Recommendations on octane rating for small power equipment(weed trimmer, chainsaws and lawnmowers) My local power shop dealer strongly recommends higher octane, 91 or 93. He states its "better" for these type engines. Whats the reality in that, if any.

Thanks

 

Compression ratio in small engines is rarely over 8:1. High octane gas is not needed or desirable.

 

High octane in 2 stroke engines is the best for them.
Why you will ask? when you add 2 stroke oil to the fuel it not only lubricates the components it also cools the burn down.
Problem is when you run a low octane fuel with 2 stroke oil you get a cooler burn. This shortens the life of the engine.

To each their own, I have run 2 strokes on low octane fuel, and had no problems. But for my chain saw it is only high octane fuel.

 

Actually the oil raises the combustion temp and speed of the flame front. Because 2 stroke engines have a very short power cycle, they need to burn the fuel quickly. Using high octane fuel lowers the volality (burnability) of the fuel and causes it to burn more slowly resulting in a lot of unburnt (or still burning) fuel to come out of the exhaust port.
Generally you don't want to use high octane fuel in a two stroke, unless it is a high compression specialty engine like some chainsaw engines.
Look on the internet for manuals for your equipment which will have the recommended fuel rating.

Like Dave said, your 4 stroke motors will normally be low compression ratio, requiring regular gas to make them run their best.

 

If I recall correctly, the manual in my weed trimmer, saw, and generator all say that 87 is all that's required.

I'd look up what the makers of your particular products recommend, and go with that.

 

News to me about 2 stroke oil increasing temps.
I always thought it was there to lubricate and then burn off.

I remember using stroke oil to cool AV GAS we use to run in our cars.

Again intersting.

 

I've always used mid grade for lawnmowers,weedeater,gen set, pressure washer, etc. I have no good reason for this, just habit

Around here mid grade is 89, 87 below that and 91 is premium

 

It is a common misconsception that higher octane rated fuels burn (hotter). Most of the time (depending on the octane increasing method) higher octane fuels actually have LESS BTU's per pound than lower octane fuels. Diesel (K2) is the highest BTU per pound fuel you can get that will flow at room temp.
Octane ratings refer to the resistance of a fuel to pre-detonation, and is directly related to the flame speed.
The reason you put 2 stroke oil in AV gas would be to increase the burn speed and heat up the burn with more BTU's.
I have never heard of this, because it sort of defeats the purpose of using high octane fuel to begin with.
The problem with AV gas it that it's mixed so that it will not boil at high altitude, so it's harder to start a cold engine because less of the fuel component will vaporize at ambient temperature. This is of course only a problem on a carbureted engine. MPI squirts it right into the intake port so it doesn't matter as much.

Diesel fuel has a VERY low octane number!!!!

 

Sthil recommends a 90 or better octane for there 2 strokes. go to their web site and check it out.

High octane prevents detonation. Detonation is a issue on high compression, high reving engines or both.

2 strokes are high reving and may also be high compression.

Oil for two strokes, primary reasons is to Provide lubrication and a cleaning agent.

Oil is NOT added to fuel to increase its burn speed.
A good quality oil will lubricate and then burn quickly so as to not foul the gas effected parts on the engine. In this process it will lubricate and clean the engine components. High detergent 2 stroke, hence why guys use it in their diesels also acts as a cleaner.

How you figure 2 stroke oil increase burn speed is beyond me, if anything due to it's inability to burn quickly would decrease the burn speed, hence when you add to much oil the engine gets fouled do to a incomplete burn of the oil.

AV GAS run in a car provides hotter temps. To hot and you will burn the valves out hence why you would run 2 stroke oil in it. To cool the fuel.

You run high octane fuel in A/C because of the higher compression ratios found in them. 87/87 92/92, 100/100, 110/110 all grades of A/C octane fuel for different applications. From low octane to high octane. For low compression to high compression engines.

Boiling fuel to atomization of fuel. I fail to see the reason why AV GAS, (Not Aviation type fuel) boiling and atomization has to do with this.

"The problem with AV gas it that it's mixed so that it will not boil at high altitude, so it's harder to start a cold engine because less of the fuel component will vaporize at ambient temperature."

You do not boil your fuel to start an engine, nor do you boil your fuel to run an engine. Please explain this more.

back to the original question.
Run a good quality fuel. IF you are running one octane of fuel and notice lack of power, hesitation or knocking from the enigine switch grades untill you get the right kind for your specific application.

When all else fails go to the manufactures website and or read the owners manual.

 

I have my Gas analysis book right here. Generally the lower the octane, the higher the BTU value. AV gas is aviation fuel. It is designed to stay a liquid in lower ambient pressures tham automotive fuels (for obvious reasons). That's just fact. Fill your car with AV gas and try to start it in the winter cold, you'll see.

To utilize the potential of high octane fuels the timing must be advanced giving the fuel more time to burn by starting the combustion cycle earlier. Advancing the timing produces more heat because the burn starts quite a bit before TDC on the compression stroke.

High compression ration engines need high octane fuels to avoid detonation, including 2-stroke engines, but any oil (including 2 stroke oil) will lower the total octane rating of the fuel considerably. So it makes sense that on a high performance 2-stroke engine like a chainsaw they would recommend hi test fuel to compensate to the effects of the 2-stroke oil.

 

Alcohol is a good case in point. Ethanol and Methanol are extremely high octane valued fuels, but they have 1/2 to 1/3 the volality (BTU's per pound) as your typical gasoline blend. They also don't vaporize will at ambient temps, so carbureted alcohol cars are a bear to start. Also the jetting must be changed to allow more fuel in because the Stoichiometric values are different than gasoline (7:1 vs 13:1 respectively).

I have heard that a Cummins can run as lean as 100:1 in a crise condition.

Make sense?

 

Actually they recommend 89 Octane. Click

 

http://www.kwf-online.de/englisch/te...h/4202_06e.pdf
I was on the Stihl European and read from there that they recommended a min of 90 octane. Do you think I can find the site now. Nope any ways the above link is a particuler saw that recommends 90.

http://www.stihl.ca/company_history_prod_1990.asp
Check out their own blend of fuel.

But I guess I was misinformed over the matter of running high octane fuel in 2strokes. Along with the use of AVGAS in standard non Airplane reciprocating engines.

I would like to know if I was to run 2 stroke oil in my car it should get better fuel milage, correct? I am now confused as to what 2 stroke oil does in a 2 stroke engine. I guess I will have to go back and take the courses again about fuel and how they work.
Not trying to be a S&^%t edit. But I am all confused.

Every manufature recommends the min fuel rating for their equipement, they have to. Doesnt mean it will run any better on it.
If I have giving any misinformation I apologize.

Good luck and run the min octane in your small engines. If you use them for work let us know how long that run for. I hope you get a full life out of them.


Cheers

 

2-stroke oil is a replacement for a standing crankcase oil level. Since a 2-stroke draws it's fuel charge into the crankcase on the up stroke of the piston, and then on the down stroke forces the mixture into the cylinder through the intake port, you can't have oil splashing around in the crankcase. 2-stroke engines use roller bearings that are lubricated buy the premix oil since they cannot have a pressurized system. All of the oil is burned, which is why two stroke engines are so dirty.

Actually, it is a waste of money to run higher octane fuel than called for by the manufacturer, and in many cases will lead to poorer fuel economy and increased emissions.

It's OK to be honestly wrong, I am all the time. The trick is to learn.
Please do the research, prove me wrong, then I will learn something new.

 

Ok here it is
My understanding of 2 stroke oil's primary function.
It lubricates bearing surfaces and cleans the gas effected parts.

High octane fuel. Used in high compression and/ or high RPM engines. This is to prevent detonation.

Avgas will run hotter then normal fuel. My cheery red exhaust on my jeep tended to agree with me. That is a fact in my case running 100/110 that I know using about 200gallons of the stuff in it.

Running low octane fuel in 2 strokes or any other engine for that matter may not harm it, or It may shorten the life of it. Depends on what fuel rating is required for optimum operation. And vice versa for high octane fuel.

Again try the different fuels and also different oils untill you get optimum performance from your equipment.