The Fuel Rating
Octane is an additive added to the fuel to prevent early combustion. So the octane rating in gasoline indicates the resistance to pre-ignition or combustion under compression. When the piston compresses the gas and reaches TPC (top dead center), the spark plug sparks igniting the gas in a small explosion. Most engines will generate different piston pressure depending on the size of the bore and its internal design. When fuel is compressed it can detonate before it reaches TPC and before the spark plug fires. This type of detonation is called pre-ignition and will create a ping or knock in the engine. Horsepower and fuel efficiency will be lost. Long-term damage to the engine can also be the result of the wrong octane.
Lower octane ratings like 87 (regular) simply mean that the fuel cannot resist a high compression engine, but if you don’t have a high compression engine like some older Harley engines, then higher octane is unnecessary. Most mechanics consider high compression engines to be 10:1 or higher. The 2004 Iron 883 owner’s manual recommends 87 octane fuel (regular), and with a compression of 9:1, that makes sense. However, in the high temperatures of summer, internal engine heat could cause early combustion and pinging, so you’ll need to determine for yourself what your engine needs. To be safe, many Harley mechanics are recommending premium gas for almost every engine regardless of compression. A higher octane fuel won’t hurt a low compression engine, but it won’t help it either if it’s not necessary. This, then, becomes a financial decision concerning wasting money at the pump.
The Wrong Fuel
Some Harley owners have determined that using regular gas (87 octane) does not harm their engine. In fact, they’ve heard no pinging or knocking which leads them to the conclusion that the premium fuel recommendation is either wrong or excessive. The problem with this rationale in newer engines (Twin Cam and Milwaukee Eight) is that Harley has sensors in the ECM (engine control module) that prevent knocking.
H-D uses an Ion Sensing System invented by Delphi (called the Delphi Ionized Current Sensing Ignition Subsystem) that senses pre-detonation or early combustion in the piston. It monitors electrical energy after every spark of the spark plug and if an early firing is detected, the ECM adjusts the spark timing in that particular cylinder to eliminate knocking. All Twin Cam and M8 engines have this misfire sensing technology which is an improvement on measuring just the engine vibration. A coil running from the igniter to the ECM gives specific output sensing to accurately detect which cylinder is pre-igniting and how often.
By measuring this ion flow of electrical current between the sparkplug electrodes after combustion, the spark plug itself becomes an engine sensor providing information to the ECM concerning cylinder misfires. The problem, of course, is that when the ECM corrects the timing it will also adjust to a weaker and less powerful engine response. Using regular gasoline may not result in pinging or knocking because your ECM is adjusting the timing, but you will be losing power and fuel efficiency. Therefore, simply listening to your engine may not be enough to determine which octane to use.
The Right Fuel
Higher octane does not always mean better engine efficiency. In other words, you may think that if a 91-grade octane is good, then a 110 octane will be better. Once again, the octane rating is indicating how much pressure your fuel can take before self-combusting. If your fuel is not under extreme pressure because your compression rating is not high enough to need it, then you are wasting your money.
Follow your Harley-Davidson owner’s manual to know what grade of fuel to use. Your Twin-Cam or Milwaukee 8 will normally need a 91-octane grade. Older Evo engines are said to run best on 87 octane and will actually perform worse with a higher amount of octane in the fuel.
Since combustion happens when the spark ignites the air/fuel mixture, too much air can create a “lean” mixture. It’s possible that too much airflow means not enough fuel in the cylinder resulting in multiple detonations and knocking. Conversely, not enough airflow can compromise combustion as well. The fuel injectors and airflow filters and valves need to be checked to make sure the cylinder walls are not damaged and that the electrical timing is efficient.
Other considerations include making sure you are using unleaded vs. leaded gas because leaded fuel can damage the 02 sensors and catalytic converters. It can also damage the plugs, leave deposits in the crankcase, and deteriorate the coating on valves.
Other additives in your fuel besides octane are also important to understand like detergents and ethanol. Today, most motorcycle engines can tolerate up to 10% ethanol which is what most gas stations sell. But ethanol is hygroscopic which means it absorbs water from the air which can cause the gas in your tank to separate leading to corrosion. Some stations are selling E-15 ethanol (15% ethanol) which causes your engine to run hotter and if left for a long period of time, can ruin your engine. Using an additive to counteract the ethanol is a consideration for Harley owners using this alcohol in their fuel.
Unless you are modding your engine and adding compression, you should normally not need 93 to 110 octane. That being said, many Harley owners love upgrading and modifying their gear, and if this is your passion, you’ll need to upgrade your octane to a higher combustion-resistant fuel. The best advice is to just check your owner’s manual and determine the best timing fuel octane for your specific compression engine.