This was pulled from a motorcycle board I frequent. The same applies to all gas engines.

BAD GASOLINE

One of the most frequently encountered problems this season is the issue of ‚Äúbad gas‚Äù. While not involving flatulence, it just as big a problem.

We have had more fuel-related problems ranging from poor starting, hesitation, etc. to just plain ‚Äúquit and won‚Äôt restart‚Äù this year than in any of the past 25 years we have been here.

A further explanation of the two different areas of concern with gasoline may be in order for those who wish to pursue it.

1.) OCTANE

This is the resistance to detonation the gasoline offers. The most common grades of gasoline are: REGULAR at 87 Octane; MID-GRADE at 87-90 Octane; PREMIUM at generally 93 Octane. Other custom-built racing gasolines can offer up to 125 Octane. Octane is an indicator of the speed at which the gasoline burns in the engine.

The higher the octane, the slower the speed of the burn; the lower the octane the faster the burn. If the burn is too fast, uncontrolled combustion can occur. This most often is heard as ‚Äúpinging‚Äù and is commonly referred to as detonation. This is because instead of burning through top dead center of the combustion stroke, the entire charge is ignited too early and explodes in the chamber and acts as a bomb. Conversely, if the speed of the burn is too slow, it continues after the useful work can be done in the motor and manifests itself as poor throttle response, reduced power output and increased emissions and fuel consumption. An engineering fact: THE MOST HORSEPOWER IS MADE AT THE THRESHOLD OF DETONATION. We have often gained horsepower on the dyno and felt improved starting and driveability going from Premium grade gas to Regular. This change was recently quantified in a customer‚Äôs Ducati M900 by reducing the cranking time to start-up from 15 to 3 revolutions, although part of the improvement is explained below.

The multi-valve combustion chambers and their reduced flame front propagation distances in the modern engines virtually eliminates the need for high-octane gasoline.

2.) COMBUSTIBILITY

This is the ability of the gas to burn. As mentioned above, gasoline needs to burn to be of any value in the motor. Modern gasoline has been subject to formulation restrictions that now make it able to sit only 4 to 6 weeks after manufacture before its usefulness as a fuel is compromised due to reduced combustibility. This is basically a separate issue from the speed of the burn as controlled by octane.

The biggest issue is the fact that the ‚Äúvolatiles‚Äù in this new formulation evaporate very quickly.

The reason these ‚Äúvolatiles‚Äù are so important is that they are the part of the gasoline that affects starting and throttle response. That is why you can often feel the difference between individual tanks of gas when it comes to starting, cold running and acceleration at lower rpms. The evaporation of these volatiles leaves a thicker, more viscous residue that becomes difficult for the carburetor to break up into combustible droplets. This sludge will adhere to the intake runner walls or goo its way into the combustion chamber.

This difference becomes more pronounced with carbureted bikes using a hemi-style engine design (Harleys, Viragos, BMW, Ducati, etc.) as opposed to 4-valve and/or fuel injected motors.

One of the recurring themes in these problems has been the use of Premium gasoline.

There are no 4-valves bikes made in the last 20 years that need premium gasoline for normal street use. Very few of the 2-valve bikes need it either. One of the problems with premium is the fact that it is not used as much and sits in the ground much longer than the other grades, with the attending evaporation of the volatiles present at manufacture. This coupled with the more efficient combustion chambers used today makes the problem even worse. A second issue is the different ways used to achieve that higher octane. Each company has a different formulation, and with the high horsepower to displacement ratios of the newer bikes, tuning for this can be an issue.

A good rule of thumb is if the bike doesn‚Äôt ping and runs acceptably with regular, DON‚ÄôT use Premium. Regular is sold faster, is fresher and will generally start easier and have better throttle response than the higher octane gasolines. Not nearly as many cars use premium as they can compensate for the lower octane of regular with built-in detonation sensors to compensate for the octane, thus reducing the amount of premium sold. Most underground tanks hold in excess of 4,000 gallons of gas, so with primarily motorcyclists buying it three or four gallons at a time, it will be there quite awhile. Also as the level in the tank drops it affords more opportunity for the volatiles to evaporate. It doesn‚Äôt matter what the owner‚Äôs manual or your friends say. We have often GAINED horsepower on the dyno when we took away octane.

We have had to make arrangements to properly dispose of all the gas that makes a bike run terribly, but works fine in a car or truck. So try a tankful or two of regular and see how the starting and performance changes. If the bike is unhappy, go back to what you were using before, but bear in mind the information here and be ready to switch if things change.

3.) Ethanol

A component of gasoline that is seeing increased use is ethanol. A member of the alcohol family, it is produced by vegetable matter (not just corn). Ethanol has three physical characteristics that distinguish it.

First, it has 30% fewer BTUs (British Thermal Units) per pound than gasoline. This means that a gallon of ethanol produces less power in an undiluted comparison with a gallon of gasoline and a corresponding reduction in power as percentage of its addition to gasoline (eg: 10% ethanol produces 3% less power with the same jetting). The E85 blend (15% gasoline, 85% ethanol) produces documented losses of mileage of 30-36%.

Second, the detonation suppression characteristics of ethanol and its cousin methanol allow it to be used to raise the effective octane of gasoline it is added to. Gasoline blenders will use this to build the higher octane numbers of mid-grade and premium gasoline. Ethanol‚Äôs reduced cost per gallon makes this very attractive. E85 is generally $0.50-0.60 cheaper per gallon than even the 10% dosed gasoline.

Third is the affinity of ethanol for water. Gas line de-icer is made of either methanol or isopropyl alcohol, chemical relatives of ethanol. This means water in the air will be drawn into the fuel. This accelerates the fuel‚Äôs degradation and decreases combustibility as explained in section 2. While the exposure to air is reduced in fuel-injected applications, it still is a factor. Combined with the alkaline reactivity of ethanol, fuel system components can suffer.

One other aspect of the ethanol content is just that. When the pump says ‚Äúmay contain up to 10% ethanol,‚Äù it may legally contain up to 20% by law, allowing for ‚Äúerror.‚Äù At this point in history, the use of ethanol is primarily a politically driven issue. The logic of the use of ethanol in the current format is flawed.

a) It costs more to produce a gallon than it sells for.

b) It accelerates the deterioration of gasoline in storage, even with the use of a stabilizer.

c) It reduces the power and mileage in all conventional applications.

To allow the proper use of ethanol, compression ratios, cam timing, ignition timing, and jetting/fuel mapping need different configurations. While cams, ignition, and fuel can be adjusted on the fly, compression ratios are not easily or quickly changed. Having run alcohol-burning combinations for 25 years, this is an empirically determined fact.

The final pinprick in the balloon of ethanol is the fact that last fall, we switched from BP gas with ethanol to Shell gasoline without ethanol and our gasoline problems have virtually disappeared, to our great delight.

So, if there is a choice ‚Äì DO NOT USE ETHANOL-BLENDED GASOLINE! Only pure regular gasoline (read the pump carefully). A good way to avoid problems such as this is to be certain you buy gasoline at a name-brand station. The neighborhood convenience store buys gas from a broker, so you have no idea what it is. That gasoline is purchased by price, not specification

ADDENDUM TO ETHANOL SECTION

One of the other less-publicized aspects of the ethanol debate is the fact that there is a $ .54 per gallon tariff on imported ethanol. Just ask your elected representative why this is. Politicians can‚Äôt regulate politics much less be trusted to properly regulate the economics and engineering aspects of normal lives. The ethanol debacle is continuing proof of that.

Remember- always buy brand name fuels to reduce the potential entertainment that accompanies poor quality gasoline.

this can be found here: http://www.motorcycleperf.com/techtips/gasoline.htm