I have heard that high octane gas is better for lawn mowers. However, the manual only says 86 octane or higher. Anyone know the truth here?
Where did you hear that?
Tester
The engine doesn’t know it’s in a lawn mower. I buy regular unleaded gasoline for my gas tools, and pour the remainder into one of my cars at the end of the season.
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Mowers are low compression (6 to 1, etc) so they should run happily on anything.
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I was short on gas for the mower and used some 104 octane race gas so I could get my small lawn cut. Should be OK, right? No, no, no…
The high octane fuel overheated the exhaust valve which closed up the valve clearance to the cam and left the valve open. The engine pretty much quit running at that point because the exhaust valve would not close and the engine would not build compression. Ran fine after it cooled down and I put regular fuel in it.
Extreme example, for sure, but it shows what can happen when you pay attention to internet myths instead of the folks who designed your engine.
The manual for my snowblower specifically warns against using high-octane gas.
Now, I understand why!
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One of the expressions that I hear nowadays is…
“Many people say…”
Translation:
“I read something that is not fact-based, on a wacko website, and now I believe it and want to spread that misinformation.”
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always good to have a wacko vaccine.
When all else fails, read the instruction manual for your mower and see what fuel the manufacturer specifies.
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@Mustangman. Thanks for your information. Mrs. Triedaq thinks I should mow the yard this afternoon. I may pour some high octane in the mower to get out of doing the job.
On a more serious note, I thought that the higher octane fuel had a higher resistance to burning. This is to prevent pre-ignition and the resulting spark knock in high compression engines. Does higher octane fuel burn hotter once it does ignite?
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It’s not the octane but it’s whether or not it is non-oxy. Non-oxy is available at some stations for boats, small engines, old cars, etc. and is the premium grade at least at my station. So that’s what I use for all my small engines but I also use the Briggs fuel conditioner without fail. Since then I have had no carb problems. Prior to that I had one carb kit/fuel pump that only lasted a week and a generator carb all gummed up in one year. Using the conditioner and the non-oxy gas has made it all go away. I did a carb overhaul this summer for a kit that had been in the mower for about 7 years and it was clean as could be. So that’s what I do and will continue. If you got a foot of snow in the drive that’s no time to have carb problems with a snow blower.
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All modern small engines are designed to run on oxy gas.
But in older equipment, non-oxy must be used.
Tester
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A few years ago a guy at my wife’s job told her premium is better because you don’t have to drain the fuel when you store the mower. I use regular and drain it anyway. I just wish I didn’t have to, I never did with our old mower.
An over simplification the characteristics of high octane fuel is that is burns a little slower and a bit hotter. The flame front progresses more slowly so it doesn’t detonate. That’s not everything that’s going on but it is close enough. In a lawnmower engine the combustion hasn’t quite finished when the exhaust valve opens so it is spitting a bit of fire and overheating the valves.
I worked with a buy whose dad operated a machine shop. He rebuilt the local airports’ lawn tractors. He always added a little extra tappet clearance because they’d run the tractors on old aviation gas they’d drained from wing tanks. Old 110/130 av gas or about 115 pump octane as cars rate it. Higher than I was running!
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Since premium road fuel has about 9% ethanol in it like regular, it goes bad at about the same rate as everything else. I always let the mower run until it died from lack of fuel.
If you want less storage issues, use Sta-Bil or better yet, non-ethanol fuel with Sta-Bil. That’s what I have in my emergency generator. I drain it once a year and add fresh. The stuff that gets drained is added to my truck.
@Mustangman. Thanks for the explanation. Some years back, I had a colleague who owned a VW bus. He bought his gasoline at a Clark station. Clark only sold one grade of gasoline.which was premium. Clark advertised that they sold high octane gas at the same price as regular at other stations and that seemed to be true. However, my colleague had to have an early valve job on his VW bus. The mechanic that did the work said it was because of the high octane fuel. I had always wondered if that was really the cause, but now it makes sense. The VW bus had a low compression air cooled engine.
I haven’t seen a Clark gasoline station in years. I wonder if the brand still exists.
Oh come on , tell where you heard that . Curious minds want to know 
Well, I didn’t say it was true, just that the OP isn’t the only one who has heard something similar.
No problem here using 91 octane non oxygenated in any of my lawnmowers, boat engines chainsaw, snowblowers or generator My boat guy said use it in everything but your car, as the car does not need it.
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All non-oxy gas is high octane.
Tester
I live in an county under mandates and have to drive 30 miles to a county that allows premium for off road use without 10% ethanol, so you might purchase 91 octane with ethanol.