# What octane should I use?



## rowdy-outty

<!--cached-Sun, 19 Sep 2010 09:15:28 +0000-->Good article I read on another forum. Thought I'd pass it on.

What octane Gas should I use?
How to “Get Gas” without leaving skidmarks 

One of the most frequently misunderstood issues among the atv community, and indeed among all folks concerned with performance and appropriate care and feeding of their motors is the subject of gasoline, and more specifically the mysterious octane number. Oil companies have gone a long way to foster this mystery, by marketing gasoline with higher octane numbers as “premium”, and inferring that the golden road to more performance, cleaner widgets, and hot chicks, is to spend the extra dough on the higher-octane stuff. Well, sorry to be the one to break this to you, but you’ve been had. So lets take a quick look at octane, and then get to the big point – what should you run in your atv? 

Now the truth is, octane is actually a chemical, which comprises an important part of the chemical soup that is gasoline. But here’s the thing – octane the chemical has absolutely nothing to do, with “octane” the pump measurement. So what is it?

In a nutshell, the octane rating of a fuel is a measure of its ability to resist detonation, ping, pre-ignition, or knock. The number we most often associate with octane is the "Anti-Knock Index", or the "Pump Octane" number. This rating is an average of two different measurements - the Motor Octane Number, and the Research Octane Number. Both of these measurements are taken using a special single cylinder test engine that has a variable compression ratio. The RON measures the knock resistance of a fuel during low RPM, light load conditions, while the MON is representative of high-speed, high load operation. As a result the MON will always be lower than the RON, but for our purposes the Research Octane Number is more significant because it more closely represents the way a low rpm V-twin motor is operated. So “Octane”, has absolutely nothing to do with the energy content or quality of gasoline. It’s just a measurement of when the gas will make a motor of a specific configuration begin to knock. 

Lets take a moment here to define a couple of things important to understanding this discussion. 

Knocking (also called pinking or pinging) in internal combustion engines occurs when fuel in the cylinder is ignited by the firing of the spark plug but burns too quickly, combusting completely before the optimum moment during the compression phase of the four-stroke cycle. The resulting shockwave collides with the rising piston, creating a characteristic metallic "pinging" sound. The fuel is normally ignited slightly before the point of maximum compression (the spark advance) to allow a small time for the flame front of the burning fuel to expand throughout the mixture, so that maximum pressure occurs at the point of maximum compression. It is only when this flame front arrives too early, for whatever reason, that the knocking effect occurs. If allowed to persist, knocking can cause vibration and damage to engine parts. 

Knocking is a different phenomenon from pre-ignition, which occurs when the air-fuel mixture in the cylinder ignites before the spark plug fires. Pre-ignition is caused by heat buildup in engine components or overheating of the air-fuel mixture during compression, and cannot be prevented by delaying spark plug firing. As such, if pre-ignition is allowed to continue for any length of time, severe engine damage can result. Pre-ignition is bad bad stuff, and changing the octane of the gas won’t affect it. 

Generally speaking, a higher-octane gasoline prevents knocking by either slowing the burn rate of the gas, or by increasing the difficulty of lighting it up. The idea here is to prevent the gas from burning too fast, and causing knocking. So the octane number has no bearing on the quality of the gas. But it does have a bearing, on how well your motor can utilize the gas and extract the maximum energy from it. However, getting too far into details of this would require a pretty darn big article, so I’m going to generalize a bit, for the sake of brevity here. 

First, consider that gas burns faster under pressure. The more pressure, the faster it burns. Therefore, a high compression motor will burn gas faster than a low compression motor. The big bore atv's are relatively low compression motor folks. So, we don’t really want a slow burning gas. 

Second, the burn timing in your motor is hugely important when considering how well the motor can develop power from a given amount of gas, and this timing is determined by the ECU, which can manipulate the spark advance of your engine. Normally, the timing in your atv differs between brands and sizes of engine but will be ~8 degrees before top dead center, but the ECU can and does retard or advance the spark timing to respond to certain running conditions of the motor, like coolant temperature, rate of increase of Rpm’s, or any of a number of other conditions. But none of these parameters are accessible (yet) by you, the itinerant tuner. So, you have no way to manipulate the spark to match the burn speed of your gas. Since utility atv's are designed by the factory to use 87 octane gas, if you run something different, the only possible way you have to tune your motor to use a different octane, is to manipulate the compression ratio of your pistons. 

So here’s the thing: Unless you have changed the pistons in your atv, a higher octane gas will have the effect of de-tuning the motor, and therefore reduce its performance. Now, the amount will probably be minimal, but it’s there. That’s the meat and potatoes guys, but there are a few more things we can infer. First, at higher altitude your compression will be slightly lower, and therefore you can get away with running a slightly lower octane gas. In fact, in areas like Denver, where the altitude is up around 5,000 feet, you’ll find gas stations selling 85 octane gas. But you’ll never find a gas station in Texas selling 85 octane. Since the altitude is much lower, the octane rating needs to be a little higher, so regular gas is typically 87 octane. 

Second, gasoline quality is dependant on a lot of things, but octane isn’t one of them. In fact, there is almost no real difference in “premium gas” other than the octane number. Federal law dictates the amount and type of most of the additives in gas. So by definition, they’re all pretty similar in nature.

Except. 

Gasoline is formulated according to climate in the US, and gasoline refiners use 6 basic formulations based on the expected temperature in the area they are expected to be sold in. Gas companies vary this by season, and by location (it also tends to be colder in Denver than in Dallas). So that gas you bought in Dallas might not give you great performance in Denver. 

These days, unleaded oxygenated gas is far superior in performance and energy characteristics as compared to the old leaded gas of yesteryear. We generally have very good gasoline available to us at the pump. 

Lastly, just remember this simple rule of thumb: You should always use the octane rating recommended by the manufacturer for your motor. But, if you are riding in a climate significantly different than sea-level at 80 degrees Fahrenheit, then use the lowest octane gas you can find that will not cause your motor to knock. 

One more thing – many cars these days have anti-knock sensors built into them (atv's don't), and those sensors are used by the ECU to manipulate the spark timing. So, if you run an octane significantly different from the manufacturers recommended octane number, the ECU will detune the motor in real time, and you won’t get any knocking – but you absolutely will lose performance. Use the recommended gas. If you’ve been running a higher octane gas, and spending the dough to do it, then I hope this little article will save you some bucks, and in the process gain you some performance. Good luck!


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## dirtydog

you would not believe how much of the world doesnt know this. i run an automotive shop and man you wouldnt believe some of the stuff i hear daily. good post i hope people read it thorough


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## Roboquad

We want more... Is there any truth in gas milage being better in a "higher Octane gas" due to a more efficient burn? and does Star-tron remove ethonal? good or bad I.Y.O.


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## dirtydog

dont know about star tron. but no truth at all about better gas mileage due to a better burn. actually it can work the opposite in some cases. really best way to understand it is higher the compression the higher the octane.


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## dirtydog

and not really sure how you could remove the ethenol from the gas . when they seperate in your tank thats when the problems start .so is that stuff supposed to be a stabilizer or something?


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## fstang24

what octane is reccomended for the brute 750?


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## Roboquad

Star tron-n- Sta-bil both came out with "ethanol treatments." As for compression I run 10 to 1, and have just switched pugs to iridium. big diff. on start up time.


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## Bootlegger

fstang24 said:


> what octane is reccomended for the brute 750?


stock motor....87 or 89 at best.


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## fstang24

i run 93 cause i think it burns cleaner, so i would be ok switching back too 87? There wont be any performance loss?


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## monsterbrute750

Great article.
I run 93 in my Brute , Rhino, son's Raptor 90 , weed eater , lawn mower , etc.....
Reason being is because 93 has less chance of ethanol seperation , causing "tarnishing" in the fuel system. So, although the article makes perfect since , it does not discuss the ethanol issue everyone is having these days.


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## gpinjason

my dealer told me to run 93 in my brute... so that's what i've been using


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## Roboquad

91 and stabil on all toys / lawn equipment. wife's LS requires 91 per instructions ant the Toyota eats any thing I feed it.


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## Bootlegger

I run ethanol free gas....we have about 6 stations here now that has No Ethanol.

Fstang23....It won't hurt a thing. I use to run 93 in all my bikes. Now I run mostly 89....you will never be able to tell difference in the two. I bet I haven't put ethanol in my bikes in a few years. The only thing 93 is good for is the High Compression built motors. Now next year I will have to use it...but until then its all 89 for me....lol. If you will looks in the manual the Brutes say 87 octane....or atleast a year ago they did.


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## fstang24

thanks boot


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## rowdy-outty

Big compression or bikes with power adders need 93 or more.


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## Bootlegger

fstang24 said:


> thanks boot


I would run 89 if it was me...!


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## dirtydog

man i wish we had ethanol free stations here . just the 10 percent ethenol we have in our gas drops you about 2 miles per gallon om most vehicles.and its in all grades. it goes bad in 3 months. if you have an old vehicle with a metal tank its really bad on them. the alchohol separates from the gas. my bike is an 06 zx-10r and man it hates that ethanol . it doesnt run the same as it does with ethanol free. of coarse here is an example . if you read the sticker on a crotch rocket tank it says to use 93 or higher or it could cause engine damage. they are abou 13to1 comp.


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## guimond47

in other words..... dont run PJ1 Gas energizer octane plus in my brute? you guys dont thing it will make a difference?


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## dirtydog

probably just wasting your money


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## kawa650

I agree dont think you'll notice a thing

I think its funny when people run 100+ octane fuel in there wheelers around here and say how much better it runs. I just laugh under my breath at them and say awesome. When in reality, most times, its all in there head because they want to believe it is to justify the extra cost of the fuel, and theres no telling them any different!!


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## 03dsglightning

Yep... No need for the added octane without more timing or comp... Ita all in the other thread.

Thanks for the read monstr


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