So, since a lot of us have more time on our hands than usual, I’ve had a lot more “basic” questions coming from guys that are finally getting their hands dirty with routine maintenance on a wide range of bikes. I’m going to share what I’ve learned to (maybe) help you out a bit, but don’t take this article to be the One True Gospel.
It’s one dude’s opinion, pure and simple. Now, maybe that dude has wrenched on a lot of stuff, but it’s still just “what has worked for me…”
First things first: Even if you never rebuild an S and S hanging off an Evo, you’ve still put fuel in the tank. “What kind of gas to use?” is an argument on more than a few forums, but here’s my thoughts:
- Use gasoline that doesn’t suck.
- Use gasoline with the proper octane rating.
- Use gasoline with as little ethanol as possible.
Now, those three may seem basic, but let’s look at what the hell I’m really talking about…
Use gasoline that doesn’t suck
We’ve all been there: some country road in the middle of nowhere and the tank is sucking fumes. You see the old-time gas station up ahead and pull in for a tankful. Depending on where you are, and what that station owner bought, you might have paid for some truly crappy gas. Why? Well, for starters, most small, independent stations – like the ones wayyyyy off the beaten path, simply buy fuel from several sources based on price and availability.
Yes, it will probably run just fine, but there’s really no way to know how old the fuel is – and it degrades over time based on a variety of things – and who actually did the refining.
That’s not to say that corporate chains sell flawless fuel. I once had a 1400cc engine years ago that absolutely refused to idle if the gas in the tank was from any BP station in my area for about an 18 month period. One day, though, I was trying to outrun a storm and had to get fuel and the only station between me and the house was a BP.
Guess what? That old engine ran just fine.
The moral of the story is to find a few local stations that “turn over” their gas supply regularly and use them.
Use gasoline with the proper octane rating
Remember that 1400? One of the challenges I had with it was the higher compression in the motor. A good rule of thumb is this: anything over 10:1 compression in an engine needs a higher octane rating.
Yes, Mother Davidson builds a few of those engines – the big Evolution and Revolution engines are over 10:1 and the Twin Cam is damn close at 9.7:1.
Bear in mind that the main problem you’ll have with too low of an octane is pinging, especially if there’s a load on the engine or you’re operating in a hotter environment – like the summer riding season down here in the South. Basically, pinging is “predetonation” where the fuel/air mixture is igniting prior to the spark plug lighting it off. Usually, more octane solves that problem, but a fellow with some mechanical know-how might be able to play around with the timing and get the engine to run fine on a lower octane (of course, you might lose a little “butt-dyno”: power, but the bike will run better…)
Use gasoline with as little ethanol as possible
Now, there’s this… ethanol is everywhere, and it really can suck for the wrong engines. Since it is an alcohol, it burns, but differently than gasoline. Additionally, it can degrade a lot of materials often seen in fuel systems – hoses, gaskets, old fiberglass or epoxy repairs on tanks (and JB Weld, too – ask me how I know…).
In short, it’s great for new vehicles, but not so great for older ones. I’ve found it interesting that Ducati considers ethanol to be a fuel additive and the use of it in their bikes will void the warranty.
For the most part, though, we’re stuck with it, so how can you work around it? Well, a higher-octane fuel might help, as well as tweaking the timing. Of course, the best bet is to try and find ethanol-free fuel local to you and run that. The real pisser in this is I can remember when ethanol came out and it cost more to use. Now, decades later, “ethanol free” – pure gasoline – costs more.
Your mileage may vary…
Like I said in the beginning, these are just some basics that work for me and the areas that I ride most often. It’s hot and humid here most of the year, and I’m sure that impacts combustion to some extent. I can certainly rationalize how the ambient humidity in the air could impact – to some degree – the ability to cool a piston (yeah, it’s a reach, but isn’t humid air just “water injection”?). Read up on what your bike is supposed to like and try it out. If you have some really old stuff, it’s worth learning what the average octanes available to the consumer were back then to try to determine what to run today.