Octane Observations

Minimichael

Well-Known Member
#1
I'd heard these old lawn mower engines weren't really designed for ethanol gas, and that higher octane gas was better for them because the fuel can burn too hot otherwise and cause pinging and extra wear and all that. And lately I've been spiking my minibike and lawnmower gas with ethanol conditioner, as well as the Lucas octane booster. I also believe the results have been positive, and notable. For one, the pinging in the old Briggs 8B flathead is gone, and until it comes back, I'm going to keep spiking.

Anyway... I wanted to post about something else that just happened to me today... once I'd warmed up my old Bonanza's Briggs 6BS to a nice and steady idle... I went to top off the tank -- but first, decided to add a quick shot of octane boost (just a healthy smidge, but yes directly into the tank) with the bike still calmly idling. Never done that before, but long story short -- not 5 to 10 seconds later, the engine changed it's tune and clearly slowed it's cycling. It was like the shot of booster had made a bee line and quickly gotten sucked into the carb's take-up tube? Yes I'll grant you, the idle dropped, but it wasn't a naturally shifting idle like a carb often does. It was like I could hear the engine encountering an obstacle, or a load, but without throttle.

I know this may be elementary to some, but it was remarkable to see it in action. It was like the combustion stroke was suddenly burning through the fuel more slowly. And that's exactly what I've been hearing octane is all about.

So, I just thought I'd write about it. Here, I grew up thinking octane was a performance enhancer, only to find out now that it's actually a depressant! Lol
 

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pomfish

Well-Known Member
#3
What also needs to be taken into consideration is when the old flatheads were made Regular leaded fuel was around 90-91 octane so even though these engines we use are considered low compression when they build up carbon deposits on the head the compression goes up and pre mature flash is even more prevalent.
Good fuel has smooth controlled burn.

Best fuel for these for sure is Pure Gas, no ethanol. The sound of the engine changes to a nice pleasant hum rather then an angry chatter and for me at least using my trusty IH Cub Cadet 12hp Kohler the fuel consumption goes down and the engine starts easier and no more backfire when shutting down hot.
Have had best luck with 89 octane pure gas.
YMMV
 
#5
The "Old Timers" I knew used to put it this way: "Regular goes BANG.........High Test goes WOOSH". That's all I really needed to know.
 

2old2care

Active Member
#6
I think I'm lucky living in Florida, as far as non-ethnanol gas goes - Many, many stations here have it.
It must be because so much outboard boating here, that it's so common, but it makes it pretty easy to get good gas for small engines.
 

Minimichael

Well-Known Member
#9
Anybody know if there's a way to test octane level? Now I'm curious what octane saturation will make my engines run best. Wondering if certain of my Briggs engines might benefit from a slower burn, whereas others maybe not? Aluminum block vs. cast iron?
 
#11
I believe one thing we have to keep in mind is how time can affect the octane rating of gasoline. Generally speaking, the longer gasoline sits unused, the more its octane rating becomes diminished; not to mention the specter of "phase separation"........something I've also had to become concerned with over my many years of owning boats. I should also mention: Insofar as phase separation is concerned; if I suspect the gas I purchase is going to sit unused for any substantial length of time, I always add a shot of StarTron fuel stabilizer to it.
Given what I've managed to learn over the years has driven me to always purchase high test gasoline. While my Mercedes calls for it, I also use it in my Tacoma, as well as every other motor in my garage. I usually keep a couple of 2 1/2 gal. cans of gas on hand...........again, always high test, and always "juiced" with Startron. Knock on wood........so far, so good.
BTW, with respect to octane; if you Google something like "degradation of octane in gasoline" you can get a pretty good education on how this stuff behaves.
 

2old2care

Active Member
#12
Octane requirement is mostly about compression ratio, with maybe some spark timing thrown in.
The higher the comp. ratio, the higher octane needed, and vice-versa.
So, an old engine with excessive combustion chamber carbon build up, will need higher octane to run better, than the same engine new.
Others may disagree, but if the engine doesn't need it, by design, I don't think it's worth it to buy higher octane, all things being equal.
This has nothing to do with ethanol content, or whether a given gas brand has cleaning additives, etc, just octane rating.
 
#13
Okay. I think 2old makes some valid points. That said, I'd submit this: If your filling-up your Ford or Chevy (that just happens to be comfortable with 87 octane fuel), that's probably fine. If, however, your filling some 2 1/2 or 5 gallon gas can that may sit for some time in your garage, and be used to fuel some small one cylinder lawn mower, mini bike, or whatever?....I'd probably suggest going with the highest octane possible. All I'm suggesting here is that (in some cases) these little motors could prove to be a lot less "forgiving" than their bigger brothers. Hey, I was just thinking; there could actually be a moral to this story. If so, it might be: "Don't be penny wise and pound fuelish". (ha)
 

Minimichael

Well-Known Member
#14
Good points, Sunny and 2old. I also understand octane dampens the flammability of the gas. So, a single cylinder engine, especially one that is only air-cooled from the outside, and might be having increased compression issues, can benefit from a slower and more predictable burn. Raising the flashpoint to where only the spark plug can cause ignition of the fuel is the whole point. My Briggs 8B was knocking and pinging once heated and running at high speed. I learned on the internet that it was likely due to the heated surfaces of the cylinder wall, piston and head (no deposits in this case, as it was recently rebuilt). These surfaces can allow pre-ignition flashing to occur, and it's the occurrence of these additional flame-fronts bumping into each other during the combustion stroke that make the ping/knock sounds. Anyway, that all went away when I started regularly spiking my 5 gal. can with the octane booster.
 

2old2care

Active Member
#15
OK - I can get non-ethanol gas around here easy, BUT, 89 octane is what they sell for that, period. All the high octane stuff here is ethanol.
To me, using non-ethanol is far more important than having a higher octane rating.
As far as storing gas, if you're not adding Stabil or something similar, all the octane rating in the world is not going to help you.
I don't think you can expect ethanol-added gas to store well long term, no matter what you add - It's always going to be a problem because of the water absorption it fosters.
Minimichael - Did you try a colder heat range plug for the knocking engine?
 

Minimichael

Well-Known Member
#16
OK - I can get non-ethanol gas around here easy, BUT, 89 octane is what they sell for that, period. All the high octane stuff here is ethanol.
To me, using non-ethanol is far more important than having a higher octane rating.
As far as storing gas, if you're not adding Stabil or something similar, all the octane rating in the world is not going to help you.
I don't think you can expect ethanol-added gas to store well long term, no matter what you add - It's always going to be a problem because of the water absorption it fosters.
Minimichael - Did you try a colder heat range plug for the knocking engine?
I have not tried a colder plug. I'm still green on the plug science. I'm green on ethanol issues too, but these discussions help a lot! And you bet I use stabil! On the ethanol front, I have started adding ethanol "conditioner" because the label says it protects against the bad effects of alcohol in gas like corrosion and degradation? Up here near Chicagoland I'd have to drive another hour into the country to find ethanol-free. But on this ethanol note, Lucas also touts keeping injectors and stuff clean, which makes me wonder now what are we fearing the ethanol can do to a naturally-aspirated in-and-out one-jug pip-squeak briggs?

So now while I'm thinking out loud here, the conditioner label implies it protects against the degradation of the fuel, as if it's warding off the breakdown of the ethanol itself. Here I had kinda assumed the conditioner product was offering to eradicate the alcohol altogether...
 

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2old2care

Active Member
#17
Are there any local marinas near you in Chicago? It's on Lake Michigan, after all :~) I'd think they'd sell non ethanol gas, for sure.
 
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