All theories aside, just the facts…
“Octane Rating” has nothing to do with the amount of power stored in the gasoline; the “octane rating” is simply a way to rate the gasoline’s resistance to ignition. All sorts of additives can be used to increase the “octane rating”… traditionally lead (tetraethyl lead) was used and is still used in aviation fuels. Interestingly, when air and gasoline are mixed (such as in an engine cylinder) its resistance to ignition increases as it is compressed… it is most easily ignited when the piston is at the bottom of the intake stroke, before compression begins.
The use of a fuel too low in octane will result in detonation… this is when heat and pressure cause the fuel/air to ignite at a secondary spot in the cylinder after the spark plug ignites it. When this secondary flame front meets the primary flame front you will hear this as “pinging” or “spark knock” in larger 4-cycle engines (like in your car); you won’t hear it in a small high-RPM 2-cycle engine (like your chainsaw). Over time, detonation can destroy an engine… piston and ring damage, cylinder scoring, and whatnot. And worse, detonation can lead to pre-ignition (fuel ignites before the spark plug fires)… which is almost always immediately catastrophic in nature. Detonation can cause a “hot-spot” on the piston, eventually hot enough to cause the fuel to pre-ignite… and typically this happens at the bottom of the piston stroke when the air/fuel is easiest to ignite. As the piston is coming up the compression stroke the fuel is rapidly expanding, pressure builds off-the-chart… something has to give, and usually that’s the top of the piston (hole in the piston top). You won’t hear any pinging or knocking from pre-ignition… you just end up with a pile of junk.
The reason your engine seems to run better with 89-octane fuel is all about the timing. All internal combustion gasoline engines make the most power when peak cylinder pressure is reached at 14[sup]o[/sup] after-top-dead-center. If you use a fuel too low in octane the fuel ignites to fast and peak cylinder pressure is reached earlier than 14[sup]o[/sup] after-top-dead-center, too high in octane and peak cylinder pressure is reached later than 14[sup]o[/sup] after-top-dead-center. The closer peak pressure is to 14[sup]o[/sup] after-top-dead-center, the more power it will make, and the better it will run. Back in the days when a good engine mechanic could tune your car engine timing “by ear”… that’s what he was doing, finding that “sweet spot” where cylinder pressure peaked at 14[sup]o[/sup] after-top-dead-center. I find that most small, high-RPM, 2-cycle engines like 91-octane fuel the best… but 89 and 93 put them pretty darn close also. One more thing to consider… two-stroke oil lowers the octane rating of the fuel.
One final warning about running a fuel with too high an octane rating. Higher than needed octane will cause incomplete combustion and excessive carbon deposits… this in turn can be a major cause of detonation and/or pre-ignition and immediate catastrophic engine failure.
Yep! I agree.
I’ve been racing motocross for most of my life and I have used every kind of fuel out there.
Your octane rating should be governed by the compression ratio in the motor you are using.
The higher the compression, the higher octane you will need.
Having said that most production motors weather they are 2 or 4 stroke are designed to run on a octane range of 87 to 93.
Since pump gas comes in 3 deferent ratings, your saw will run on any of them.
But optimal octane will depend in several factors, like timing, compression, air fuel mixture and oil fuel mixture, if used in 2 strokes.
Just because the pump has a 93 rating doesn’t guarantee its 93 octane.
Many stations will sell what ever they can get just to make sales.
If they need 93 and none is available, they will put 87 in the 93 pump and sell it as 93.
Buying race gas is an option, but an expensive one. Also, race gas that is oxygenated has ethanol in it.
I use regular 87 and mix it with Sunoco 114 in a 50/50 mix with a 50/1 Amsoil oil mix.
Sunoco 114 has a rating of 114 octane, but its working octane is actually around 110.
Mixing it with 87 gives me around a 97/100 octane rating. This works great in my saws.
I have ran strait 114 with a 50/1 oil mix with no problems what so ever. Smells good too!
Any pump gas rating should run fine in all your saws.
Also your motor running better on 93 may not have anything to do with octane, but could be the atmosphere. Any motocross racer will tell you that 2 stroke motors will run better when the weather is cool and the air is dry.
I forgot to add that your saw will not have more power with a higher octane fuel. But it will affect the throttle response and rev faster with a higher octane fuel.
