Re-tuning for different octane and fuel
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Tomos770
ArboristSite Guru
Which octane rating saw needs is determined by its compression ratio....
If a saw was built for a 95RON fuel....you wont achieve anything with a 100RON fuel....
And with less then 95RON.....you will get only less power and/or knocking
If a saw was built for a 95RON fuel....you wont achieve anything with a 100RON fuel....
And with less then 95RON.....you will get only less power and/or knocking
I agree that you should use enough octane and more octane than needed is a waste. BTW Stihl recommends 89 octane in the US.
Lower octane fuel generally has slightly more BTUs/gallon than higher octane, and thus slightly more power, everything else being equal. High octane lets you run a higher compression ratio or leaner fuel/air ratio without risk of detonation.
However changing the compression ratio requires engine disassembly and possibly machine work, and most people don't tune their saws to the edge of detonation. The normal working chainsaw tune is quite rich, just lean enough to stop four stroking under load. A motorcycle racer would be appalled at how rich saws run. But a rich tune is safer because it takes a large change to make the tune dangerously lean. Given all that, you're unlikely to need to tune the carb for a change in octane.
Ethanol burns at a stochiometric ratio of 9:1 vs 14.7:1 for gasoline. Which means that you need more ethanol per unit of air. The carb meters the same amount of liquid with E0 and E10. Therefore carb tuning that is perfect for E0 will be slightly lean on E10.
However it's a fairly small change. The affect on fuel mixture from seasonal changes in air density due to temperature is likely greater. If you're not already tuning your carb for the seasons you're probably not going to need to retune when going from E0 to E10 or vice versa.
But if you are concerned or want to do it right, get a tach and use the procedure in the shop manual to tune the carb. Even without fuel changes it's instructive and useful if you have not yet developed a feel for what the correct tuning is like.
Lower octane fuel generally has slightly more BTUs/gallon than higher octane, and thus slightly more power, everything else being equal. High octane lets you run a higher compression ratio or leaner fuel/air ratio without risk of detonation.
However changing the compression ratio requires engine disassembly and possibly machine work, and most people don't tune their saws to the edge of detonation. The normal working chainsaw tune is quite rich, just lean enough to stop four stroking under load. A motorcycle racer would be appalled at how rich saws run. But a rich tune is safer because it takes a large change to make the tune dangerously lean. Given all that, you're unlikely to need to tune the carb for a change in octane.
Ethanol burns at a stochiometric ratio of 9:1 vs 14.7:1 for gasoline. Which means that you need more ethanol per unit of air. The carb meters the same amount of liquid with E0 and E10. Therefore carb tuning that is perfect for E0 will be slightly lean on E10.
However it's a fairly small change. The affect on fuel mixture from seasonal changes in air density due to temperature is likely greater. If you're not already tuning your carb for the seasons you're probably not going to need to retune when going from E0 to E10 or vice versa.
But if you are concerned or want to do it right, get a tach and use the procedure in the shop manual to tune the carb. Even without fuel changes it's instructive and useful if you have not yet developed a feel for what the correct tuning is like.
Actually it’s the other way around. When switching from E10 to non ethanol fuel you’d need to lean the mixture out slightly.
I think the alcohol 93 explodes faster, better/hotter when fresh out of the pump?
Ethanol has oxygen in it so requires a richer jet setting to make up for more oxygen than what is available in the air. Going from rec fuel (0 ethanol) to ethanol laced fuel will make the saw run lean(er)
Somebody has been reading data sheets
87 and premium have roughly the same BTU content assuming like ethanol content. They may tune different or the same, just depends on the batch of fuel.
Outside air temperature has a much greater influence on tuning than things like fuel or oil ratio. You should be tuning your carb regularly.
Outside air temperature has a much greater influence on tuning than things like fuel or oil ratio. You should be tuning your carb regularly.
No. Someone filled their gas cans with E85 and saws started scoring because they wouldn’t tune themSomebody has been reading data sheets
Oh my
Altitude is certainly a factor. I’m at about 7000 ft. Here in New Mexico I use 86 octane in my bikes, saws, and cars with no pinging in any of them. When I’m up in Colorado I use 85.
The lowest I could get when I lived in Oklahoma was 87. Never an issue with that either.
Unless you have a very high compression ratio and/or advanced timing you most likely likely won’t ever need the higher octane fuel. All octane does is tone down the volatility of the fuel to make it more resistant to pre-ignition. It in no way gives you more power. If your engine runs without pinging on the lowest octane fuel available then you’re good to go. If it pings try the next octane level but it’s much more likely that you have a problem such as an air leak or incorrect timing.
The lowest I could get when I lived in Oklahoma was 87. Never an issue with that either.
Unless you have a very high compression ratio and/or advanced timing you most likely likely won’t ever need the higher octane fuel. All octane does is tone down the volatility of the fuel to make it more resistant to pre-ignition. It in no way gives you more power. If your engine runs without pinging on the lowest octane fuel available then you’re good to go. If it pings try the next octane level but it’s much more likely that you have a problem such as an air leak or incorrect timing.
I tune daily throughout. regardless of fuel. Elevation air quality muffler filter all play a roll. Tune is constantly monitored and changed if when needed.
You could hand me a saw ill tune it not giving a care in the world whats in the tank
You could hand me a saw ill tune it not giving a care in the world whats in the tank
Smitty Smithsonite
ArboristSite Operative
I have to run premium in my '07 flex-fuel Silverado, especially when I tow. The thing will knock itself to pieces on 87. According to the owner's manual, 87 is all she needs! So much for computers and knock sensors. 
One time I had to do 50 mph on the highway until I got to a gas station to mix some premium in, when I let my wife drive it, and she filled it with the cheap stuff. It was BAD. If I had an iPhone back then, it would've picked the sound up for sure. I couldn't give it more than 1/4 throttle, or it sounded like rocks in a cement mixer.
Only 125k on it now, but it'll be getting rebuilt this summer. I'll be bolting on a supercharger, and I don't trust this engine and the crap AFM system as it is. All that will be gone. Forged pistons will make me cringe less on crappy fuel. Plus the 20 or so potential oil leaks inside the 5.3 will be plugged up.
One time I had to do 50 mph on the highway until I got to a gas station to mix some premium in, when I let my wife drive it, and she filled it with the cheap stuff. It was BAD. If I had an iPhone back then, it would've picked the sound up for sure. I couldn't give it more than 1/4 throttle, or it sounded like rocks in a cement mixer.
Only 125k on it now, but it'll be getting rebuilt this summer. I'll be bolting on a supercharger, and I don't trust this engine and the crap AFM system as it is. All that will be gone. Forged pistons will make me cringe less on crappy fuel. Plus the 20 or so potential oil leaks inside the 5.3 will be plugged up.
I've never heard a modern fuel injected engine ping and I've never put anything but 87 in my trucks.I have to run premium in my '07 flex-fuel Silverado, especially when I tow. The thing will knock itself to pieces on 87. According to the owner's manual, 87 is all she needs! So much for computers and knock sensors.
One time I had to do 50 mph on the highway until I got to a gas station to mix some premium in, when I let my wife drive it, and she filled it with the cheap stuff. It was BAD. If I had an iPhone back then, it would've picked the sound up for sure. I couldn't give it more than 1/4 throttle, or it sounded like rocks in a cement mixer.
Only 125k on it now, but it'll be getting rebuilt this summer. I'll be bolting on a supercharger, and I don't trust this engine and the crap AFM system as it is. All that will be gone. Forged pistons will make me cringe less on crappy fuel. Plus the 20 or so potential oil leaks inside the 5.3 will be plugged up.
I woukd check your timing and make sure you don't have something going on wonky there.
I bought a new 2001 Isuzu Rodeo for my wife. I drove it home and loaded it up for a trip from Oklahoma to California. By the time I hit the interstate I could tell that it was pinging. I nursed it along until I had burned enough fuel to put some higher octane fuel in it. It didn’t help. ObviouslyI have to run premium in my '07 flex-fuel Silverado, especially when I tow. The thing will knock itself to pieces on 87. According to the owner's manual, 87 is all she needs! So much for computers and knock sensors.
One time I had to do 50 mph on the highway until I got to a gas station to mix some premium in, when I let my wife drive it, and she filled it with the cheap stuff. It was BAD. If I had an iPhone back then, it would've picked the sound up for sure. I couldn't give it more than 1/4 throttle, or it sounded like rocks in a cement mixer.
Only 125k on it now, but it'll be getting rebuilt this summer. I'll be bolting on a supercharger, and I don't trust this engine and the crap AFM system as it is. All that will be gone. Forged pistons will make me cringe less on crappy fuel. Plus the 20 or so potential oil leaks inside the 5.3 will be plugged up.
this Rodeo should run just fine on low octane fuel that’s been stored in an open pickle barrel for a year.
I nursed that damn thing all the way to California and back and then immediately took it to the dealer. They hooked it up to a computer and said that it needed an ignition map upgrade. WTF! You would think that a brand new car would be up to date. They updated the mapping and it never pinged again.
I just thought I’d throw that out there. If you haven’t taken it to the dealer for an update it might be worth a shot.
I keep a note book for my saws that notes tune settings ( Hi Lo Idle) with different mixes. That is fuels, oil, and ratios.
If I need to change fuel I can can reset carbs fast and be very close to optimum tune.
If I need to change fuel I can can reset carbs fast and be very close to optimum tune.
All that is fine, but a swing in temperature is has a much larger effect than what your tracking.
