91 Octane and small engines

[Ben]

Are there any detriments to using 91 octane gas in small engines (lawn mowers, power washers, generators, etc)? Now that I live where I can get ethanol free gas, I'd like to start using it in my power equipment. While I can get ethanol free 87 octane in the city, the local coop sells ethanol free 91 octane.

Ten minute drive versus thirty minutes, so with travel, price difference is a wash. Plus I always seem to realize I need power equipment gas when the cans are empty, so quick trip into town is more convenient. I'm just wondering if the higher octane would cause any long term issues in the engines?

[Nick1911]

I don't see how it would.

[charby]

What I have used for years.

[MillCreek]

I envy that you have easy access to ethanol-free gasoline. The pure gas website is hit or miss as to if the listed local stations have pure gas and I have driven to them more than once to find they did not.

[TechMan]

Ben, I am with charby, I have used it for years (10+) in all my small engines.  No issues.  My 91 octane gas is ethanol-free as well, comes from a fuel store about 5 mins from my house.  They charge an arm and a leg for it but to have it ethanol free, I'll pay.

[Ben]

I envy that you have easy access to ethanol-free gasoline. The pure gas website is hit or miss as to if the listed local stations have pure gas and I have driven to them more than once to find they did not.

Puregas is the site I used. It appears you need to go by the comments when available as more recent comments indicate a likelihood of the station still having ethanol free gas. I guess I got lucky on the local coop, because none of the other small towns near me have it (at least according to puregas).

Sounds like the 91 is A-OK. My only potential worry was (I assume) higher energy output that might wear components. Part of my googling on it had me on a Corvette forum where they debated some of that in detail.

[Brad Johnson]

Should be fine, though there is the possibility it might run a little rough at times. The reason is that small equipment engines are usually not very highly engineered in terms of fuel homogenization, intake efficiency, combustion, flame propagation, etc., and rely on the more volatile nature of lower-octane fuel to run properly. It seems kinda counter-intuitive that an engine will run a little rough on better fuel but it is a legit thing.

That being said... it won't hurt a thing, though you may want to occasionally treat for carbon buildup.

Brad

[TechMan]

Puregas is the site I used. It appears you need to go by the comments when available as more recent comments indicate a likelihood of the station still having ethanol free gas. I guess I got lucky on the local coop, because none of the other small towns near me have it (at least according to puregas).

Sounds like the 91 is A-OK. My only potential worry was (I assume) higher energy output that might wear components. Part of my googling on it had me on a Corvette forum where they debated some of that in detail.

Ben, knock on wood, the only thing I have had to do with my small engines is change the oil.  I have had my Honda mower and string trimmer for 13 years and I have run either 93 with 10% ethanol or 91 pure gas and nary an issue.

[Hawkmoon]

Sounds like the 91 is A-OK. My only potential worry was (I assume) higher energy output that might wear components.

What higher energy output? It's gasoline. The energy output is the same. All octane does is resist pre-detonation (ping).

[BobR]

For my small engines and motorcycle I just use pump gas (corn fed) and treat the gas with Star Tron Enzyme treatment. It seems to work, my small engines start right up after being stored. I also drain and run the fuel out of the engines when storing over summer or winter. A splash of enzyme treatment, a splash of Sea Foam when running out of gas to store and a splash when reviving them seems to work just fine for me.

bob

[bedlamite]

What higher energy output? It's gasoline. The energy output is the same. All octane does is resist pre-detonation (ping).

ethanol is 76000 btu per gallon
gasoline is 119000 btu per gallon

Running ethanol mix requires a richer mixture than gasoline, which may or may not be possible  depending on the engine, so they end up running lean and hot, this is why a lot of people can't get small engines to last. Ethanol is also corrosive, add Sta-Bil Marine if you run ethanol fuel.

There is no downside to running premium if it's ethanol free.

[HeroHog]

Higher octane fuel is more resistant to detonation. It does NOT "add power" itself, it DOES allow engines with higher compression/more aggressive timing to run without destroying themselves due to detonation. In a small engine, it just costs more per gallon.

[Brad Johnson]

Sounds like the 91 is A-OK. My only potential worry was (I assume) higher energy output that might wear components. Part of my googling on it had me on a Corvette forum where they debated some of that in detail.

HH beat me to it. Octane is a measure of detonation resistance (actually resistance to auto-ignition), not of energy content. Doesn't matter if is 86 octane or 93. Same BTU per gallon.

The real downside to using oxygenated fuels (read: containing ethanol) is the lack of ethanol-resistant materials in the fuel system and the relatively short storage life of ethanol-containing fuels. For some reason, small engine manufacturers steadfastly refuse to use ethanol-resistant seals and hoses, resulting in greatly reduced service lives for float bowl seals, needle seats, and fuel lines. The alcohol also tends to attract moisture, leading to corrosion. Finally, ethanol-containing fuels tend to degrade quickly over time, clogging carburetors and such. Green Death is a real problem for equipment stored over long periods. It's the bane of folks with short lawn seasons.

Brad

[Hawkmoon]

ethanol is 76000 btu per gallon
gasoline is 119000 btu per gallon

Running ethanol mix requires a richer mixture than gasoline, which may or may not be possible  depending on the engine, so they end up running lean and hot, this is why a lot of people can't get small engines to last. Ethanol is also corrosive, add Sta-Bil Marine if you run ethanol fuel.

There is no downside to running premium if it's ethanol free.

But Ben asked about the difference between 87 octane ethanol-free gas and 91 octane ethanol-free gas.

It's the same gasoline, just more or less detonation-resisting additive.

[HeroHog]

PS: A quick data point: It takes MORE alcohol to produce the same energy as a smaller volume of gasoline. Where alcohol shines is in it's resistance to detonation and that it runs cooler allowing, again, more compression and timing.

[230RN]

Should be fine, though there is the possibility it might run a little rough at times. The reason is that small equipment engines are usually not very highly engineered in terms of fuel homogenization, intake efficiency, combustion, flame propagation, etc., and rely on the more volatile nature of lower-octane fuel to run properly. It seems kinda counter-intuitive that an engine will run a little rough on better fuel but it is a legit thing.

That being said... it won't hurt a thing, though you may want to occasionally treat for carbon buildup.

Brad

That sorta answers my first question.... not that I've mown my own grass for decades:  Why would you want to run 91 in small engines anyhow?  What are they, 7:1 compression ratio or thereabouts?

Sorta like Model T level engineering.... no?

Educate me:  It seems like  the main issue is whether the fuel is alcohol free or not.  Is 91 more likely to be alcohol free?  My life is simple since the car manual (1997 Subaru) says 87 octane, so that's the extent of my decisioneering.

Terry

[HeroHog]

The old flat heads were but most modern engines are overhead valve.

[230RN]

I just re-read Brad Johnson't Reply #12 which answers most of my questions.  Thanks.  Don't know how I missed it first time through.

[Hawkmoon]

Educate me:  It seems like  the main issue is whether the fuel is alcohol free or not.  Is 91 more likely to be alcohol free?  My life is simple since the car manual (1997 Subaru) says 87 octane, so that's the extent of my decisioneering.

The issue is that Ben can find alcohol-free 91 octane locally, but to get alcohol-free 87 octane he has to travel.

Entiende?

[brimic]

In my area, 91 octane is just the pump drawing from both the 87 and 94 octane tanks and mixing them roughly 50/50 as it dispenses.
The only gasoline that doesn't have ethanol is the 'ethanol free' 94 sold at the rare gas station- they sell 94 octane at every station, but the closest ethanol free 94 is about 50 miles away from me.
The only time I've ever bought 91 is when I bought from a craptastic gas station nearest my house. I normally get about 30 mpg with 87, but only get about 27 from the 87 sold at this particular gas station.

[Brad Johnson]

What are they, 7:1 compression ratio or thereabouts?

Sorta like Model T level engineering.... no?

If it's not an OHV engine, yes. Even moreso than you may realize. Non-OHV small engines are a low-compression flathead with oil-slinger lubrication and magneto-driven ignition. Short of main bearing/bushing inserts rather than babbitt, they're operationally identical to a Model T engine (albeit minus 3 cylinders).

Brad

[Brad Johnson]

In my area, 91 octane is just the pump drawing from both the 87 and 94 octane tanks and mixing them roughly 50/50 as it dispenses.

That's the system used most everywhere.

Brad

[Ben]

The issue is that Ben can find alcohol-free 91 octane locally, but to get alcohol-free 87 octane he has to travel.

Affirmative. :)

It costs 2.5 gallons of gas to get 10 gallons of ethanol free 87 octane, so the extra cost of the premium locally is cancelled out by fuel savings. Plus it's easier to fill all the diesel and gas cans up at once at the coop, where I also get tax free diesel.

[charby]

That's the system used most everywhere.

Brad

Iowa for the most part has switched to 85 octane fuel which 10% ethanol is added for 87 and 15% added for 88. We do have 87 octane no ethanol (most stations) and 91 no ethanol (near recreational areas, boat or ORV)

Used to be 87 octane and 10% ethanol was added to make 89 octane. 5 or 6 years ago they made the switch to 85 octane because it was said it was cheaper to make, and cheaper prices at the pump. Sometimes there is a 30 cent difference between no e 87 and 87 w/e. no e being much more expensive. 91 no e is 50-70cents higher. 87 no e is achieved by mixing 85 with 91 at the fuel terminals

[230RN]

Thanks for the info.  A lot of catching-up for me.