Octane

[RTJ]

COULD THE DIFFERENCE IN 93 OCTANE AND 86 GHANGE THE PERFORMANCE OF A CONTRARY 4 STRCKE

[GPracer2500]

First, I'm not sure what you mean by "contrary four stroke".

But assuming that bit of confusion is irrelevent: All else being equal, a change in octane rating is not likely to change performance unless the lower octane fuel fails to meet the minimum required octane rating for the engine. In other words, the only time an engine is aware of octane rating is when it doesn't have enough. You can't add or loose performance soley due to an excessively high octane rating.

It is possible that the two fuels might require slightly different jetting. The octane rating itself won't have anything to do with that, but the method of achieving that octane rating could. For example, if the 93 fuel achieves that rating due to ethonal being added, that might cause a tiny change in performance and might call for a small jetting adjustment. But it isn't likely to be a big enough difference to cause a good running bike to suddenly run like junk. Hardly anyone rejets often enough to keep things that spot-on. Just keep in mind, in that example the ethanol is responsible for the difference between the two fuels, NOT the octane rating.



[^^That's me trying to answer briefly. But here's what I really want to say....]

Trouble arrises when folks consider the affect of octane rating on performance. Most forget (or don't realize) that octane rating is only one of many characteristics that distinguish one fuel from another. Too often, any performance difference attributed to different fuels is automatically chalked up to octane rating. That's a big mistake because it ignores all the OTHER things that make one fuel different from another. There are a whole slew of specifications that make a fuel what it is. Unfortunately, octane rating is the only one most poeple see or are aware of so it gets blamed or praised for everything good or bad about a fuel.

I couldn't tell you how many times I've read or heard people say that high octane fuels burn slower and/or the best performance will be had using the lowest octane fuel required. My theory for the popularity of this myth is as follows:

Octane rating is a measure of a fuels resistance to detonation (true). Detonation is an abnormal combustion phenomenon characterized by the gasoline exploding in an uncontrolled way (also true). What folks seem to miss is that under normal circumstances (normal combustion) the fuel doesn't explode at all--it burns (deflagrates). Burning fuel vs. exploding fuel inside a combustion chamber might seem like the same thing but they are very, very different. Under normal operation NONE of the gasoline actually explodes. Octane rating only comes into play when conditions inside the combustion chamber become such that some of the fuel can explode instead of burn. Octane rating is ONLY a measure of how easily a fuel will reach the point where it will explode. It says NOTHING about how a fuel burns. You can have two fuels with identical octane ratings that burn at different rates and/or produce more or less power.

If you're not making the important distinction between burn (deflagration) and explode (detonation) then "harder to explode" gets incorrectly morphed into "harder to burn", which leads to "burns slower" and that's how you can arrive at "the lowest possible octane the engine will tolerate makes the best power". It's specious logic that reveals a misunderstanding of what octane rating really means and the mechanisms of combustion.

[ETA]I've seen people report they're dyno results that "prove" that high octane fuels make less power. What's usually missing is any reference to a/f ratios. You can't just swap fuels and compare power without confirming the jetting. Different fuels will jet differently--especially if you're going from a pump gas to a race gas. But even if the test was done correctly and jetting was adjusted, all it shows is that particular fuel did not perform well in that engine. Perhaps the fuel was a street oriented fuel (no TEL) that was only able to achieved it's octane rating by compromising other properties of the fuel. Or maybe it was a fuel intended for drag race cars that had a boiling point distribution curve designed for a narrow range of RPM. Who knows. Fuels must be matched to the application and it doesn't all come down to octane rating. Just because one high octane fuel does one thing does not mean any conclusions can be drawn about octane ratings in general.

A fuel's octane rating can be achieved in a variety of different ways. The most common are to increase the percentage of aromatic hydrocabons, add TEL (lead), or add an oxygenated like ethanol. They each increase octane rating but may have different impacts on the other performance characteristics of the fuel. I mention this to show that octane rating cannot be correlated with burn speed or anything else other than detonation resistance. [ETA]Octane rating is measured through the practical laboratory testing of fuel samples to demonstrate what the fuel can take before detonation begins. It doesn't measure anything else.

It's kind of like saying: I put on 25 lbs of weight so I must be in worse shape than I was before (the logic being that more weight = worse physical condition). Well, what if that 25lbs of weight was all muscle? See what I'm saying? The measurement of a change in body weight doesn't have to mean better or worse physical condition just like the measurement of deto resistance (octane rating) doesn't have to mean anything about burn characteristics.


[ETA] I added a few things marked by the tags.

[zrpilot]

Wow GP, thanks for the very informative post. The one question I have is: How does the average joe, such as myself, tell (other then detonation ) which gas is best in my quad? Seat-of-the-pants dyno?

[2004exrider]

He stole that from me:blah:

[GPracer2500]

Originally posted by zrpilot
Wow GP, thanks for the very informative post. The one question I have is: How does the average joe, such as myself, tell (other then detonation ) which gas is best in my quad? Seat-of-the-pants dyno?

[The guts of my answer are in bold]

It's not easy. The short answer is there's not a whole lot you can do to distinquish one pump fuel from another other than simple observation of how if behaves in your machine. Expect the differences to be mostly attributable to varience in jetting requirements. Perphaps the best thing you can do is to look for the highest quality fuel you can find. But judging the "quality" of pump fuels isn't easy. I mostly just try and steer clear of the discount gasolines, stick with the name brand stuff, and try and get it from the same place each time. With pump fuels, you can pick which station we buy from but the fundamental properties of the fuel available in each region of the US (and time of year) is dictated by the EPA and/or local authorities. We don't have much choice. Getting spec sheets for racing fuels is easy. Getting spec sheets for the pump fuel at your local station is next to impossible.

Getting the jetting right is going to get you a lot farther towards having your machine running tip-top than choosing brand X over brand Y. Other than just being good at jetting, there is one (fairly) reasonable thing you can do to help match your jetting to the fuel. Purchase a hydrometer. A hydrometer is a device for measuring specific gravity. SG is a measure of density. For testing gasoline, idealy you'd want a hydrometer set that includes a hydrometer that measures in the .700 to .800 range (the typical range for gasoline), a thermometer, and a graduated cylinder.

Here's a kit sold by VP Fuels (sorry, small picture):
http://www.vpracingfuels.com/hydrometerkit.jpg

That kit is expensive at over $200. But you can make your own kit for probably a quarter the cost, or less. Hydrometers kind of look like fat thermometers:
http://www.southwest.com.au/~jfuller/liquids/hydrometers.jpg

A graduated cylinder is just a tall glass container for measureing samples:
http://i6.ebayimg.com/02/i/05/db/71/04_2.JPG

SG measurement is directly connected to the tempurature of the sample. That's what the thermometer is for. The standard sampling temp for SG measurements is 60deg F. To compare two samples they must be at the same temperature. To compare a sample to a spec sheet it will need to be a 60deg F. Check Ebay or science class supply shops for good deals.


Knowing the SG of a fuel tells you two things. One, if you know the SG then you'll have a strong lead on whether a fuel is going to want comparitively richer or leaner jetting. A/F ratios are measured in weight. But carbs and EFI meter fuel by volume. So if you've got a fuel with a high SG then you'll need to deliver less volume of it to provide the engine with the same quantity by weight. Example: fuel A has a SG 10% higher then fuel B. All else being equal, fuel A will need jetting 10% lower (leaner) than fuel B for each to wind up with the same A/F ratio. By "all else being equal" I'm specifically talking about oxygenated or not. The fuel A vs. fuel B example would only hold up if both fuels either are or are not oxygentated. When two fuels vary supstantially in composition (like the presents or abscense of an oxygenate like ethanol) then other factors beyond SG can have a dramatic influence on jetting. Take VP-U4 vs. VP-C12: They have almost identical SG specs but because U4 is a carefully engineered with oxygenates to add HP, U4 and C12 shouldn't be expected to jet the same. A label on the gas station pump will indicate if the fuel containes an oxygenate or not. In most cases I'm aware of, all the fuels at the pump will be oxygenated or none of them will. SG isn't the end-all be-all factor in jetting, but in many cases it can get you pointed in the right direction.

The second thing SG info can do is help you evaluate the consitency (quality) of the fuel from any given source. If the fuel you get from gas station A always tests out between, say, .740 and .750 then that's pretty good consistecy. If fuel from gas station B swings between a wider range then you'd know to stick with station A to get the most consistent fuel fill-up to fill-up.

SG is often used by pro racers as a proxy for detecting fuel contamination and consistency. Generally, lower end race fuels will have a SG spec that's given as a range. Batch to batch it can be a little different. The best fuels will have a spec given as a set number. If your fuel tests out at anything other than that number you know the fuel has been contaminated.

Now, all that said--I bet hardly anybody is going to take the search for the best pump fuel that seriously. Frankly, I don't (although I'd like to get a hydrometer set just because I'm into that sort of stuff). Unless you're racing and finding the perfect tune for each day means the difference between winning and loosing, it's just too much trouble to confirm jetting everytime you ride. Deciding what you think is probably a good quailty fuel and sticking to it usually works best when you're just out to have fun.

You already answered the question of octane rating. If deto can be detected either audibly or through plug examination then use the next higher octane rated fuel until the deto stops. If your not confidenent in your abilities to test for deto then error on the side of having more rating than necessary.

[RTJ]

:confused: gosh i'm really mixed up now

[GPracer2500]

Sorry, I get carried away sometimes.

What's the problem you're having?

If you're not having a specific issue then the simple answer is no, you won't see a performance difference--UNLESS your engine is detonating with 86. Detonation is also known as "knocking", "pinging", "spark knock", and a few others names.

[wilkin250r]

Good job GP, you confused the snot out of him. :blah:

[pats2007450er]

i all way's got mine from BP 93 octane but know im race gas with klotz nitro.