stroke.
