saw gas 91 octane?

[minesmoria]

I heard that you should use 90 octane gas or higher for the saw..

It keeps the engine coller.

[SasquatchMan]

Octane rating has nothing to due with power, cooling capabilities, or anything of any interest to a saw, save for one.  That is higher octane gas has a higher flash temperature, and thus it will not explode prematurely in a hot engine, causing what is known as pinging.  If you run a low octane gas in a hot application (like a two stroke motor) you will find the timing of the motor is poor, and performance is subideal.  

The answer is yes, use high octane gas, because your engine will work better with high octane gas in it.  It allows your saw to operate correctly.

[Timber_Framer]

Interesting, I always thought that it caused your machine to run cleaner thereby allowing the saw or other 2-stroke to work more efficiently.

[SasquatchMan]

Well, it sort of does allow the machine to run cleaner, in that it allows the engine to work properly - therefore, you find fewer deposits on the plug or the spark screen.  Having the gas fire at the right time IS more efficient.  But these bonuses are just the bonuses of running the machine correctly.  Higher octane does not "add" them.  If you could get 95 octane, there would be no improvement in performance, for most folks, over 91 octane, but some do notice a difference between 87 and 91 in terms of pinging etc.

[Timber_Framer]

Thanks for clearing that up

[Rocky_J]

Some saw manufacturers (such as Echo) recommend 89 octane or higher. Your mix oil will also slightly lower your octane rating. Also, some gasoline additives can evaporate or deteriorate in as little as 4 weeks, lowering octane rating. Therefore I've used premium gas for my mix fuel for the last 20 years and it is extremely rare for me to ever have any issues as far as poor running machines. I've worked with guys who only use regular gas for mix fuel and many will have carb or operating problems fairly regularly on older machines. So I prefer to use my own mix fuel.

[weimedog]

The octane discussion goes something like this:

Higher the octane the lower the flash point and a slightly lower burn rate. The trick to figuring out what octane has to do with the tune of the motor is understanding the balace between the setup of the motor vs. the fuel it needs to allow that setup to work well. Two parameters in particular are important. Compression and timing.The higher the compression and/or the more advanced the timing will tend to want a higher octane fuel. The power gained thru the higher compression and/or more advanced timing is greater than the power lost to a slower burning fuel. Conversly if the compression is low and the timing is retarded then the lower octane fuels help offset those machanical tune issues with a little hotter and faster burning fuel. Think of the fuel as a method to fine tune your motor. You want to use as low an octane fuel as possible to get the most out of a particular motor setup.

With older and less precise ignition systems higher octane fuel was needed to allow for the varience in timing as those thing swung back and forth in their mechanical caracteristics. Kept them from pinging (pre-ignition) when they got to the advanced side of their varience.

Pre-ignition is litteraly the flame front of the burning fuel hitting the piston before the piston gets to top dead center. The Flame front whacks the piston on the way up....you hear the "ping" in a two stroke or the "clatter" in a multi cylinder four stroke as it smakes the valves back to their seats too. Left running that way too long and the combustion chamber temp rises and eventually thing start to fail. (You see a grey on the plugs from the melted aluminum off the piston)

To run around the block on this discussion again....if the compression is too high for a given octane when the combustion chamber is hot from running AND the incressed pressure raises the temp of the fuel air even more..sometimes the compression ratio can cause the tempurature of the fuel air mixture to cross that ignition threshhold even before the spark plug fires..like a deisel. Since this is before the prescribed time based on ignition timing...PING!!!!. So you can raise the octane level (IF the compression is within reason) until the fuels flash point is high enough to push the ignition temperature due to presure rise past the time the spark plug fires...now the motor is happy and no more ping on that cylinder. AND since the higher compression ratio allows more engine power for a variety of other reasons..it more than offsets the power loss due to a slower burning fuel..you have a snappier engine.

So then the tuner begines to fine tune the ignition timing to try to get that last ouce of energy&power out of the choosen fuel/compression ration combo and advances the timing right to the edge of preignition and that more aggressive timing adds a bit more favorable power characteristics.....whew I hope you all get the idea

Conclusion? If in doubt...use higher octane.

The next real issue we all have to work around is the new alcohol fuel additives and their impact on things like diaphrams and fuel lines. Especially with us old saw collectors. I just picked up this REALLY nice old Homelite Super XL 925...its fuel lines were like putty.

Anyone use 100LL av gas??

[Timber_Framer]

I've used AV gas in my Harley but not ina saw.
It was real fun in the hog ;)

[Avalancher]

One question along the same lines. Does anyone use/have problems using gas dryer in their saws? I end up cutting a lot in the rain, felling and milling(yah, I know I should have enough sense to come out of the rain) and even though I use one of those safety cans that the lid slams down on the spout the second you release the handle I still end up with a little water in the can after a day sawing. Last weekend I ended up with a carb full of water, and it ended the day early for me. I thought about throwing some of that B12 Chemtool that works great for all my other stuff(all four stroke tools)but I got to wondering if it might have any effect on a 2 stroke. Any ideas?

[Minnesota_boy]

Chainsaw carburetors use a thin rubber diaphram (most of them) to pump the gas from the tank.  Some additives that are safe to use on a 4-stroke engine will eat that diaphram.

[weimedog]

From what I have seen in the past, certain alcohol based products attack the rubber in things like Diaphrams. I saw that when I was working on outboard motors. I THINK I am seeing in again as I go thru old my old Homelites. Lots of melted fuel lines and filters.

[rebocardo]

Dry Gas chemical bonds with the water so you burn it along with the gas, from what I understand. I have never researched it. I would not use it in a two stroke.

What I do know about is fuel separators. You can buy one for water at NAPA for under $30 and use it on your fuel and once the filter fills up, unscrew the clear housing and dump the water out. If you saw gets water in it, you can dump it out and recycle the fuel.

From what you describe, you need a gas can that comes with a handle and you use it to fill your saw just like you do your car. That way the water does not get into the can itself and it makes filling the saw quick and easy. They sell for $30 and I think northerntools.com carries them.

[tony_marks]

very inteesting there wiemedog..take a while to digest it all..
  i dont really understand,, your first sentence. higher octane means lower flashpoint.. i thought the reverse was true. thus preventing preignition in hot motors.. ???

[Captain]

Nice post, Wiemedog, right on the money.  This coming from an Automotive Engineer.  

While most fuel manufacturers put more detergents in their higher octane grades, by law all grades must have a baseline of detergent additives.  

Many people do not understand the concept that you should burn the lowest grade fuel that you can in your particular application.  In the case of automobiles,  lower octane fuels that have a faster burn rate can produce more power and economy in the right application.  If your engine can not burn the fuel without retarding the timing, the oppesite is the result.  My wife's car gets the best fuel economy on regular grade 87 octane fuel.  With that said, it still gets a tank of top tier "super" unleaded every couple of months for the detergents.

Captain

[Ianab]

you are right Captain.. if an engine is designed to run on 87 octane then it's compression and timing will be set up to work best on that. Running a higher octane will not give any andvantage, and may actaully be less effecient.
Problem with saws is if they go out of adjustment and run lean they can die. Higher octane gas gives a bit more leeway maybe?
My crappy little Toyota work wagon runs runs 96 octane, I believe it will run regular, but only because the computer adjusts the timing to prevent knock
Not the best idea when it's 11 to 1 compression  

For my saws I run the high grade gas too

ian