What Octane Gas in the VTX? By Tapper - VTXOA
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post #1 of 102 (permalink) Old 11-09-2008, 04:43 PM Thread Starter
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Post What Octane Gas in the VTX? By Tapper

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

by: Brian "Tapper" Davis

One of the most frequently misunderstood issues among the VTX 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 VTX?

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 damn 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 VTX is a 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 VTX is pretty close to 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 the VTX is 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 VTX, 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 (the VTX doesn’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!
Last update: 2005-06-22 15:43
Author: Tapper

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post #2 of 102 (permalink) Old 11-11-2008, 10:33 AM
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Great article. Thanks for posting. Honda's owners manual says use regular gas. This manufacturer's recommendation comes from brainiac engineers with graduate degrees earning fat 6-figure salaries.
Good enough for me LOL.

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post #3 of 102 (permalink) Old 11-23-2008, 09:55 AM
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Good article Spike.

I had always believed higher octane gas was more powerful gas. Just common knowledge, right?. Everyone knows it.
But after seeing enough knowledgeable people in this forum claiming otherwise.
I checked it out. Wikipedia is a good source.

I have always run 91 octane.
So this summer I ran 87. My riding buddies still think I'm nuts.
What I found was as follows.

Power - hard to tell any difference by seat of pants dyno.

Gas mileage - has dropped slighly. This is a hard thing to judge
as one does not always ride the same road or the same speed.
But over the summer I never hit the highest mileages I got the summer before using 91 octane. On trips where I rode very easy (steady 55mph)
I went about 6 to 10 miles less on a tank from what I had experienced on 91 octane. The more aggressive I rode the smaller the difference became.

Spark Plugs - Without a doubt burned cleaner with 87 octance.
My plugs were souted from burning 91. They cleaned up from using 87.
I could also see the tips of my cobra pipes were not as dark.
This means my engine is burning cleaner and a cleaner running
engine is a longer lasting engine.

I have gotten a lot of good advice in this forum.
But if I had to choose what was the best advice
I would say it was to use 87 octane instead of the 91.
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post #4 of 102 (permalink) Old 11-28-2008, 10:16 AM
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Quote:
Originally Posted by bigsmoke View Post
On the contrary, the manuel says to use 86 or better. The lowest at the pump is 87.

As far as octane goes, I have noticed that when I do choose to run 89, the scoot does run better, and that's not just once a year. I the higher octanes have more detergent, then wouldn't that mean that the carb remains fairly clean, thus more power, quicker reaction, and better acceleration. I'm no engineer, but if you feed one guy fruits, veggies, and all that good stuff, and the other donuts, candy and red meat, I would believe that that latter would conk out first. JMHO
Same as my line of thought but I use Super Unleaded. I'm not an expert nor have I run an independent experiment like Trackman. I'm in this thread to learn. One thing I found inconsistent is that Trackman said

"Spark Plugs - Without a doubt burned cleaner with 87 octance.
My plugs were souted from burning 91. They cleaned up from using 87.
I could also see the tips of my cobra pipes were not as dark.
This means my engine is burning cleaner and a cleaner running
engine is a longer lasting engine."


But from other postings and from what I understand, higher octane burns cleaner because of higher cleaning agent. Am I not understanding this?

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post #5 of 102 (permalink) Old 11-28-2008, 12:12 PM Thread Starter
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Higher octane does not always have more detergents. Only SOME do like Shell. ALL fuel grades have detergents in them. Use a bottle of Techron, recommended by Honda and many other vehicle manufacturers, once a year. Fact is, higher octane will NOT make this bike run better. In order for a car to use it to it's advantage it must have an ECU that will be able to retard the spark in order to burn the fuel in the combustion chambers completely. The VTX ECU does NOT adjust this and you will leave unburnt fuel in the combustion chambers resulting in fouled plugs,poor mileage and less power. This is not conjecture, just use Google to search "What fuel octane should be used" and you'll get many fact based articles from all over
http://www.google.com/search?hl=en&q...e+used&aq=f&oq=

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post #6 of 102 (permalink) Old 11-28-2008, 08:01 PM
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Quote:
Originally Posted by Chicago-Spike View Post
Higher octane does not always have more detergents. Only SOME do like Shell. ALL fuel grades have detergents in them. Use a bottle of Techron, recommended by Honda and many other vehicle manufacturers, once a year. Fact is, higher octane will NOT make this bike run better. In order for a car to use it to it's advantage it must have an ECU that will be able to retard the spark in order to burn the fuel in the combustion chambers completely. The VTX ECU does NOT adjust this and you will leave unburnt fuel in the combustion chambers resulting in fouled plugs,poor mileage and less power. This is not conjecture, just use Google to search "What fuel octane should be used" and you'll get many fact based articles from all over
http://www.google.com/search?hl=en&q...e+used&aq=f&oq=
Thanks Spike,
I'll use regular unleaded gas from now on and use the cleaner once or twice a year.

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post #7 of 102 (permalink) Old 12-25-2008, 06:10 PM
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higher octane ,slower burn .You will get the most power out of your motor with the lowest octane you can burn ,with no detonation.High compression motors burn premium to slow the burn .motors 101
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post #8 of 102 (permalink) Old 01-05-2009, 11:39 PM
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yeah, 87, the cheap stuff.
I have worked with gasoline all my life. I installed removed and remediated underground storage tanks for gas stations. I did all of there testing and found the same thing in all the stations that you go to is that the gas all comes from the same place. They all have their own additives when the truck comes to pick up the gas. One thing is for sure though, Shell gas burns like hell when you get some on you. I was soaked in it one day and I went screaming for momma.
Wintertime, they load up the gas with propane to boost octane. Texaco has a slant on things though, I use to have to pump water out of some of the tanks around town during winter and spring. I never touched Texacos gas. Wierd at the time, later I found out they use dryers in there fuel in the winter time. Mystery solved.

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post #9 of 102 (permalink) Old 02-21-2009, 12:09 PM
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As with anything else,everyone has an opinion.I always use premium and always will in my bike.On the other hand when gas prices spiked I tried to save some money by using a lower grade in my car and the results were lower gas millage.If premium gas makes your engine run cleaner and get better gas millage, I will pay the extra buck now rather than hundreds later ............
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post #10 of 102 (permalink) Old 02-21-2009, 01:52 PM Thread Starter
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Quote:
Originally Posted by bills View Post
As with anything else,everyone has an opinion.I always use premium and always will in my bike.On the other hand when gas prices spiked I tried to save some money by using a lower grade in my car and the results were lower gas millage.If premium gas makes your engine run cleaner and get better gas millage, I will pay the extra buck now rather than hundreds later ............
That is the point, Premium DOES NOT make the engine run any cleaner or get you better gas mileage on a bike. The ECU must be able to adjust the timing curve to take full benefit from premium fuel. The bikes ECU does not adjust the timing therefore the VTX runs worse, gets less fuel mileage and will actually foul the plugs if you run premium. All those things you listed are complete myths and they are futhured by the gas companies to have people spend more money. In your car, go ahead and run premium. The VTX that does not have an ECU that will adjust the timing, run regular. Been tested here before, regular makes the VTX run better and get better mileage

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