Octane Boost

[Dagger]

Ok Fish, I'll jump in here. 1st a definition over commonly misunderstood terms.
1st..what is octane?
A measurement of a fuels resistance to detonation,the higher the number, the more resistance to detonation.
in other words...the higher fuel is harder to light !
high octane gas is not "hotter" than low octane!

now, the octane requirement of an engine is determined by a couple of things.
Compresson ratio
ignition timing.(if static...not varible)
and to a lesser extent...port timing on a 2 cycle

an engine designed to operate on 87 octane fuel will NOT have more power when operated on 90. in fact if there is any change..it will be most likley be slightly lower horspower results.
now this is if you keep ignition and compression parameters the same...dont forget this part.

any engine of given displacement will make more power when a higher compression ratio is utilized..simply due to higher BMEP
(Brake mean Effective Pressure)
lets call it cylinder pressure.
In other words..when a fuel is compressed more, and ignited..a higher pressure explosion will occour..pushing harder on the piston...etc.

Now..on all designs, you can increase the compression ratio and get more power then right?...correct....untill the fuel starts to "pre-ignite"..or light off before we are ready for it to. When this happens the whole cycle is thrown off an several things happen...All detrimental to the engine, and which thing causes the 1st failure is simply a matter of where the design is weakest.
could be the heat causing that meltdown as described in an above post..could be bearing failure due to impulse transfered down the rod to big end...or maybe a main...what fails 1st might be different in a different engine.

A manufacturer could easily give you a 4 cube saw that has twice the power if he could design it to run on ..say...140 octane..
raising the compression and adjusting the ignition timing would be a simple thing..not affecting the production cost of the engine.

But a manufacturer cant do that cause you cant buy that fuel..so they design an engine to run on the octane available in the target market area...I assure you, its as simple as that!


If none of the mentioned parameters on your saw have not been changed...then use what the owners manual recommends..that suggestion did not come from the sales dept..but from the design teams original target specs when the engine was 1st just a drawing on an engineers table.

Dont compare the results you get in your car when changing octane fuels either..That engine probably has a computer controlled ignition curve..mixture ratio..and so on that adjust to different sensors to accomplish different things...some set up around gas mileage as the priority...some set up with power as a priority. Nothing there will give you the results that your 2 cycle chainsaw with these parameters fixed.

Now for those who want to split the hairs..you might be able to raise the octane level used and see if it works better,,trouble with this is, in a 2 cycle, pre-ignition..or detonation is a very hard thing to hear , and takes a well-trained ear to detect. So you are treading on really thin ice.

In a car...you might just change the head gasket to a thinner one and raise the compression and use higheroctane fuel and get a power increase..hot-rodders use to do this all the time..but in a saw..you cant do this..and dont sit around thinking about how to shave off the cylinder base to get this effect because you do that and you have changed the port timing..unless you intend to change that also.

Some of you may be familiar with the WW2 story of the Rolls-Royce Merlin in the mustang and spitfire and how that engine was superior to the allisons from the USa and the Axis fighters engines. Well, the underlying reasons behind its superiority was that it was designed around a fuel octane level that was higher that available in germany and the USA..and without that fuel availbility...there would be no advantage. the Allisons were not designed for the fuel..so they would not run on it. The Axis powers engines were designed to run on 80 octane, and german oil companies did not have the ability to produce anything higher..but thats a story fer another thread!!!!

 

[sawracr]

Fish, I have a couple orange Poulans that run on 110 octane Nascar fuel, they don't ping and they eat Stihls for lunch!!!! But my 090 does'nt run to bad on high test.
P.S. as I recall a guy named Ron Hartill ran Poulans all over the Country in logging competition and kicked-butt.

 

[bwalker]

You guys seem to be confused about detonation and preignition. They are not the same. Pre ignition is caused by something in the combustion chamber being heated to the point of being incandesent. This causes the mixture to be ignited before the spark plug fires and results in the audible ping(sort of like severly advanced timing). Pre ignition in my expierance is usually caused by lean jetting although this is mostly on mx bikes. Detonation on the other hand is the massive explosion in the combustion chamber after the plug has fired. This event takes place toward the end of the combustion event and is very destructive. It can result in a seizure, melted pistons, head damage, broken plugs etc. If a pressure rise of a normal combustion event was graphed it would look something like a gently slopeng bell curve. The curve for the ressure rise of detonating combustion event would look simular at first but just as the curve peaked there would be a severe pike in the graph.
As for the octane issue. Octane has nothing to do with how hot (or cool for that matter)or how fast the flame speed is of a certain fuel. These properties are controlled by the distilation curve and the hydro carbon makeup of the fuel. Octane is simply measure of a fuels ability to resist detonation. In the aircooled motors we are dealing with it can never hurt to have to much octane as a insurance policy. MY advice is run premium in all two strokes. I may not be needed most of the time but when it is it my mean the differance between a running piece of equipment and one the is detonated itself to death. BTW octane boosters generally are not worth a ????. The do not increase octane in a consistant or linear manner and the can lead to combustion chamber deposits that actually cause pre ignition.

 

[stihltech]

octane

Pretty sure all current Stihls use 89 octane. Makes a big difference on my 009. Any thing more is just a waste.I was always taught that higher octane meant a slower burn which will make less detonation. But lower octane will light quicker. Sounds like a fine line. I will just stick with the 89. It works and cuts firewood. At least that is what I use my saw for.

 

[Stumper]

I don't think that the octane rating makes very much difference in the way the saw runs(speaking only in terms of "normal" gasoline ratings). Higher Octane fuels are much less prone to boiling in the tank/vapor lock. The premium fuels also may have more detergent additives-which I frankly don't know much about but I have one Echo manual which states that that is the reason for recommending mid-grade or higher fuel.

 

[bwalker]

I was always taught that higher octane meant a slower burn which will make less detonation. But lower octane will light quicker. Sounds like a fine line

Octane does not effect flame speed/ignitability or BTU.

will just stick with the 89. It works and cuts firewood

Whith the compretion ratio of a stock saw you will not have any problems unless you get a bad load of fuel which does happen from time to time. Where premium becomes critical is for those people who have tuned saws. Start playing with porting/compression and its a good idea to run premium. The way I look at is premium cost only a few cents more than mid grade, why not use it as cheap insurance.

Higher Octane fuels are much less prone to boiling in the tank/vapor lock

Octane has no effect on vapor lock. A fuels 10% point is the largest influance on vapor lock.
BTW not matter what octane you use try to avoid fuels that contain alcohol. If your state has RFG try to use one with mtbe in it.

 

[Tony Snyder]

orange Poulans?

Can someone clarify the orange Poulan thing. I know that the 2036 and 2040 Jonsereds are made at that plant, but which Huskys?

I have not been able to get anything made there to run well, trimmers or saws, so I won't reorder any of them anymore. They just cause bad customer satifaction for me.

 

[Dennis]

poulans

Tony....if I remember correctly and I dont have the saws on the shelf to confirm...the 2035 is the only Jonsered made by Poulan..the 2036 and 2040 are swedish made...you will note the very different design features of the 2035 to 2036/2040...the Husky 136 and 141 are both made by Poulan I think...its been so long since I have sold the smaller saws...and then the 340 goes back to Sweden....

 

[stihltech]

octane

OK, way back in the 70's when I went to Alfred Tech, A guy from Quaker State gave a demo. 2 small troughs. 1 held a small stream of low octane gas, the other, high octane. He lit each one. The low octane went up in a big flash. The low octane started at on end and burned to the othe much more slowly. That is how I was taught about octane. More even burn for more power and less detonation or PING. Not to scientific, but real.

 

[bwalker]

Stihl tech, Check out this article. The writter does a much better job than I explaining the concepts involved.
http://ericgorr.com/techarticles/Fuel_Basics.htm

 

[bwalker]

Stihltech wrote:

The low octane went up in a big flash. The low octane started at on end and burned to the othe much more slowly. That is how I was taught about octane. More even burn for more power and less detonation or PING. Not to scientific, but real.

The faster a fuel burns the better. Detonation takes time. Cut the time by speeding up the combustion event and you decrease the chance of deto occuring. This is why two strokes have squish bands. They aid in speeding up the combustion event. Check out this article. The writter does a much better job than I of explaining the concepts involved.
http://ericgorr.com/techarticles/Fuel_Basics.htm

BTW this qoute comes directly from the article "It's a commonly held misconception that higher Octane fuel slows down the flame speed which keeps the engine from knocking. Flame speed is a function of fuel chemistry, not the Octane rating. The component make up of the fuel will determine the flame speed whether it's a high octane fuel or not

 

[Stumper]

bwalker, I assume that you have a scientific basis for your statement that Octane has no effect on vapor lock. Okay..........but I have consistently experienced problems when air temperatues are over100 degrees and the problems went away with premium fuel. So what is the 10% rate you mentioned and does it differ between grades of fuel in any consistent way?

 

[bwalker]

The 10% point is part of the distaillation curve at which 10% of the fuels componenets evaporate. The reason you may not have expieranced VL with premium could be because the premium had a higher 10% boiling point then the regular. It however had nothing to do with octane. If you are really having provlems with vapor lock use avgas. It has a high 10% point to combat vapor lock in aircraft. This high 10% point leads to crappy throttle reponse in a two stroke however. Not that throttle respoonse in a saw matters much.

 

[Newfie]

Stumper,

Does the vapor lock/ boiling gas probably have to do with being "rocky mountain high"?

 

[stihltech]

OH NO!

You mean a gas company lied with hocus pocus? I saw it with my own eyes. It smelled like gas, it looked like gas. So why make racing fuel?

 

[bwalker]

You mean a gas company lied with hocus pocus?

It appears so. Look at one of the major oil company websites for physical properties of there gasoline. You will find all have about 19000 btus and have simualer flashpoints.

So why make racing fuel?

Because its harder than hell to tune a race motor with the inconsistent, epa mandated crap we get.

 

[Stumper]

Newfie, Actually most of my vapor lock troubles occurred at lower elevations during hot weather. For several years I've used premium during the summer and haven't experienced much trouble. I don't care if it is octane or 10% rate (I believe you bwalker) the premium solved the problem for me.

 

[treeclimber165]

I can vouch for the 'premium in hot weather' thing. As someone who has done treework and lawn work in FL for the last 20 years, I can tell ya about fuel boiling in the tank. I've found 2 things that prevent it. 1 is premium fuel with Stihl (or comperable) oil, 2 is to set your saw in the SHADE when you shut it off and set it down.
Glad my lil POS Echo 3400 has a purge button on the carb.