A buddy of mine who rides a VTX and I got into a discussion about recommended octane ratings for our bikes. We disagreed on residue left behind by higher octane fuels. He pointed me at a site with a posting about the proper juice for his ride and I thought it was worthy of posting here, since I've seen the subject come up now and then.
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How to “Get Gas” without leaving skidmarks

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 increa

 

Thanks for the info.

 

There was a gent in a different forum I belonged to (...Honda...) that dynoed his bike with 87 then 94 octane. He lost horsepower on the Premium stuff...

 

How do you adjust the bike to run on 87?

 

How to “Get Gas” without leaving skidmarks

if reading this dosn't put you to sleep......nouthing will [sm=nxsmile.gif]

 

I put 7:1 pistons in my ol' Shovel.

 

Believe it or not my owners manual tells me that all I need is 87 octane

 

That is a great article, very informative. I have argrued this point many times with many people, and they all still believe that premium gas is " better " than regular gas. I'm printing it out and putting it up on the wall!

 

Somebody can correct me if I'm wrong in my understanding that the Sportster doesn't have an ECU that adjusts timing to conditions, but rather a mapped ignition curve. Some of the aftermarket stuff will allow you to improve on the flashed map, but it still won't adjust for conditions. My '04 1200 manual says use 91 octane and I don't argue, but it sure hurts at the pump these days with what gas prices have gone to. Still, paying the price for four gallons in the bike is a whole lot more accetable than dumping 20 gallons in my Mountaineer! Gotta use the plastic with that thing--who carries that much cash around?

By the way, my local gas station had a notice that they were switching from MTBE to E10 (ethanol), and they apologized for the price increase because of it. I believe the MTBE was mandated by this state and they've decided it wasn't such a good idea after all. Saw an interview on TV with a petro rep who said they have to produce 46 different kinds of gas to meet different state reqirements. Let's see, if it takes one acre of corn to produce 300 gallons of ethanol.... hmmm, guess I'd better dig up the lawn and plant corn.

 

The bottom line here is to use what the manufacturer recommends. My owners manual call for 93 octane or higher. Is this the average recommendation for all Harley-Davidsons, or are some models different?