Why do bike motors make so much more power?
- Thread starter Mike-M
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ft. churchill
Addicted to ArboristSite
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.
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?
spencerpaving
ArboristSite Guru
Square motors are usually very happy engines. ..but depends on the application
ft. churchill
Addicted to ArboristSite
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.
Thanks for clearing that up.
ft. churchill
Addicted to ArboristSite
I was noticing that ya never did really get a good answer. I was just over at bills saw shop which has some great spreadsheets of stihl and hoosky data. Only one saw is a square bore... The stihl 009.
I was dreaming of building something big like...MS660 over size 56mm on a husky 2100 to get around the no cylinders available.
070 58mm on a 2100 yields 106cc
090 66mm on a 2100 yields 137cc
Part of the problem is I cannot find out what the rod lengths are, that makes a drop on deal with bolt pattern changes, and or clearances issues much worse if I have to carve out an adapter to correct rod length/squish height.
I was dreaming of building something big like...MS660 over size 56mm on a husky 2100 to get around the no cylinders available.
070 58mm on a 2100 yields 106cc
090 66mm on a 2100 yields 137cc
Part of the problem is I cannot find out what the rod lengths are, that makes a drop on deal with bolt pattern changes, and or clearances issues much worse if I have to carve out an adapter to correct rod length/squish height.
ft. churchill
Addicted to ArboristSite
Damn it there always seems to be a fly in the ointment.
SawTroll
Information Collector
Profess
....and then there are the the rather small box type mufflers, that severly restrict what can be done....
Usually it is the Husky saws that can be modded better though, as the cylinder design tend to leave more metal to work with in the critical places - and I don't think it is a coincidense. Professor GB likely is behind it, and the long transfer quads....
....and then there are the the rather small box type mufflers, that severly restrict what can be done....
Usually it is the Husky saws that can be modded better though, as the cylinder design tend to leave more metal to work with in the critical places - and I don't think it is a coincidense. Professor GB likely is behind it, and the long transfer quads....
ft. churchill
Addicted to ArboristSite
None of us here on AS have ever seen pistons melted, or rings seized due to high heat. 
SawTroll
Information Collector
None of us here on AS have ever seen pistons melted, or rings seized due to high heat.
I believe I have seen both - but neither is common.
SawTroll
Information Collector
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.....
Are you pressing the crank apart and changing rods and re truing the cranks?
ft. churchill
Addicted to ArboristSite
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. There is certainly alot of power to be found elsewhere first in a chainsaw motor before experimenting with rod lengths. I seen a big dolmar race saw with dual double pumper carbs on it and dual huge reed boxes grafted onto the side of the jug. That and expansion chambers are where the most gains will be found once the porting is at the limit for that design. Both not very practical for cuttin' timber for a living or stocking up your family with firewood for the winter.
So far Iv'e just build ported oversize bore engines, with all the typical mods the guys here trot out, but I've dreaming of a Frankenstein saw. Mixed and matched components to get around a obstacle. For instance the Husky 2100 has quite a reputation against other saws from its era. But parts are hard to find and arm and a leg expensive. Especially the jug and factory piston. What if you could graft on a different jug and piston, from another more modern saw that parts are still available for?
jar944
ArboristSite Operative
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)
Then all you need to do is account for the piston being 16mm+ higher than stock
jar944
ArboristSite Operative
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.
jughead500
Sasquatch
I can't believe all you guys missed it.here is the reason bikes make more power.
F.W.P.T
ArboristSite Operative
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.
jar944
ArboristSite Operative
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.
ft. churchill
Addicted to ArboristSite
I have been in the scheming stages for a while now and these are some of the issues I'm locking horns with. You can build a spacer ring and put it under the jug to come up with the 16mm added deck height, BUT will that even fit under the handle bars. What does the balance feel like after you welded a step in the bars?
And I need a huge junk parts bin of throw away parts to measure and see what will fit as I cannot find specs on the rods anywhere.
Besides any saw sitting around here for more than a year ends up getting rebuilt and ported with all of the usual tricks. AND I'm thrilled with the results of 10% to 30% gains in cut speed,quicker spool up , and cool exhaust note. I also value the clean compactness of a stock appearing saw as I use them to cut wood up on the mountain.
Here's some wood SierraWoodsman and I cut last winter. Gosh darn his saws that have made that famous trip out to visit Mastermind in Tenn. sure run hard. I was testing some of my work against Randy's work, but the playing field was not level. I had three saws with no porting , just muffler mods and the 385 also had a gasket delete. We ran my 346, 372 and 385 against the chimp's fully ported worked over saws in 346, 550 372 and 390. To further tilt the field Walt square grinds all of his full chisel chains. His chains alone are a big gain over my round filed full chisel chain.
