Octane?

[ride-to-live]

What do you mean by octane? Cant 440ex's run on regular gas like unleaded?

[buster024]

Octane is a rating (petrol rating), that rates how quickly fuel burns. The higher the number, the slower the fuel will burn. "Regular gas" (unleaded) can have many different octane levels. You can have regular, super etc. Super unleaded will typically be around 92-93 octane. You can buy race type unleaded that has a higher octane rating. I don't have any experience with 440's, so I couldn't advise you on what is best for your bike (I'm sure it's AT LEAST super unleaded).

[Sparky_20]

with a 440 id run a mix of race fuel and 93 ocante to be safe

[GPracer2500]

OK, I'm dividing this up into two answers: the quick n' easy answer and the more in-depth answer. If the quick n' easy answer is enough for you then great. If you want more, read the whole thing.

Quick N' Easy Version: The "higher" the tune you put on an engine the more demanding it is of the gasoline it burns. The higher the octane rating of a gasoline the better it can withstand being used in a high compression engine without causing abnormal combustion. Abnormal combustion will hurt power output and may damage the engine. In most cases, if you're using a gasoline that is of too low octane rating for the engine then under high load, high heat, WOT conditions it will make an audible "knocking" or "pinging" noise.

For most people with a 400EX this largely comes down to the compression rating of the piston. Below 10.5:1 static compression premium pump gas is very likely to be OK unless you're running your engine under ultra-severe conditions. 11:1 can be a gray area. Most people report 91 or 93 octane being acceptable but it won't always be depending on your particular situation. Anything over 11:1 and you'll need gas with a higher octane rating than you can get at the pump (i.e. some form of race gas).

The way the octane rating numbers work is that the higher the number the more resistant the fuel is to the abnormal combustion phenomenon known as detonation (detonation = a fancy name for engine knock or ping).

More In-Depth Version: I'm pretty sure what you're actually asking about is octane rating (octane itself is actually an alkane hydrocarbon--a chemical found in or created from oil). Octane ratings are a measure of a fuel's resistance to the abnormal combustion phenomenon known as detonation. The higher the number the greater the detonation resistance. Detonation is known by other names as well, the most common being: knock, engine knock, spark knock, and pinging. All those names describe the same thing. I frequently abbreviate the word detonation and just call it "deto".

Octane ratings are determined by testing fuel samples in a special variable compression test engine. There are several test protocols, the most common being Motor Octane Number (MON) and Research Octane Number (RON). The test for MON is more stringent than RON so MON will almost always be lower than RON for the same fuel sample. The number you see on gas pumps in North America is an average of those two ratings. Many (most?) racing fuels use MON alone since MON is a better measure of deto resistance in high performance, hard-working engines. Anytime you're talking about octane ratings you should specify which rating you're talking about (the exception is if you're talking about pump gas--then it can be assumed you're using [MON+RON]/2).

Each engine will have a minimum required octane rating (MROR). This MROR is a function of many, many factors. The one we tend to focus on the most is the compression rating of the engine. But there are lots of factors that go into MROR. There are so many that I don't feel like trying to list them all at the moment. Maybe later.



Here's a something I wrote recently for something else but it fits here too. It's mostly about the "high octane fuel burns slower" myth:


Octane rating is not a measure of the general combustion properties of fuel. It is only a measure of one very specific kind of combustion--detonation. Octane rating is ONLY a measure of detonation resistance--absolutely nothing else (not HP potential, not burn speed, not energy content, etc, etc). Octane rating says nothing about deflagration; deflagration being the normal propagation of the flame front, started by the plug's spark. Deflagration is the burning of air/fuel mixture in a normal, controlled manner. Detonation is a spontaneous explosion of parts of the air/fuel mixture (specifically the end-gases that reside around the edges of the combustion chamber). Both are types of combustion, but one is normal (deflagration) and one is abnormal (detonation). The chemical kinetics associated with each type of combustion are different.

Here's an excerpt from a paper written by Westbrook and Pitz titled, The Chemical Kinetics of Engine Knock. They are talking about specific hydrocarbon fuels:


...A flame burns through a mixture of any of these fuels in air at essentially the same rate, and other properties of the five fuels are also very similar. Yet the range of octane ratings for the five fuels is dramatic, and the fuels respond at drastically different rates to knocking conditions. Because every other combustion feature of these fuels is virtually identical, only their chemical structure can possibly explain the differences in octane rating and knock tendency....

The point of the above excerpt is the burn rates of hydrocarbon fuels are not directly connected to their tendency to detonate (i.e. their octane rating). They simply do not correlate. You can have two fuels with the same octane rating that are wildly different in every other way. Similarly, you can have two fuels that behave nearly identically except for their octane rating.

Here's the reference if you'd like to read the entire paper:

The Chemical Kinetics of Engine Knock.
C.K.Westbrook, W.J. Pitz.
Energy and Technology Review, Feb/Mar 1991. p.1-13.

Lawrence Livermore National Laboratory published Energy and Technology Review. They still publish its successor, Science and Technology Review. LLNL is one of the worlds leading centers for combustion modeling thanks to their responsibilities with our nuclear weapons. Westbrook and Pitz (Westbrook in particular) have been leading researchers in the field of combustion science since the early eighties. I highly recommend their work to anyone interested in the science of combustion!

The notion that high octane fuels burn slower may be the most common misconception about octane ratings among folks who know something about gasoline. To believe that is true reveals a misunderstanding about what detonation actually is and why it occurs. During every combustion cycle there's a race going on between the deflagrating flame front and the pre-flame reactions that are going to work on the end-gases. If the flame front can't get to and consume the end-gases in time, those end-gases will chemically decompose into new chemicals that are so unstable they will ignite all by themselves. That is detonation--when the end-gases have enough time to get thoroughly "beat up" by the ever increasing heat and pressure from 1) the piston that is still on its way up (remember the spark occurs before the piston reaches TDC) AND 2) from the expanding gases of the mixture that's already been burned behind the advancing flame front. Actually, the faster the flame front can get to the cylinder walls the less chance there is of detonation. This is why larger bores are more prone to deto than smaller bores for any given compression ratio. It also helps explain how combustion chamber design can have such a strong impact on minimum required octane rating. If you can't speed up the laminar burn rate of a fuel then maybe you can speed up the front with carefully orchestrated turbulence. This is also why ignition timing plays such a crucial role in octane requirements. The sooner you start the ignition, the more time the end-gases are going to have to sit around the edges of the combustion chamber and wait before the squish band can help push them towards the flame front.

None of that should be misconstrued to mean a faster burning fuel will always have a higher octane rating than a slower burning fuel. In some cases a fast burning fuel may have a chemical composition that is very resistant to the pre-flame reactions that cause detonation. But in other cases an equally fast burning fuel may have a very low resistance to these chemical reactions.


I certainly don't blame anyone for believing the high octane fuels burn slower myth, because it's been crammed down our throats for decades by people (many of whom with an air of authority) who knew just enough about gasoline to have some details but not enough to quite get them right. I'm a firm believer in NOT paying attention to any "facts" about gasoline no matter where it's coming from unless the source is published, cited research. I use to believe all sorts of inaccuracies about octane ratings and detonation until I spent countless hours looking for core research by actual scientists (not engine tuners or race gas company reps) that explained what was happening at a fundamental level. I mention that not because I like sounding pedantic--I don't--but because I've had lots of conversations about this topic in which I've met considerable resistance. I'm not pulling all this out of my ***. Nonetheless, I'm open to criticism and friendly debate. I won't expect others to keep an open mind without keeping mine open too.

This next part was in response to someone who reported seeing a dyno test on a CBR600F4. He reported that the a lower octane fuel made fractionally more HP than a higher octane fuel. He was suggesting that the octane ratings where responsible for the difference.



My issue with data that shows some engine produced more power on a dyno with one fuel over another goes in several directions:

1) Too often it seems no steps where made to ensure the a/f ratio was optimized for each fuel. If you're trying to distinguish a 1 or 2 percent difference in HP then you've got to consider the varying stoich's and densities for each fuel.

2) If such matters were taken into consideration, then I'm still not clear what the results are really telling me. All I could take from it is that those particular fuels performed as they did in that test. How am I to know that octane rating had anything to do with it? Answer: I don't. Does it tell me anything that can be applied to gasoline in general? Answer: no. Too often we like to chalk up every good or bad thing about a fuel to it's octane rating--because that's the only damn thing we know about the fuel in question. That thinking is a mistake. There are a whole manner of specifications that distinguish one fuel from another. The only way octane rating could impact HP levels is if A) the engine was actually detonation on one fuel and not the other, or B) if the fuel achieves it's octane rating by sacrificing some other property of the fuel. But in that case, the octane rating is only indirectly responsible--it's really the other property of the fuel that is to blame.

There are so many different ways to achieve a particular octane rating that it's impossible to say the X octane rating performs good or bad across the board. Some fuels might add octane rating by way of ethanol, some by replacing paraffinic hydrocarbons with aromatic hydrocarbons, and still others by using an additive such as TEL. Each strategy is going to have a different impact on the fuel as a whole even if each method gets you to the same octane rating.

I'm not trying to tout high octane over low octane or anything of the sort. I'm just trying to show that we must be careful when trying to evaluate a fuel based on octane rating alone. If you use octane rating for anything other than a measure of deto resistance then you're likely holding the rating responsible for things that are really caused by other things.

[mod440ex]

the size of the motor doesnt have has much effect on what octane you should run has how much compression you have, anything over 11.5:1 should have at least a 50/50 mix

[hondaman440ex]

I have a 440 with 12.5:1 compression and run 110 VP race fuel. It is really a 107, but I don't have spark knock. And it is also leaded. Boy does that $6.50 a gallon smell GOOD!!