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Timmer

Octane Rating.... What Is It?

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Timmer    0

The most important thing to know when tuning the fueling of a motorcycle(or any other vehicle) is the stoichiometric air/fuel ratio of the fuel that you are using for that vehicle. This is pretty easy for us motorcycle enthusiasts because all modern motorcycles run on gasoline.

 

Another important property of fuels is their octane rating. When selecting a fuel at your common local gas station, octane rating is the difference between all the fuels you can choose from at the pump.

 

Octane Rating: a measure of the anti-knock(anti-detonation) properties of a fuel. A lower octane rating(number) means the fuel is MORE LIKELY to knock. A higher octane rating(number) means the fuel is LESS LIKELY to knock.

 

I've already posted some info about knock/detonation here if you need a refresher:

http://forums.superb...?showtopic=3279

 

Octane Rating Equivalents of Different Fuels

Gasoline(Petrol) = 85-95 octane

"Race" Gas = typically around 110 octane

Diesel = 15-25 octane equivalent

E100(100% Ethanol) = 108-113 octane

E85(85% Ethanol, 15% Gasoline) = 102-105 octane

E15(15% Ethanol, 85% Gasoline) = 92-101 octane

E10(10% Ethanol, 90% Gasoline) = 88-98 octane

Methanol: 108-112 octane

Nitromethane: this is hard to quantify because nitromethane is an oxygenated fuel and they are also blended in different proportions with gasoline. Adding nitromethane actually decreases the equivalent octane rating.

 

 

One interesting fact here...

If you look at the fuel pumps at your local gas station, you will probably see a small note on the pump saying "may contain up to 10% ethanol". This means that some of the fuels at the pump are actually E10 instead of pure gasoline, or somewhere in between!

 

Also....If you are running fuel from VP, Sunoco, or some other fuel manufacturer you can find exact info on the octane rating of their specific fuels(along with other technical info) on their websites.

 

I just wanted to post some reference info on this stuff for some further posts I am going to do on fuels. Please fire away with any questions you guys have on this stuff.

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Cobie Fair    13

Good stuff Timmer, clear and to the point. I know you have some other stuff coming, very interested to see that.

 

From the practical standpoint, what does this mean to the average rider, say just a street rider? If it doesn't knock he should just run '87?

 

How about the average track rider? I had seen articles a while back in ROADRACING WORLD where it was recommended not to use race fuel, there would be no benefit, in fact might be worse? Admittedly though, this was a few years back.

 

Any comment on that?

 

CF

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Stewal    0

Here is a question i've been wondering...

 

How much gas is in the fuel line from pump to nozzle? Is it worth paying 10-20c more per gal trying to get 2-3gal of high octane if half of it is 85oct thats in the line?

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Timmer    0

All good questions Cobie.

 

Most street riders and track riders that I have met are running a stock engine and want to get the most miles out of their engine they can. If that is the case, I would suggest using 90-92 octane "premium" unleaded gasoline. This is typically the most expensive gasoline grade that you see at your local gas station. It is also the fuel that manufacturers typically specify on the fuel tank sticker on most modern street bikes. Knock is one thing that kills engine life, and running higher octane gasoline will ensure that your engine won't knock.

 

You might see a small horsepower gain by running lower octane fuel, but the trade-off of running your engine closer to knock is probably not worth it. I don't have any concrete horsepower numbers on how much of a power gain you would get, but I don't think it would be much.

 

Race Fuels

When talking about race fuels, it gets a little more complicated. Race fuels come in a bunch of flavors.

 

The most common things they do to make a "race fuel" are:

 

1. Octane boosters - They put additives in it to raise the octane rating. Race teams use this when they have modified the motor to run higher compression ratios. Higher octane fuels have really good anti-knock properties, but the by-product of this is they burn slower.

 

AND/OR

 

2. Oxygenate it. - They put additives in it which oxygenate it. Oxygenate: to enrich with oxygen. When this type of fuel burns it releases oxygen, providing more air to burn in the air/fuel mixture. Since extra oxygen is released, you will have to add extra fuel(with a Power Commander for example) to get the proper air/fuel ratio you want.

 

We know that higher octane fuels burn slower, ultimately causing slightly less horsepower on a stock motor. We also know that oxygenated fuel produces more oxygen in the combustion chamber which will "lean" it out if no fuel is added. This would also produce less horsepower.

 

So theoretically you could actually see less horsepower by running race fuel on a stock motor. It would of course depend on what modifications(if anything) have been done to the bike.

 

Does that make sense?

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Timmer    0

Stewal,

 

I'm curious to know that myself!

 

I don't know how much gas would be in the fuel line. I can't imagine it would be much more than 1 gallon though.

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ktk_ace    0

@ Timmer

 

Good read !!

 

but i have read somewhere else that higher octane fuel (to a certain extend) also results in less " backpressure " on the intake phase of the combustion phase in ICE engines

 

said reason is because the HO(hi octane) fuel does not vaporise as much on the intake valves . Cant find a paper or scientific study thou.

 

Its true to a certain extend from my butt dyno (I tried RON 92<3-7%>95 and <+1-2%> 98, it all makes a differnce)

 

Theres quite a noticeable torque improvement from ROn 92 to 95 but not much from 95-98 too on my stock engine (4V hi reving single cylinder engine, idles at 1350, rev limiter kicks in at 9250 rpm)

 

 

Not much if its slower revving(as in bone stock) as methinks theres much less heat on the intake valves at low revs; heat gets carried away much more efficiently too (less heat builtup/energy density)

 

 

I personally use a PEA (poly-ether amine)based cleaner on my gas pump fuels to keep the intake valves clean (carbon residues therotically increase aformentioned backpressure = lower efficiency)

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Timmer    0

Thanks for the input ktk_ace. :)

 

I'm not totally clear on the point you were making on less "back pressure" on the intake phase. Perhaps you could find an article about it or clear that up a little more for me?

 

 

Check out the list of race fuels that Sunoco offers...

http://www.racegas.com/fuel/index

 

There are a bunch of other technical details that Sunoco lists about their fuels that could affect certain behavior of an engine. To be honest, there some technical details they list that I don't have full understanding of. The main thing I wanted to get across here was the octane number.

 

You can see that some Sunoco's fuels have the same octane number, some of them are unleaded or leaded, and some of them are oxygenated, but they are all different from each other. I'm sure the engine behavior you would get if you tried all these fuels in your bike would be VERY different!

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Cobie Fair    13

So theoretically you could actually see less horsepower by running race fuel on a stock motor. It would of course depend on what modifications(if anything) have been done to the bike.

 

Does that make sense?

 

OK, so what's the trade off? Less horsepower, but is one balancing that against the knock?

 

CF

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Timmer    0

If we are strictly talking about different octane fuels...

 

The trade off here is a lower octane fuel will make more horsepower but be closer to causing the engine to knock. Alternatively, a higher octane fuel will make less horsepower, but will also be more knock resistant.

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SMA90804    0

I was always told to check the manufacturer recommended octane rating for a stock bike.

Octane rating mainly based on how the motor was engineered, so if the bike has no major modification, wouldn't it be wise to use the octane recommended by the bike's manufacturer?

To my knowledge, simple rule 'o thumb is: higher octane is only needed for high compression engine or when knockin occurs. Using a higher octane than the engine requires is a waste of $.

Please feel free to correct me if any of my statements were incorrect.

 

Cheers,

SMA

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Hotfoot    48

I was always told to check the manufacturer recommended octane rating for a stock bike.

Octane rating mainly based on how the motor was engineered, so if the bike has no major modification, wouldn't it be wise to use the octane recommended by the bike's manufacturer?

To my knowledge, simple rule 'o thumb is: higher octane is only needed for high compression engine or when knockin occurs. Using a higher octane than the engine requires is a waste of $.

Please feel free to correct me if any of my statements were incorrect.

 

Cheers,

SMA

 

I think that's a sensible approach as the manufacturer will likely recommend the octane rating that will give the engine the best life. However, motorcycle enthusiasts being the way we are, there are often questions raised about how to get more power from an engine, and it is interesting to note that going for higher octane fuel will likely NOT increase horsepower. And for those that hear recommendations to use the "cheapest possible gas" or use the lowest octane rating (to get more HP) it is useful to know how the octane rating affects the engine and what the trade-offs could be.

 

Or, you could get into discussions about race fuels, and that is a WHOLE other can of worms. :)

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mugget    3

Just coming back to this thread now as I've had some new questions raised after talking with someone who was adamant that all engines should always be fuelled with the highest possible octane rating, that they will always run better with a higher octane fuel. 

So - is it actually true that a higher octane fuel will "burn slower", or do the additives only change the volatility? Meaning that higher octane fuel is more resistant to pre-ignition, but that once there is spark both low/high octane fuels will ignite just as easily and burn just as quickly??

This also lead me to the subject of carbon deposits - if an engine has minimum requirements of 91 octane and we used 98, will that cause more carbon deposits? From what I have read some people say yes because the higher octane fuel won't have a complete/proper burn - others say it won't make a difference because the 98 will all burn just the same?

Any info would be much appreciated!

Cheers

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