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Hey guys,

I was checking out pure-gas.org and all the stations in the states around me all have 91 octane pure gas, none has 87. I know on bigger engines the octane affects the timing and can cause knocks, etc... how does it affect small engines? Would the advantages of using ethanol free be offset by the disadvantages of the high octane?

Thanks a ton
 

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Higher octane is actually better since it reduces spark knock AKA detonation.
 

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I was told to use low octane gas in small engines. High octane burns to hot and there is no benefit from using it. That said I won't use ethanol either.
 

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So what would the alternative be?
I guess if I Lived out there I would use as low of grade of ethanol as I could find, Being as I'm in Iowa I can get 87 octane with no ethanol at every station. I think the low grade of ethanol is 89.
 

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High octane gas does not burn any "hotter" than regular. Regular gas under detonation burns "hot"- hot enough to damage valves and pistons. Think of high test as a log and regular as kindling.
 

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High octane gas does not burn any "hotter" than regular. Regular gas under detonation burns "hot"- hot enough to damage valves and pistons. Think of high test as a log and regular as kindling.
Guess I've done no major research on this subject. I know I had a L head briggs I toasted the head gasket out of and was using premium gas and a Very well known shop owner mech of Simplicity tractors in Anamosa Iowa told me not to use premium gas . I never lost another head gasket and I quit wasting the extra money for high test for my Lawn mower.
 

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Octane ratings are just indicative of that gasoline's resistance to pre-ignition.

The higher the octane rating, the slower the flame front moves upon ignition, and the higher the flash point.

87 octane, in a high compression hot engine...as it is pulled into the HOT combustion chamber and the piston moves upwards and compresses the gas/air mixture...the 87 octane fuel can become unstable and actually ignite before the spark occurs, or at the instant the spark occurs, the flame front moves so rapidly that the piston is at or still before TDC.

When this occurs, you get the explosion happening at or before TDC, this is called detonation...it basically hammers the piston to death.

A higher octane fuel is more resistant to becoming unstable in the hot combustion chamber under compression and will tend to only ignite when the plug sparks, the flame front is also slightly slower, meaning you get a better expansion in the cylinder.

For instance..White gas (coleman fuel) is like 60 octane. In a small engine, it is **** on parts. The fuel is designed to be very unstable to ignite with great ease, but stable enough to be under low pressure and not explode.

AvGas is over 100octane, it can be used in a race engine with well over 13:1 compression. @ 13:1 compression, the fuel/air mixture is literally under 13X more pressure than regular atmospheric pressure, 87 or 93 would detonate simply from the pressure/heat...similar to a diesel engine.

Diesel fuel is easily combusted under compression...thus why it works so well for its intended purpose, no spark required...put it under enough compression that the air becomes heated..inject diesel and it instantly combusts.

Running a high octane fuel in an engine that doesnt require it serves no purpose at all, it can only do harm. The main thing ive seen is the potential to overheat the exhaust valve due to the slower burn rate, high engine speed, and short strokes of small engines, also power can actually decrease with higher octane fuel when it is not required.
 

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Running a high octane fuel in an engine that doesnt require it serves no purpose at all, it can only do harm. The main thing ive seen is the potential to overheat the exhaust valve due to the slower burn rate, high engine speed, and short strokes of small engines, also power can actually decrease with higher octane fuel when it is not required.

HUH?? Red, Although I agree with pretty much everything you say, this is one area that I think you are mistaken. Only harm that will come from using a higher octane fuel when you don't need it is to your wallet. It does not affect EGT's enough to cause any damage. And as for a loss in power, that is also negligible. I would like to see some data showing where you acquired these facts. I'm not trying to be a you know what but I have done quite a bit of research on this in the past.
 

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I dont have a data logger, but on my race kart, running a calculated 10.1:1 compression ratio, my CHT was consistently 25* higher in a semi-controlled environment when i ran 93octane vs 87 octane fuel, from the same station. I was consistently in the 340-355 range with 87, when I went to 93 my CHT went sharply to the 365-380 range and even with a jet change 1 step richer, it didnt signifigantly drop until I went back to the 87, then it was 330-345.

When I give a range, that means that under reduced load or reduced speed I was at the lower temp, under acceleration it peaked at or near the upper temp.

I also noted that at night, my header glowed about 6" farther down the tube on 93 vs with 87.

If you run a stock engine with no exhaust, or with a stub pipe, on 87 you should note some fire out the pipe under power, if you switch to 93, you will note more fire out the exhaust, having only changed the octane rating, this is due to the fuel still burning as it gets pushed out the open exhaust valve..as we know, any fuel that is not burned on the power stroke is wasted power and lost efficiency.

Ive found a race kart is a perfect platform to test engines with, you directly control load, speed...and you can clearly hear the engine, feel the heat coming off the exhaust and head, and with a Digatron tach/cht/timer in front of you, you can, in real time, follow how the engine is running.

I did a TON of product testing of over the counter power boosters a while back, all of them I tested did nothing, or actually cost a bit of power, but some stuff called Boost107 actually did seem to increase my power, ever so slightly.


Do the open exhaust port test, its the easiest to see the difference in the length of the unburned fuel flame.
 

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And Im not saying that using a higher octane rating than you require "WILL" harm your engine, but it certainly isnt helping it any, and EGT's should be slightly higher under load with a higher octane fuel than you need.
 

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Aviation gas and automotive gas are rated by different octane formulas. 100LL av gas has less than 100 octane when rated by the automotive formula.
 

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Very thoughtful perspectives on this always controversial subject. I think the OP had a very specific question related to ethanol, rather than the traditional octane question. And I'd echo his interest. Here in PA it is very hard to get reliable info on ethanol content. Most pumps are labeled something like "Contains up to 10% Ethanol", and most pumps dispense 2, 3, or 4 grades of gas, typically 87, 89, 91, and even a few 92 octane. Common wisdom here is there is less ethanol in the higher grades (octane), based on how hard/expensive it woud be to blend a 10% ethanol fuel with enough octane boosters to get 91 or 92 octane. And therefore many small engine repair shops, especially 2 cycle shops recommend 91 octane. Not so much for the octane rating, but to ensure lower to no ethanol. Of course, they also recommend a fuel stabilizer and an ethanol remediation additive.
FWIW, for my chainsaws and line/brush trimmers I use 91 octane from a local brand name station and add Startron Enzyme Fuel Treatment an the new synthetic 50:1 2 cycle oil. Might be over kill for the chainsaws that run pretty much every week cutting firewood, but the brush cutters take long winter breaks.

And I am first to admit I have no petroleum engineering degrees, so please consider my post opinion if you prefer.
 

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i buy my OPE fuel 15 gals at a time from the same station, always 87 octane and test each batch for ethanol, i have found 4%-6% and the last batch was 0%.

the pump just says ,"contains ethanol". state law says it canot be more than 10%.
when i find ethanol i use stabils ethanol treatment.
 

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Common wisdom here is there is less ethanol in the higher grades (octane), based on how hard/expensive it woud be to blend a 10% ethanol fuel with enough octane boosters to get 91 or 92 octane.
That's actually kind of backwards. Ethanol itself has a higher octane rating than gasoline. In the lower grades the ethanol is the booster, so to speak. They can start with gasoline with octane ratings below 87 an add the alcohol. But you are correct about the higher grades having less alcohol. At least with one oil company exec I spoke to, I was told their premium 93 octane had no ethanol and the midgrade was a blend. They supplied basestock to multiple gasoline companies who would then mix in their own additive packages as the delivery trucks would pick up at the refinery.



i buy my OPE fuel 15 gals at a time from the same station, always 87 octane and test each batch for ethanol, i have found 4%-6% and the last batch was 0%.

the pump just says ,"contains ethanol". state law says it canot be more than 10%.
when i find ethanol i use stabils ethanol treatment.
I'm surprised to hear that the ethanol was testing that low. From the reading I've done on one of the oil forums, it seems most people that actually test their fuel find the ethanol content is actually much higher than the 10% allowed.
 

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Just to add a bit on my experience about octane ratings and my old cavalier's behavior...

I had a '89 Cavalier with a 2.0L 4 cyl back in high school. These engines are relatively low compression and not an overly hot ignition either... on cold damp mornings with older plugs those cars let you know they were due for plugs, it'd cough and sputter for a couple minutes warming up and if you dared put it in drive and hit the gas it usually would conk out on the spot!! One time I decided to try hi-test in it just to see if it would help out the power/economy/etc. (it had a good set of plugs at the time). The next morning (it was a warm summer morning) when I started it it coughed, sputtered, and died a couple times before it heated up enough to reliably ignite the hi-test; it was behaving as badly or worse than with old plugs on a chilly damp morning!! After heating up for a minute all was good and I couldn't discern any other difference in the hi-test fuel. Needless to say, I NEVER ran higher than 87 in that car again!!
 

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I haven't found anywhere nearby that sells non-ethanol 87. I've been using 93 in everything for years and haven't had any issues. My 1989 Toro still starts right up and runs strong and has great compression. All I've ever done to that engine is change the oil and spark plug, I've never had to clean the carb out or anything. My pressure washer and tractor are both from the mid 80's and both work perfect too, I've had to clean the carbs in those over the years, but compression is still perfect and no smoke. I sure haven't found a downside to running 93 octane, and any extra expense is sure worth it to not have the ethanol.
 

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Very thoughtful perspectives on this always controversial subject. I think the OP had a very specific question related to ethanol, rather than the traditional octane question. And I'd echo his interest. Here in PA it is very hard to get reliable info on ethanol content. Most pumps are labeled something like "Contains up to 10% Ethanol", and most pumps dispense 2, 3, or 4 grades of gas, typically 87, 89, 91, and even a few 92 octane. Common wisdom here is there is less ethanol in the higher grades (octane), based on how hard/expensive it woud be to blend a 10% ethanol fuel with enough octane boosters to get 91 or 92 octane. And therefore many small engine repair shops, especially 2 cycle shops recommend 91 octane. Not so much for the octane rating, but to ensure lower to no ethanol. Of course, they also recommend a fuel stabilizer and an ethanol remediation additive.
FWIW, for my chainsaws and line/brush trimmers I use 91 octane from a local brand name station and add Startron Enzyme Fuel Treatment an the new synthetic 50:1 2 cycle oil. Might be over kill for the chainsaws that run pretty much every week cutting firewood, but the brush cutters take long winter breaks.

And I am first to admit I have no petroleum engineering degrees, so please consider my post opinion if you prefer.
A few years ago, it was only thr Low test gas tht has ethanol in most stations, but today, if one has it, almost 100% likelyhood all grades have it.

National limit is 10%...but Ive personally tested over 30% from all the major grades of gas.
 

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This may or may not relate to our smaller engines but it is well proven that using HT in lower compression bigger engines costs HP, I did not agree and had to try it. I have a 383 in my wet ride it on a good day will run 65.5 on 87, my CR is 9.125. I decided my own testing was in order, I have two tanks I emptied one and put 10 gallons of 93. My other tank was straight 87, I ran it up and did my 65.5 on the 87. I switched tanks and ran it a few minutes to clear the lines of the regular, I ran it up and only got 63.2. Switched back again ran it a few minute to clear the HT ran it and got right back up to my 65.5. Needless to say I will not run HT unless nothing else is available at the dock.

Phil
 
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