tgerloff92
ArboristSite Operative
How the hell did randy get 225 put of a 440?! That's impressive right there. I am guessing its not an everyday work saw? That or the ignition timing isn't advanced as well?
What fuel for compression?
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rbmopar
ArboristSite Operative
Ok...... The point at which a chainsaw has enough compression to require race fuel is well past the level at which the increased compression is beneficial to power. The exact compression that will cause detonation is very situation specific and cannot be given a general answer.
Discalimer: I should add that when you get into true race saws with two piece heads, pipes, alchohol, etc. I don't have a clue. Only talking about my experiences with work saws.
Discalimer: I should add that when you get into true race saws with two piece heads, pipes, alchohol, etc. I don't have a clue. Only talking about my experiences with work saws.
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tgerloff92
ArboristSite Operative
Is there a general number for most applications, I would think 85 octane would pre-maturely detonate in any motor at say 250 pounds? Next question? Have you ever heard of compression being high enough that the inertia of the engine/flywheel was not enough to overcome this?
Trx250r180
Saw polisher
it has a 460 jug on it ,its a good runner ,can bury a 32 inch bar in fir and the power is smooth through the cut ,he cuts the squish band to make the combustion chamber smaller ,then cuts the base down ,then does the porting ,piston is not altered at all ,is 52 mm bore ,ign timing was advanced a little also
What knowledge?? I haven't seem much in this thread at all, just guessing at best.
Since Stihl and recomends 89 or higher octane most try to stay with 89 or higher but most can't go there without getting into a ethonal based blended fuel so that is where the AV gas comes into play for most plus the added benifit you can store it for a much longer period of time.
I don't see it from a performance perspective.
you can read until your eyes bleed as there's a ton of variables with this and oyu just have to find what you are comfortable with. 110 octane seemed high to me so that is why I am trying the 100.
As for compression with your direct question there has been a picture lately of a saw that showed signs of detonation on the piston as I recall here on this site. I'm not sure what was determined as to the cause. I would guess timing was to advanced? Like others have said few saws really need it based directly from high compression alone.
I don't see it from a performance perspective.
you can read until your eyes bleed as there's a ton of variables with this and oyu just have to find what you are comfortable with. 110 octane seemed high to me so that is why I am trying the 100.
As for compression with your direct question there has been a picture lately of a saw that showed signs of detonation on the piston as I recall here on this site. I'm not sure what was determined as to the cause. I would guess timing was to advanced? Like others have said few saws really need it based directly from high compression alone.
tgerloff92
ArboristSite Operative
I know how he gets it that high, but reasonably that's a whole mother story. That man spends a lot of time on each and every saw. That considerable of a drop means that much more aluminum must come out of the transfers, and intake and exhaust ports. That adds up time quickly!
tgerloff92
ArboristSite Operative
I second that. But it is fun to see people's reasoning and thoughts.
user 64030
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I thought this as well, but can't find any regs on it myself... I did find a sight that discussed EPA requiring AV gas to go lead free, but the same article said EPA has no jurisdiction on AV fuel it is FAA controlled. Small airports w/out a tower typically have a 24/7 pump I can walk up to the one near me and pump w/ a credit card night or day, doesn't even ask a tail number.
It does burn cooler higher octane raises the flash point and lowers the stored energy thus less heat... and yes it has a longer shelf life and is required by FAA to be ethanol free. Ethanol in an airplane is real bad as at 10K ft or higher ethanol that has attracted moisture will turn the carbs on the plane to a block of ice.... and the plane into a earthbound rocket ship.
dw
user 64030
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Yea, I drive out to hte sticks (well I drive around the corner I'm already in the sticks). Place has 100LL AV and Jet-A. I asked the guy what the heck can land here that needs Jet-A, he said we just had a jet take off minutes ago... wowsers. Only a 4ft fence around the airport, walk through gate and I park 40' from the pump, walk in and pump (or used to).
dw
dw
tgerloff92
ArboristSite Operative
Ha our airport is SO small. I know the owner and have fine a few favors for him, so I may go see if he won't make me a deal and test a batch out
STOIE
ArboristSite Operative
Compression RATIO is not based primarily off psi... it does come into play but, lots of other things do also.
For instance FI engines generally have low CR's but, pull much higher cylinder pressures. (as the initial manifold pressure is higher than atmospheric)
N/A can have low cylinder pressures but, high CR's.
A fuel with octane X doesn't knock at temperature Y, and a fuel with octane A doesn't prevent knock at temp B...
Ignition advance has much much to do with this, and again is not purely dependant on octane... it also depends on vapor rating and air/vapor/temp ratios as well as burn rates.
Different fuels have different burn characteristics, some burn slow to begin with (huge advance), then rapidly expand shortly after, which produce an "accelerating" motion to the combustion, rather than a det motion.
Some burn different with different AFR's, some run better at certain ambient temps, pressure, run time...
There is literally sooo many variables and you need to know them all before you can even begin to determine what will work best.
I recently tried my newly acquired stumpbroke 576AT on C16 117MON VP leaded race fuel and did "appear" to be getting more top end power, but, this is dependant on my ambient temp, pressure, run time, mods done to saw etc.
I know the fuel has a low vapor rate, which makes it somewhat harder to start with the race fuel.
But, when it heats up after a period of running, it appears to stay more stable, the cooler running fuel seems to keep the saw in it's "sweet" zone.
I would not even begin to try this with a non-AT saw at this stage as getting the timing, AFR bang on would be a damn hard task.
For instance FI engines generally have low CR's but, pull much higher cylinder pressures. (as the initial manifold pressure is higher than atmospheric)
N/A can have low cylinder pressures but, high CR's.
A fuel with octane X doesn't knock at temperature Y, and a fuel with octane A doesn't prevent knock at temp B...
Ignition advance has much much to do with this, and again is not purely dependant on octane... it also depends on vapor rating and air/vapor/temp ratios as well as burn rates.
Different fuels have different burn characteristics, some burn slow to begin with (huge advance), then rapidly expand shortly after, which produce an "accelerating" motion to the combustion, rather than a det motion.
Some burn different with different AFR's, some run better at certain ambient temps, pressure, run time...
There is literally sooo many variables and you need to know them all before you can even begin to determine what will work best.
I recently tried my newly acquired stumpbroke 576AT on C16 117MON VP leaded race fuel and did "appear" to be getting more top end power, but, this is dependant on my ambient temp, pressure, run time, mods done to saw etc.
I know the fuel has a low vapor rate, which makes it somewhat harder to start with the race fuel.
But, when it heats up after a period of running, it appears to stay more stable, the cooler running fuel seems to keep the saw in it's "sweet" zone.
I would not even begin to try this with a non-AT saw at this stage as getting the timing, AFR bang on would be a damn hard task.
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SawTroll
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tgerloff92
ArboristSite Operative
tolman_paul
ArboristSite Guru
As I've said on other threads, the two strokes with the highest efficiency are pretty much square i.e. equal bore and stroke. Saw manufacturers have long used what I consider fairly over bore designs i.e. much larger bore than stroke in order to build a more compact and lighter engine, at the sacrifice of power per cc.
Husky certainly wouldn't have gone to a longer stoke bore ratio if they hadn't needed to, as weight rises fairly rapidly when you increase the dia of the crank shaft and the lenth of the rod, steel parts, and lengthening the cylinder vs. shrinking the piston, aluminum.
