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High Octane on small engines?

40K views 36 replies 27 participants last post by  thecause17 
#1 ·
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
 
#3 ·
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.
 
#7 ·
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.
 
#9 ·
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.
 
#10 ·
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.
 
#11 ·
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.
 
#12 ·
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.
 
#14 ·
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.
 
#16 ·
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.
 
#15 ·
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.
 
#17 ·
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!!
 
#27 ·
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!!
I noticed from all of these posts that small things can make a big difference. Timing off, what kind/brand of fuel, amount of ethanol, jetting, water or dirt in the fuel, etc. It's hard to say what is actually making your engine run poorly.
In Red's case, he eliminated a lot of variables when he used his cart to do the testing. The only other thing that might be interesting, would be to make some tuning adjustments with the 91octane to see if the heating problem could be removed.
 
#18 ·
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.
 
#20 ·
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
 
#21 ·
High octane produces less power. That is what it is designed to do. It is because of the heat generated from the higher compression engines, that you get more power. Confused? Ok, the lower the octane, the more volatile the fuel is, the more powerful the explosion. The more any fuel is heated, the faster it will burn. More heat causes higher volalite reaction. This is why you need extra fuel on a cold engine, when you choke an engine, you are putting in exta fuel until the engine warms up, because the fuel is not volatile enough. Higher compression engines produce more heat, which needs less volatile fuel. (Higher octane) But, when you put higher octane in a lower compression engine, it will produce less power than if you ran 87 octane because of the volatility. But, if you run higher octane in a lower compression engine long enough, it will eventually have more power. And the reason is this, it will start to carbon up bad enough to actually raise the compression. Then if low octane fuel is used, then it will ping because of the "higher" compression caused by the carbon. I believe that any damage caused by higher octane fuel when "not" needed is either from carbon coming loose, or someone putting in the lower octane fuel in a "carbon compression raised" engine and pinged itself to death.
 
#22 ·
club
the last two post are basically what i want to say
i ran a Pro Stock garden tractor puller for ten years.
a big block onan 60 c.i. T 260 / P 224
start at 7.0 comp. ratio. advance at 21 degrees.
after many mods 9.25 comp ratio.
i ran a mix of 103 and 87 octane. advance at 27 to 29 degrees.
this was by slotting the ign. module
now the rest of the story.
for eight of those years i was the scale/tech. inspector.
a local ( now well known) puller always used ava fuel in the stock off lawn class.
our rules stated pump gas only.
you can SMELL the ava. fuel.
GUESS, what? he passed the tech. every time.
he would have needed 13 to 14 comp ratio and about 36 to 42 degrees
of advance to do any good.
SO, run 87 octane, that is all you need.
thank you. boomer (the used onan engine parts guy)
p/s this is a LONG story for me
 
#24 ·
The verdict is simple. A motor that is designed to run on 87 will see no benefit from running on 91. If you let your gas sit for long periods you run the risk of a slight drop in octane but at that point you are running stale fuel and if there was ethanol in the fuel it is likely that there is some phase separation (water) present.

I run the 91 ethanol free in all of my seasonal equipment but only at the end of the season for storage. Other than that there is no downside to using the 87 ethanol on equipment that is regularly used. The exception to that would be old equipment that does not have ethanol compatible components in the fuel system. In that case it's time to replace the lines anyways.

Basically, use what the motor was designed for. Race fuel will mot make your car into a race car but race cars are designed for race fuel and require it. Your lawn mower operates the same way.
 
#25 ·
Better late than never,I'll jump in.I'm an old Aircraft Mechanic and am quite familiar with AV-Gas.The LL100(low lead) is just the latest version of aircraft gasoline,the LL100 replaces the former 100/130 fuel,as well as 80/87,it meets the requirements of 100/130,with about half the lead content.There was also a 115/145 grade,and that still be bought in tank truck lots,and can be found at the Reno air races,and at some airports,but not many.The 100 number is the "lean" rating,the 130 number is the "rich" rating,actually the 100 is the MON,or "Motor Octane Rating". The 100LL and 100/130 and 115/145 fuels are made for supercharged engines,I know that 100LL will work fine at 48 in.hg(1.6 atmospheres,24PSI)for 5 minutes ,much above that,115/145 is required,and some of the military or racing engines,also use water injection too.The "slow burning" can be overcome with more spark advance,and richer mixture,and it will bring a return to "lead fouling" ,which some of you younger mechanics may have never seen.As to Ethanol,remember that when using a carburetor,10% Ethanol will lean the mixture 5%,and so on for any mixture of gas/Ethanol,100% Ethanol requires around twice the jet AREA.Pre-ignition and detonation,are both abnormal burning conditions,pre-ignition occurs before the spark plug fires,detonation occurs after the spark plug fires,pre-ignition can cause detonation,but detonation can't cause pre-ignition.
 
#26 ·
I agree with what you said Red, and some of the rest of you guys. I recall back in the late 80s before this ethanol laced fuel, using my dad's lawn equipment, he wanted to use premium (91/92) in everything. I kept noticing on a 3.5hp briggs engine on our edger that the plug would have black carbon deposits on it. The carb was perfectly in spec, air filter was clean, etc. It ran just fine, but the plug never had that tan light-brownish color, was always black and carbon covered. I tried using 87 gas in it for a few months and the plug color finally got to where it should be, no longer carbon fouled. This thread reminded me of that. I speculated that it was due to the higher octane fuel either not combusting completely, or just not quick enough since this was a low compression engine. I don't know if it would have harmed the engine, but it was obviously wasteful and didn't have any advantage over using 87. To the OP I would almost say it would be worth it to use the least octane rating you can get away with that doesn't contain ethanol. The damage caused by ethanol fuels seems to me to outweigh the damage that could be caused due to using slightly higher than oem recommended octane ratings.
 
#28 ·
After reading the entire thread,I have come to only one answer---Ya'll caused me a **** of headache.:fing20: Well maybe two--I agree with the statement-Use what the manufacture recomends for fuel.After all,they designed the eng.
 
#29 ·
I think all carberated engine dont run as good on ethanol laced gas as theydid on the good old leaded gas we had back before they started selling us oxogenated gas and alcohol laced swill..

..you can re-jet the carbs and fool with the timing on an old engine,it'll help some,but they were meant to have the fuel we ran back then,todays gas just dont cut it in them really...todays cars with EFI and computer controlled injection and timing that can compensate for variations in fuel compositions are a must now,seeing the fuel from one pump to another is often different,as far as how much alcohol actually got added to it...

I read in an old Briggs and Stratton manual that "leaded gasoline of at least 65 octane" is the minimum requrement for satisfactory engine operation...I think one of my old flatheads I had on a go-kart ran better on coleman stove fuel than it did on todays ethanol gas...
 
#30 ·
I am also in a slightly confused state. Usually use non-ethynal gas as it is still available in this part of Kansas, but I have been using Startron lately just in case. 87 octane most generally, but sometimes I mix in a high grade. Depends if I get gas in the Truck/Murano at the same time as Mower gas. The delema come from my 317 I just got and the recommendations for it. JD book says 87 and not higher grades. The Kohler engine manual says 87 or higher? JD book says not to use additives like Startron/and or Seafoam. Kohler doesn't say you can't. Anyways I will still use Startron and Seafoam in the JD as I am a firm believer in their use! I have more worries over what Viscosity of Oil to use. I know it says 10w-30 for below 32 degrees but we get those temps, but a few days later it can get and stay above 32 degrees? Sorry, off-topic, I will post that question in another thread. :banghead3
 
#31 ·
The fella that I use for two stroke help has been working on them for 45 years. When my old Homelite chainsaw was getting hard to start, he told me to check the holes in the spark arrestor/muffler. Sure as he said, they were mostly blocked with heavy carbon deposits. He told me to clean the holes out with a wire brush, reassemble, and use only 91 or better premium octane in my oil/gas mix. He said that the little 2-strokes love higher octane fuel and leave less carbon deposits in the motor. I've been following his advise for three years now and the ol' Homelite has been a two pull starter ever since.
 
#32 ·
Everyone can test their fuel for ethonal content eaisly. all you need is a 25 mL graduated cylinder and food coloring.

Put 4 mL of water in the cylinder, then add 20 mL of fuel for a total of 24mL in the cylinder. add a drop of food coloring and cover and shake to mix it all toghther, let is sit for 3 minutes. record where the colored fluid (water / alcohol mix) meets the pure gasoline. Subtract 4 and multiply by 5.

Example: 7mL of colored water (7-4)5= 15% alcohol by volume

Whereas alochol needs more volume to get the same result as pure gasoline, alcohol has a higher anti-knock rating. the benifit of using a higher octane is you can run more compression and more ignition timing thus making more power. if you do not run a higher comp. or more timing advance then it does no good.

Alcohol mixes with water and is hydroscopic meanig it asorbes water out of the air over time so that is why you need to use a fuel stablizer if you store your equipment with fuel in the system to prevent rust and corrosion. the best bet is to drain your fuel systems and run you engines untill they will run no more to insure all fuel supply is exhausted.

The higher the antiknock value the more the fuel resists burning prematurely. combustion chamber temperature determines what antiknock fuel you need to use. that is why most chainsaws recomend 90 octane or higher for limbing or other high rpm manuvers. with 2 strokes having an ignition event every crankshaft revolution as opposed to only 1 every 2 revs for a 4-stroke they run a higher combustion chamber temperature that 4-strokes. so if you run a 2-stroke at 9000rpms then it is firing the combustion chamber 9000 times a minute verses a 4-stroke runnig at 3600rpms it is only firing 1800 times a minute. More firing, less time to cool down inbetween firing events so you run a higher CC temperature and require a fuel that wont just flash burn the millisecond it enters the CC and you melt your engine.

Bassically Unless you modified the timing advance or compression ratio there is no need for a highter octane fuel unless the engine manufacture specifies. If you just run it to "fix" an ethonol problem then use a fuel additive for best results.
 
#33 ·
I know this thread is old, but I came across it in a search and thought maybe it'd be helpful to share a video I made of using different octane gas in a small engine.



As far as ethanol content, as far as I know, both grades I used contained ethanol, so I don't believe that accounted for the performance variance. In some areas, Shell V-Power is listed as ethanol free, but not here. In fact, I'm located in Delaware and according to the link below there's not a single station in the state that's ethanol free.

https://www.pure-gas.org/index.jsp
 
#34 ·
I know this thread is old, but I came across it in a search and thought maybe it'd be helpful to share a video I made of using different octane gas in a small engine.

https://youtu.be/uYX_W0v2y8w

As far as ethanol content, as far as I know, both grades I used contained ethanol, so I don't believe that accounted for the performance variance. In some areas, Shell V-Power is listed as ethanol free, but not here. In fact, I'm located in Delaware and according to the link below there's not a single station in the state that's ethanol free.

https://www.pure-gas.org/index.jsp
In canada the shell V-power has it posted right on the pump 'contains no ethanol' . I've tested it as well and there is none.
 
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