BIKES: Which Gas Is Best For Your Bike?

Motorcycling Lifestyles

BIKES: Which Gas Is Best For Your Bike?

Learn more about which kind of gas to use for your bike.

E Don Smith | Posted October 30, 2011 Charlotte, NC

VP 110 Fuel.

When it comes to which gas you need to use in your bike there is a lot of misinformation and a lot of people are purchasing gas that actually makes LESS horsepower instead of more. Since we have so many misinformed riders out there we felt it was time to give you the straight scoop so you don’t waste money on products that don’t work.

The octane of fuel is the number that you see posted on gas pumps when you go to fill up your bike and most stations carry three grades of gas 87, 89 and 93. Some stations such as the one shown here offer five different octanes. Since 93 is a higher number than 87 and since 93 costs more than 87 it must be better right? Before we can answer that question we first need to understand what octane is, then you can decide.

Pump gas choices.

Simply stated octane is the gasoline’s resistance to detonation (knocking). It is not more powerful nor does higher octane fuel contain more BTUs (energy). Higher octane fuels are simply less prone to detonate in high compression engines. Many high performance cars and motorcycle such as those with turbos and higher compression engines specifically state they require 91 or 93 octane gas. This is one more reason this subject is confusing to many consumers. If high performance engines and turbo engines require premium fuel, then many people incorrectly assume it is better for a standard engine as well.

In order to better understand this dilemma we first need to talk about detonation and engine pressure inside the combustion chamber. When you look at the specs for a given engine you will see a compression ratio stated such as 12.5:1 or 13.2:1 etc. This means that the volume of a cylinder is 12.5 times greater when the piston is at the bottom of its stroke as opposed to the volume when the piston is at the top of its stroke. That means the contents of the combustion chamber is under a lot of pressure when the piston starts toward TDC (top dead center) of its stroke. Stated a different way, a four cylinder engine displacing 1000cc means that each cylinder has 250cc, of volume, but when the piston reaches TDC an engine with a 12.5:1 compression ratio reduces that volume to a much smaller 20cc volume. Yikkes!!

When the piston starts approaching TDC and the chamber is charged with fresh gas and air it becomes very flammable and wants to ignite from the pressure and heat alone. If the charge ignites too soon, the force of the explosion tries to force the piston back down the cylinder before it reaches TDC instead of moving forward as it should. This is detonation or knocking and can be very bad thing for your engine. So much so that many engines are fitted with knock sensors. If the engine senses detonation it slightly delays the signal to the spark plug therefore reducing your power (slightly) and eliminating detonation. Remember this is happening at over 10,000 rpms. When an engine retards the spark it is only changing it by a few degrees of the cranks rotation, any more than that would result in a lot of power loss.

During the power stroke of an engine, the piston only has a fixed amount of time (degrees of rotation of the crank actually) to be forced down by the burning air/fuel mixture. If the engine misses some of that rotation time with no spark to create the proper ignition needed to push the piston, then the engine has lost some opportunity to make power. Pretty soon the piston reaches the bottom of its stroke and the cams start opening valves in order to exhaust the spent gasses. At the point that the valves start opening the party is pretty much over and cylinder pressure drops and the cycle starts over.

In a properly setup engine with the proper grade of gas, the engine sparks at the exact ideal point of the cycle and then the charge ignites. The force generated by the burning gas and air forces the piston down which twists the crank shaft that is connected to your back tire via the drive chain. The octane of the gas is nothing more than its ability to prevent itself from igniting under the pressure and heat of the combustion chamber before the proper time which is when the spark signal ignites the charge. In other words higher octane gas is HARDER to ignite than lower octane, which is why higher compression engines and those with turbos normally require higher octane gas.

The use of higher octane fuels allows higher compression/cylinder pressure and more spark advance without detonation, and that is what yields more power, not the gas alone. Many professional racers such as those running turbo charged drag bikes/land speed bikes may run over 30psi of turbo boost which puts a tremendous amount of air and gas inside the combustion chamber. As a result they run gas such as VP Racing C-16 which has a motor octane of 117.

Now back to the original question of which gas you need to run in your bike. If you have a basically stock sport bike with no internal engine modifications you can run the gas specified in your owner’s manual. In most cases that is 87 octane. Some bikes specify 89 or even 93 so check your manual and use that grade of gas. If you think you are doing yourself and your bike a favor by running a higher octane than required, you are not. In fact you could be actually reducing rear wheel HP a small amount. The power drop in 93 vs. 87 or 89 is not huge, but the difference in 108 or 112 octane (non oxygenated fuel) and normal pump gas can amount to a 5 % or more LOSS in a standard engine. So if you are any of your buddies are buying aviation gas or other high octane fuels for your basically stock bike, you are not only wasting your money, you are going slower too.

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