Well which engine would rev more short stroke.
Tell me about two saws if everything was the same but one had a longer stroke and the one had a square bore. Wouldn't the shorter stroke have more rev and the other more low to mid torque?If two saws had the same rpm at the crank, the saw with the short stroke would have a slower piston speed. One of the rpm limitations is piston speed as too fast of piston speed and it starts to transfer metal to the bore due too high heat. So of the two saws, with different stroke lengths, the shorter stroke saw will have a much higher safe rpm limit based on piston speeds. Of corse porting, carb, exhaust style all determine what rpm the saw will operate at.
Rod length does not change the stroke at all. It has no effect on that at all. But it has a big effect on piston to crank leverage. The longer arm of a long rod allows the piston to push on the cranks stroke at a more favorable angle. Think of it like a longer crowbar generates more force than shorter one when pushed down upon with equal force by the user. The other place long rod vs. short rod is at TDC. There is a portion of the combustion cycle where the crank stroke moves thru 10 to 20 degrees or so at TDC, but the piston moves very little up and down in the bore. Long rods increase the time the piston is in the dwell zone and allow more time for the combustion high pressure gas to push down on the crank, and this happens while the piston is exposed to the highest peak cylinder pressures. Couple this with the increased favorable leverage angles and an increase in power can be found. Longer rod lengths help engines make more power at a given rpm limit.
The problem is we can not easily change our rod lengths. You have to cut off the rod, and use a spacer under the jug, and adapt a longer rod from a homelite or mcculloch style engine. Piston pin heights in relationship to squish clearance may/probably have to moved as well.
Square motors are usually very happy engines. ..but depends on the applicationTell me about two saws if everything was the same but one had a longer stroke and the one had a square bore. Wouldn't the shorter stroke have more rev and the other more low to mid torque?
That would also be true..... Longer stroke saws, everything else being equal, would tend to reach a lower max RPM and have more torque. A shorter stroke saw, everything else being equal, would spin at a higher RPM and produce a higher peak hp value. A "square" bore would have the bore and stroke measure the same. Most saws are over square in that the bore is larger than the stroke.Tell me about two saws if everything was the same but one had a longer stroke and the one had a square bore. Wouldn't the shorter stroke have more rev and the other more low to mid torque?
Thanks for clearing that up.That would also be true..... Longer stroke saws, everything else being equal, would tend to reach a lower max RPM and have more torque. A shorter stroke saw, everything else being equal, would spin at a higher RPM and produce a higher peak hp value. A "square" bore would have the bore and stroke measure the same. Most saws are over square in that the bore is larger than the stroke.
You can't make horsepower without extracting the heat. Air cooling limits the horsepower. Liquid cooling adds weight. Chainsaws are very weight sensitive tools. They are all air cooled.
None of us here on AS have ever seen pistons melted, or rings seized due to high heat.
Lots of "homeowner class" saws die because of over taxing too weak engines., with too much bar and chain on them. This is hardly a secret.....exactly, because the horsepower is limited in the design process. They need to keep a cap on it. The cooling fan is the horsepower limiting device on an air cooled engine. If it cannot dissipate the heat the engine will die.
Are you pressing the crank apart and changing rods and re truing the cranks?If two saws had the same rpm at the crank, the saw with the short stroke would have a slower piston speed. One of the rpm limitations is piston speed as too fast of piston speed and it starts to transfer metal to the bore due too high heat. So of the two saws, with different stroke lengths, the shorter stroke saw will have a much higher safe rpm limit based on piston speeds. Of corse porting, carb, exhaust style all determine what rpm the saw will operate at.
Rod length does not change the stroke at all. It has no effect on that at all. But it has a big effect on piston to crank leverage. The longer arm of a long rod allows the piston to push on the cranks stroke at a more favorable angle. Think of it like a longer crowbar generates more force than shorter one when pushed down upon with equal force by the user. The other place long rod vs. short rod is at TDC. There is a portion of the combustion cycle where the crank stroke moves thru 10 to 20 degrees or so at TDC, but the piston moves very little up and down in the bore. Long rods increase the time the piston is in the dwell zone and allow more time for the combustion high pressure gas to push down on the crank, and this happens while the piston is exposed to the highest peak cylinder pressures. Couple this with the increased favorable leverage angles and an increase in power can be found. Longer rod lengths help engines make more power at a given rpm limit.
The problem is we can not easily change our rod lengths. You have to cut off the rod, and use a spacer under the jug, and adapt a longer rod from a homelite or mcculloch style engine. Piston pin heights in relationship to squish clearance may/probably have to moved as well.
Are you pressing the crank apart and changing rods and re truing the cranks?
Pressing apart, pressing together and retruing is the easy part. Finding a suitable rod is not. Saw rods are considerably smaller than all 2t mx bike rods (even the 50cc bikes) The smallest easily available rod is 75mm C to C, 22mm BE, 14mm SE. Compare that to a 7900 rod ( just because I have one in front of me. 63.2mm C to C, 20.4mm BE and a 16mm SE.Are you pressing the crank apart and changing rods and re truing the cranks?
Hell no. I hear those take talented men and fancy cuss words to get them apart.
As someone who builds car and chainsaw engines, this is how I would approach doing a longer rod in a motor IF I had too. I'd leave the crank in one piece, find/make a two piece rod to fit on the crank.
Also keep in mind service life before a rebuild. Dirt bikes need top end rebuilds each year whereas saws can go hundreds of hrs before a rebuild.
Pressing apart, pressing together and retruing is the easy part. Finding a suitable rod is not. Saw rods are considerably smaller than all 2t mx bike rods (even the 50cc bikes) The smallest easily available rod is 75mm C to C, 22mm BE, 14mm SE. Compare that to a 7900 rod ( just because I have one in front of me. 63.2mm C to C, 20.4mm BE and a 16mm SE.
Ignoring width issues the small end is a non starter. Going the next size up rod 80x22x17 now you need to bush the small end back down to 16mm and you still have a 1.6mm larger crank pin (which could work as basis for a 1.5mm offset pin & 3mm stroke increase)
The rod has to be 1pc. You don't have space in the case for a 2pc rod. Also even if there was, you would have a very hard locating a split cage needle bearing.
Most 2t bikes will go 100-300 hrs between pistons depending on use. However you are correct if you were to run a 15hp 65cc saw @13000 rpm constantly you would likely need a new piston every 20-40 hours of run time, and a crank every 100.
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