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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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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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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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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.:dontknow: Am I not understanding this?
 

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Discussion Starter #5
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=what+fuel+octane+should+be+used&aq=f&oq=
 

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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=what+fuel+octane+should+be+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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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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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.

Board is awesome!
 

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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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Discussion Starter #10
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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Just have to put my .02 in. Higher octane does not have more power, Like stated before. All octane does is control detonation and preignition. The low compression of our engines do not need anything higher than 87. I come from a racing background. We run stuff 13:1 and up. We run 110 to 120 octane. The main factor in the fuel we run is compression. We run racing fuel. This is what the maker of our fuel says on their web site about octane.
FOUR FUEL PROPERTIES
Listed below are the four basic qualities of fuels. As in everything, there are trade-offs. You can't make a racing fuel that has the best of everything, but you can produce one that will give your engine the most power. This is why VP produces different fuels for different applications. The key to getting the best racing gasoline is not necessarily buying the fuel with the highest octane, but getting one that is best suited for your engine.

1. OCTANE: This does nothing more than rate a fuel's ability to resist detonation and/or preignition. Octane is rated in Research Octane Numbers, (RON); Motor Octane Numbers, (MON); and Pump Octane Numbers (R+M/2). Pump Octane Numbers are what you see on the yellow decal at gas stations, representing the average of the fuel's MON and RON. VP uses MON because this test method more accurately simulates racing conditions. The conditions under which fuels are tested using the RON method are not as demanding, thus the number is normally higher than the MON rating. This leads many other fuel companies to rate their fuels using the RON in an effort to make them appear more resistant to detonation. Don't be fooled by high RON numbers or an average -- MONs are the most relevant ratings for a racing application. Be aware, however, the ability of fuel to resist detonation is a function of more than just octane.

2. BURNING SPEED: This is the speed at which fuel releases its energy. At high RPMs, there is very little time (real time - not crank rotation) for fuel to release its energy. Peak cylinder pressure should occur around 20° ATDC. If the fuel is still burning after this, it is not contributing to peak cylinder pressure (which is what the rear wheels see).

3. ENERGY VALUE: An expression of the potential energy in the fuel. The energy value is measured in BTUs per pound, not per gallon. The difference is important. The air:fuel ratio is expressed in weight, not volume. Generally speaking, VP's fuels measure high BTUs per pound and thus, have a higher energy value. This higher energy value will have a positive impact on horsepower at any compression ratio or engine speed.

4. COOLING EFFECT: The cooling effect on fuel is related to the heat of vaporization. The higher a fuel's heat of vaporization, the better its ability to cool the intake mixture. A better cooling effect can generate some horsepower gains in 4-stroke engines, and even bigger gains in 2-stroke engines. They want to sell you the fuel that is going to perform the best for you. So when they say higher octane isn't always the best choice i tend to believe them.
 

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Found out yesterday that the gas here in Houston is hell on carbs. We have 10% Ethanol in the gas here and was told by one dealer that Honda does not warranty the carbs malfunction due to the gas here, and was show how the gas here turns the carbs green over time. i wonder in this case would a higher octane help this issue.
 

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Found out yesterday that the gas here in Houston is hell on carbs. We have 10% Ethanol in the gas here and was told by one dealer that Honda does not warranty the carbs malfunction due to the gas here, and was show how the gas here turns the carbs green over time. i wonder in this case would a higher octane help this issue.
I find that hard to believe. Especially since the E10 is mandated by law in most locations. The issues you'll run into with the ethanol would be phase separation if you get water into the tank. Higher octane will not combat the effects of ethanol. Ethanol will not affect performance, but it will reduce your mileage due to less energy in a volume of ethanol than gasoline.
 

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Found out yesterday that the gas here in Houston is hell on carbs. We have 10% Ethanol in the gas here and was told by one dealer that Honda does not warranty the carbs malfunction due to the gas here, and was show how the gas here turns the carbs green over time. i wonder in this case would a higher octane help this issue.
Complete and utter BS line by that dealer! In Illinois and the surrounding corn states, 10% ethanol is ALL we can buy and has been this way for over 20 years. If what your dealer said was true, we would have no working vehicles up here. Ethanol actually burns cleaner than straight gas and cleans out your carbs and injectors. You only have trouble if you leave the ethanol sitting in th ebike for long periods of time(over a month ) without adding a fuel stabilizer like Seafoam.
 

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Well, I just rode out 2 tanks of 85 octane gas, which is regular in this area. I can't give you any technical data, but my bike feels more responsive and seems to have more power. I'll give it a few more weeks, then pull the plugs and see what they look like, but so far I'm impressed.
 

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You only need to add Seafoam, if you are going to park your bike and leave it for over a month at a time. That doesn't suprise me that the dealer never heard of this when you are in Texas. You can pretty much ride all year and never store it. But up here in Illinois, we store bikes for the winter, when fuel will be sitting unused for 3 or more months, this is when we add a stabilizer to the tank right before we put the bike on a stand and cover it up for the winter 8)

Spike, we've had this discussion before. If your going to moderate the board please at least pass proper information.

http://www.seafoamsales.com/motorTuneUpTechGas.htm

SEA FOAM MOTOR TREATMENT for Gas Engine Applications

100% Pure Petroleum
Use in All Engines in All Seasons
2 Cycle, 4 Cycle, and Diesels
Treats 3 Critical Areas: Gas Tank, Fuel Systems, and Crankcase
  • Cleans fuel injectors
  • Cleans carburetor jets
  • Cleans carbon
  • Stabilizes fuels
  • Upper cylinder lube
  • Removes moisture in fuel
  • De-icer<LI class=plain>Frees sticky lifters <LI class=plain>Frees sticky rings <LI class=plain>Removes moisture in oil <LI class=plain>Cleans P.C.V. systems <LI class=plain>Cleans catalytic converter odors
  • Oxygen sensor safe

How Many Mechanics Use SEA FOAM In Tune-Up of 4 Cycle Gasoline Carbureted or Fuel Injected Engines
Autos, Trucks, and Tractors
  1. <LI class=plain>With engine warm, slowly pour 1/3 to 1/2 pint through carburetor or throttle body throat. (If vehicle is port injected slowly pour SEA FOAM through direct manifold vacuum line that will feed all cylinders, possible sources are P.C.V. valve or brake booster line.) This will pull SEA FOAM down on top of the pistons and to the back of the intake valves to dissolve carbon. Turn ignition off. Restart engine after 5 minutes. If severe carbon build up is apparent, use more Sea Foam as previously directed. Make sure exhaust is well ventilated when using Sea Foam in these various ways as fumes will be extreme for a short time. <LI class=plain>Pour 1/3 to 1/2 pint into oil crank case to clean rings, lifters, dirty parts and remove moisture. <LI class=plain>Pour 1/3 to 1 full pint into fuel tank to clean injectors, carburetor jets, fuel lines and remove moisture.
  2. Immediate Results: Smoother idle, increased R.P.M.'s better throttle response and improved performance. See label on can for detailed results for use in each area.
FUEL TANK, CARBURETOR, INJECTION and OIL CRANKCASE.
For Peak Performance, Use SEA FOAM Every 2,000 to 5,000 Miles

  • A 100% pure petroleum product for use in all gasoline and diesel type engines, both 2 and 4 cycle. OXYGEN SENSOR SAFE.
  • Cleans dirty engine parts internally by removing harmful gums, varnish and carbon. WORKS AND PERFORMS INSTANTLY.
  • Removes moisture from oil crankcases and fuel tanks.
  • Stabilizes and conditions fuels. Use for engine storage.
  • Cure hesitations, stalls, pings and rough idle due to carbon buildup.
  • Helps pass emissions test. EPA Registered.
When Used Thru Injection or Carburetor

  • Cleans carbon build up
  • Cleans intake valves and pistons
  • Gives smoother idle
  • Cleans catalytic converter odors
  • Cures hesitations and pings
  • Restores power and pickup
  • With warm engine running, SLOWLY pour 1/2 pint through carburator, throttle body or direct manifold vacuum line that will feed ALL CYLINDERS. Possible sources are P.C.V. valve or brake booster line. Turn ignition off. Restart engine after 5 minutes. Be sure exhaust is well ventilated. Fumes will be extreme for a short period of time.
  • For use in injector cleaning machines, use 50% SEA FOAM and 50% fuel.
  • Fill diesel filters with SEA FOAM to clean injectors fast.
 

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Discussion Starter #17
Mark, what did I say that was wrong? The poster and I were talking about STORING a bike or having it SIT UNUSED for a while. That is why I said he only needs to add for THAT reason. We were not talking about cleaning out the fuel system, just stabilizing the fuel 8)
 

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You only need to add Seafoam, if you are going to park your bike and leave it for over a month at a time.

Sorry I guess my comment did come off as being critical. I was trying to say that Seafoam and some of the other products that you've mentioned in the past are used for storing and everyday use to keep the fuel systems clean.

Now as for Stabil (sp), I know we've all have been using it since god made dirt but I've have to say though I don't think it works. I've been told my many auto and MC mechanics NOT to use Stabil because it has a tendency to gum up the works. Just an example...the Goldwing I used to own..I use Stabil religiously.......I religiously replaced the fuel and air filters...oil etc. I started to have trouble with the carbs so I brought the bike to Honda and they disassembled my carbs and showed me the red crud in my carbs. I've had the same experience with my chain saws.

in Illinois, we store bikes for the winter, when fuel will be sitting unused for 3 or more months, this is when we add a stabilizer to the tank right before we put the bike on a stand and cover it up for the winter
 

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Discussion Starter #19
I know, but we were only talking about storeage use here, not cleaning the system ;)
I use Seafoam to clean my small engines out. I use Techron to clean my cars/trucks and bikes out
 

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I run 85 (Utah) in mine and it runs a lot better than when I have run 91. With 91 my bike tends to surge a bit, and just in general doesn't run as good.
 
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