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View Full Version : Stihls view on what OCTANE to use in the 4-mix and 2-cycle engines...


dcplace2004
09-29-2005, 03:45 PM
I called Stihl and they said that minimum 89 octane but 93 would be the better choice...just like the other LCO and Stihl said, the better the grade, the less impurities, prevention of pre-detonation, less heat, less wear and tear on engine...although there is extra cost for 93 octane, the benefits outweigh the cost, and I stand corrected from past posts...

John Gamba
09-29-2005, 03:57 PM
I called Stihl and they said that minimum 89 octane but 93 would be the better choice...just like the other LCO and Stihl said, the better the grade, the less impurities, prevention of pre-detonation, less heat, less wear and tear on engine...although there is extra cost for 93 octane, the benefits outweigh the cost, and I stand corrected from past posts...


Thank you :waving:

geogunn
09-29-2005, 04:12 PM
...the better the grade, the less impurities....

I would like an explanation of the "better the grade, the less impurities".

does a higher octane rating equal a "better grade" of gasoline?

and how does a better grade of gasoline, which is higher octane by this definition, equal less impurities?

is sthil saying they put dirt, crud, cr_ap or something else in the lower grades you wont get with a "higher" grade?

their explanation doesn't sound right to me on the impurities thing.

GEO

grass_cuttin_fool
09-29-2005, 04:18 PM
I wonder if the higher the grade the less alcohol in the gas? Seems I remember some people having issues with the oil not mixing with the winter formula of gas.I asked stihl in an email and the guy acted like it was a deep dark secret and no explanation of why 89 octane was needed other than that is what is stated in the manual

Richard Martin
09-29-2005, 05:35 PM
I called Stihl and they said that minimum 89 octane but 93 would be the better choice...just like the other LCO and Stihl said, the better the grade, the less impurities, prevention of pre-detonation, less heat, less wear and tear on engine...although there is extra cost for 93 octane, the benefits outweigh the cost, and I stand corrected from past posts...

I'm not buying most of this because it is pure bunk. The only reason to use 89 octane is for detonation. Did you know that using too high a octane rating can also be harmful? Higher octane gas is harder to burn, it resists preignition because of this. If you use too high an octane gas it can lead to carbon deposits and increased emissions because all of the fuel will not burn. High octane gas needs an appropriately high compression to burn properly, just like deisel fuel.

nmurph
09-29-2005, 06:03 PM
octane and impurities have nothing to do with each other. a higher octane rating does not necessarily equate to more power or a more pure fuel. octane is an additive that increases the detonation point of fuel. it does not increase the caloric value, which is where "power" comes from. that said, a higher octane of fuel can allow the spark to be advanced, or the compression rate to be raised, resulting in an increased efficiency of the burning of the fuel and a corresponding increase in power. if an engine is optimized for a certain grade of fuel, increasing the octane will not increase the power unless the engine has a management system that is designed to adjust for these new parameters. conditions such as engine load, temperature, engine condition, ambient temperature, etc. influence detonation. i don't know what specific conditions small engines are designed for, but i think you could safely assume they are built to endure some harsh conditions and i would think that small engines are optimized for those conditions, with the grade of fuel they recommend.

bill w
09-29-2005, 06:10 PM
I'm not buying most of this because it is pure bunk. The only reason to use 89 octane is for detonation. Did you know that using too high a octane rating can also be harmful? Higher octane gas is harder to burn, it resists preignition because of this. If you use too high an octane gas it can lead to carbon deposits and increased emissions because all of the fuel will not burn. High octane gas needs an appropriately high compression to burn properly, just like deisel fuel.


No, it is not harder to burn, it is merely more stable (like dynamite is more stable than nitro glycerine). It has a spark plug so it doesn't need higher compression to burn. Now, if the engine had higher compression, it might need higher octane.

dcplace2004
09-29-2005, 06:24 PM
but that is not in etched in stone, obviously...I would stick to 89 then...middle of the road and on the safer side...I would not go 87 though...

Richard Martin
09-29-2005, 07:00 PM
No, it is not harder to burn, it is merely more stable (like dynamite is more stable than nitro glycerine). It has a spark plug so it doesn't need higher compression to burn. Now, if the engine had higher compression, it might need higher octane.

Truth be known nitro is the active ingrediant in dynamite. I stand by my statements.

bill w
09-29-2005, 07:13 PM
Truth be known nitro is the active ingrediant in dynamite.

That's why I used it as an example. :D

mowtech
09-30-2005, 02:31 PM
The octane rating of gasoline only tells you how much the fuel can be compressed before it spontaneously ignites. It does not tell you anything about the energy content--all gasolines being relatively equal in the amount of energy contain. Anyway, when the gasoline ignites by compression rather than because of the spark, the ignition is explosive. This is called both detonation and knocking. Obviously this detonation can damage an engine and lower performance. Lower octane gas like regular 87 octane gasoline can handle the least amount of compression before spontaneous ignition.

The compression ratio of an engine determines the octane rating of the gas that must be used. One way to increase the power of an engine of a given displacement is to increase its compression ratio. The advantage of higher compression ratios is higher horsepower for a given engine weight. High performance engines have higher compression ratios and therefore require higher octane fuel.

Gasoline is made up mostly of two hydrocarbons, these being Heptane with seven carbon atoms and octane with eight carbon atoms. Heptane actually is prone to detonation while octane handles compression very well. Historically, the per cent of octane in the gasoline would determine the amount of compression the gasoline could handle, hence the octane rating. Eighty seven octane gasoline contained 87 per cent octane and 13 per cent heptane.

It was found that octane ratings could be raised by adding tetraethyl lead instead of increasing the amount of octane. This was cheaper to produce and was called as you remember leaded gas or ethyl. Catalytic converters could not handle the lead so the lead had to be taken out of gasoline. Today octane is obtained by the blending of the different hydrocarbons with the lower the ratio of hydrogen to carbon the higher the octane rating. Alcohol in the fuel also affects the octane rating. Octane ratings are now determined by tests.

All gasoline no matter the octane levels are required by the EPA to contain about the same amounts of additives (detergents) to clean injectors and valves. Low octane gas certainly does not contain more impurities. High octane gas has no advantage in additives or quality.

The only time higher octane gasoline may give you an advantage is if for some reason your engine is running too hot. If the engine is too hot, detonation can occur even at the lower compression ratio of a standard engine (itís the heat of compression that causes the spontaneous ignition, add some more heat and there you go). Your engine can run hot for several reasons these being your carburetor is out of adjustment or dirty, you have excessive carbon build up in your engine, you are operating in high ambient conditions and or very dry conditions. In most of these cases you should be able to hear the detonation and know that the higher octane can help.

Dirty Water
09-30-2005, 04:52 PM
Guys, I covered all of this for topsites a while ago in this thread:

http://www.lawnsite.com/showthread.php?t=115554&page=1&pp=10&highlight=octane

Why does this keep coming up?

There is also no difference in purity between the two grades, however the higher grades have additive's added in the tank (cleaners etc), these can be bought for a few bucks at wal-mart seperately, and you only should add them to a tank on each oil change.

mowtech
09-30-2005, 06:10 PM
Guys, I covered all of this for topsites a while ago in this thread:

http://www.lawnsite.com/showthread.php?t=115554&page=1&pp=10&highlight=octane

Why does this keep coming up?

There is also no difference in purity between the two grades, however the higher grades have additive's added in the tank (cleaners etc), these can be bought for a few bucks at wal-mart seperately, and you only should add them to a tank on each oil change.

Higher grade gasolines (higher octane) do not contain any more additives (other than the octane boosters). Check out the web sites of the major gasoline companies and you will see they make no claims of additional additives in their premium grade gasolines.