What octane

[bendjoseph]

I am running 89 octane in my Stihl MS391, would a higher octane be better?

[Cut4fun]

Straight from stihl info.

STIHL recommends  89 octane or higher unleaded gasoline

[clww]

I run high-test (preferably ethanol-free) in all outdoor power equipment.

[shinnlinger]

I have heard in the summer months, super is ethanol free, but I don't know if this is regional.

[beenthere]

I've not heard that one, but may be the case in some states.

For 10¢ a gallon more, I can get regular without ethanol. Use it in the car and for all my gas needs.

[tyb525]

Almost every engine manufacture recommends regular 87-89 octane nowadays. An engine performs the best on the lowest octane you can run without it detonating. Run whatever the manufacturer designed it to run on.

[Ianab]

Yeah, simply going to a higher octane than needed wont gain you anything, unless you are doing to get ethanol free gas, which may be a valid reason.

Running a Lower octane then spec is bad as you can get pre-detonation, and blow up an engine. Running higher than needed has no effect, except you pay more.

We only have 2 gas grades here in NZ, Regular and High Octane. It all comes from the same refinery no matter what gas station you buy it from. Most cars are fine on the regular grade, although my little Toyota runs 11:1 compression and does NOT like low octance 

I use the higher grade gas in my saws, just to be safe.

Ian

[DandB Mack]

Ethanol free would be a better gain than moving up in octane.  Chances are, you won't notice a difference going from 89 octane to 92/93/94.

[cgk60]

I run the highest I can get 91 octane.

[thecfarm]

I have no idea if I can even find non-ethanol at a gas station here in Maine. I use the high test in all my small engines. I have no idea if it really matters or not. Just makes me feel better. May all be in my head,but something needs to be there. 

[DouginUtah]

I have no idea if I can even find non-ethanol at a gas station here in Maine.

Norridgewock airport

[aquacanis]

 8)The only thing I can tell you is I have been running high octane fuel with much less
trouble and a lot easier starting.  It may be my imagination but my saws seem to rev a little faster and cut with more power.  Could this be my imagination?  I live in 10% 87 octane country.

[thecfarm]

DouginUtah,I never checked around. Only about ½ hour from me. I never have any trouble with the new gas,YET. Probaly Bowman airport which is only 10 minutes from me has it too.

[John Mc]

To low an octane can cause detonation (basically, the gas "explodes" in the cylinder, rather than a controlled fast burn, as it should. If you get enough of those tiny "explosions", say goodbye to your piston/cylinder). Once you have enough octane to prevent detonation, higher octane does not mean higher power, at least not without making physical changes to the engine (such as piston/cylinder modifications to change the compression ratio).

If you are noticing higher power by switching to higher octane gas, there are a couple of things it could be:

• your imagination
• you were running to low an octane before, and ow you're above the minimum to prevent detonation on your saw
• your old gas had ethanol, and the new gas is ethanol-free (some engines just don't run well on ethanol blends. Also ethanol has slightly less energy content than gas)
• Your old gas had ethanol or some other fuel oxygenate in it, and your higher octane fuel did not, or vice-versa. Switching from a fuel with an oxygenate in it to one that does not (or switching back the other way) would require a mixture adjustment for best performance. Sometimes you can ignore this and get away with it. Other times, if your mixture adjustment was off one way or the other with your original gas, switching to the other may have moved you in the "right direction" on mixture, improving your power.

One reason for using higher than the minimum rated octane for your engine: most automotive gas loses octane as it ages (due to some parts of the fuel evaporating off). Ethanol blended fuels tend to lose this more quickly than ethanol free gasses. If you are not using it right away (or if you bought it from a station that doesn't turn around a load of gas quickly), the gas in your can may not have its original octane rating. Starting with a bit extra can give you a bit of margin for safety. (Note I'm not advocating going out and buying 100+ octane aviation fuel... Just considering 91 or 93 octane if you are not turning over your fuel supply quickly.)

John Mc

[Al_Smith]

Although it's often a subject of lively debate I'm not so sure you can actualy get detonation on a stock compression ratio saw engine .You probabley could on one with the compression ratio jacked a tad though .Not a big deal to run high test if you wanted to .It's at best 20 cents a gallon more .

[tractorfarmer]

I agree with John MC. Get 93 or 94 if you can. I was told to use the highest grade not because of the octane, but because of the added chemicals that help the life of the engine. They also said there were a few brands to stay away from, but most of the gas comes from the same refinery anyways.

[John Mc]

Although it's often a subject of lively debate I'm not so sure you can actualy get detonation on a stock compression ratio saw engine .You probabley could on one with the compression ratio jacked a tad though .Not a big deal to run high test if you wanted to .It's at best 20 cents a gallon more .

What is the compression ratio on a typical stock chainsaw engine?

If I remember correctly, does temperature have something to do with detonation as well (or am I confusing that with pre-ignition)?

John Mc

[Al_Smith]

I'm not exactly sure but I do believe on most Stihls according to my IPL's around 9.5 to 1 .

I'd imagine on a few I've modified it approachs 12 to 1 .Well it's enough I had to install D-handles to keep my fingers from getting ripped off .

Then again you have to remember that's "static " compression or in other words that which is induced just by pulling the engine over .Dynamic compression is higher because of the way the rings seat when the engine is running .

To more confuse the issue things such as flame front propigation enter into the mix when the talk is about actual detonation .

Now preignition is another issue all together .Often caused by a lean mix and a source of ignition such as a carboned up piston that develops  a "hot spot " .

Then further adding to the mix is actual ignition timing which can cause quite another lively discussion .Examples of that being most piston ported Macs are at 26 degree advance ,most Stihls at 27 .Some can benefit by a slight advance if running at speeds in excess of factory stock speeds and some it becomes a detriment if the timing is jacked up .---and it goes on ,and on and on --

[Al_Smith]

They also said there were a few brands to stay away from, but most of the gas comes from the same refinery anyways.

That is a fact .It will be however at what additive mix or ratio that the praticular brand of gasoline is sold for .FWIW the additives and the dyes are added at the bulk loading terminals prior to shipment to the end seller ,the neighbor hood gas station.

It's all done by a computer operated mixing contraption .

[stumper]

Norridgewock Airport here I come.  It is 30 to 40 minutes away but I'll stop the next time I am in the area.

I run high test.  I have not noticed any added power but I do notice the saw starts easier.  The other thing is I here is ages slower.  Not a big deal in my small saws (372's) but I generally do not run my big saws (394 & 3120) more then a couple of times a month and then ony to cut one or two stump tops off so aging is a concern.