Tech Stuff

Everything You Always Wanted to Know About Octane


Octane is one of the most misunderstood aspect in the performance aftermarket. Put simply the “octane rating” of a fuel is the fuel’s ability to resist detonation.


OK, so what is detonation? Detonation is when the fuel ignites at a time before or after the sparkplug fires. Pre-detonation happens when the fuel is ignited during the compression stroke rather than the power stroke. Post ignition happens when the fuel ignites during the exhaust stroke.


Pre-ignition occurs when the fuel doesn’t have enough octane and is ignited due to the heat generated in the compression stroke. Post detonation occurs when the fuel has too high octane and or has low oxygen and burns ion the exhaust stroke. Both types of detonation are called “knock” or “pinging”, (this is due to the unique sound the pistons make as they “rattle” in the cylinder).   Either will damage your engine.

OK, so how does this hurt my engine? When your engine has the proper octane the fuel burns in a very controlled fashion and burns evenly across the face of the piston. When detonation occurs the fuel is ignited by hot spots on the piston and burns in an uncontrolled fashion. When this happens the force of the detonation isn’t even across the piston face and the piston moves slightly side to side, causing the tell-tale “pinging” or “knocking” sound.


So, what does this mean to you? Well, too little octane and you will get pre-detonation. Too much octane and you get post detonation. To run your engine safely and for maximum performance you need just enough octane. This is where you need to have a long conversation with your engine builder and access to a dyno if you can.


There really are no specific rules of thumb when it comes to tuning your engine for the proper octane. Unfortunately, every engine is slightly different. There are some general rules around octane requirements for certain compression ratios, boost levels or nitrous injections. These are a good place to start and you will generally be safe. To get maximum performance, you need to tune your specific engine in your specific application.


We have all seen the selection of octane boosters at the auto parts store. For the most part people don’t think they do much and are “snake oil”. The fact of the matter is they all work, but the buyer needs to understand how strong and effective they are.


The off-the-shelf octane boosters are designed to raise the octane of the gasoline a few points. This is where you need to be careful. A “point” of octane is 1/10th of an octane “number”. That means that if the octane is raised by 4 points and you start with 91 octane, you now have 91.4, not 94!


Now these products have their place and are effective at what they do. If it is especially hot out or you are towing something or you are at higher altitude or you have a stock car that pings at wide open throttle then an octane booster will give the additional octane you need to keep the engine happy.


If you are trying to get the octane up to that of racing fuel you need to buy a “race fuel concentrate”.  A “race fuel concentrates” raise the octane by as many as 20 octane numbers or 200 octane points. In addition, a “race fuel concentrate” needs to increase the chemical energy and chemical oxygen of the base fuel.


Why is this important? Well, if you have high octane and low chemical energy you are effectively leaning out the engine at Wide Open Throttle. This can create detonation even though the octane is sufficient for the engine.


If there is high octane and low chemical oxygen in the fuel, not all of the fuel burns on the power stroke and instead some will burn on the exhaust stroke. This can raise cylinder head temperatures and will burn the valves. It can even cause detonation in the exhaust stroke.


If you are in the market for a “race fuel concentrate” make sure it does all three of these things.


[NOTE: RACE-GAS Race Fuel Concentrate is the only product on the market that does these three things. RACE-GAS was tested head to head with commercial racing fuel and actually exceeded the performance of racing fuel in both torque and horsepower. Got to www.race-gas.com and click on test data to see the third part dyno test with RACE-GAS. In the video you can watch the many engine dyno tests that were done by POWERNATION: Engine Power, POWERNATION: Extreme Off Road, POWERNATION: Truck Tech, Monster Garage and All Girls Garage.]


There is a great deal of confusion about octane ratings in the performance aftermarket. Octane booster and race fuel concentrate capitalize on this confusion to sell their products. What you don’t know can hurt you!

There are three octane numbers that you need to be aware of:


  • Research Octane Number or RON: RON is the octane of the fuel at idle. That means the engine has no load on it at all. This octane number will always be the highest.
  • Motor Octane Number or MON: MON is the octane of the fuel under load or at open throttle. This number will always be the lower of the octane numbers.
  • Anti Knock Index or AKI: AKI is the average of these two numbers. The formula to calculate the AKI is (MON+RON)/2. This is the number that you need to pay attention to.


Most race fuel concentrates don’t tell you what octane number they blend to. This is because they blend to the Research Octane Number or RON. They do this because it is easy to create a high RON number and it allows them to use cheaper feedstocks like kerosene, alcohol and lower blended fuel aromatics. Remember the RON is the lowest of the three and is the octane at idle! I don’t know about you but I don’t do much racing at idle!

In this example from 76 you can see that they use the AKI Standard, not RON


The standard used by the fuel industry in the United States is the anti-knock number or AKI. They use this number because it best represents the way a motor is run, at both idle and under load. If you are going to use a race fuel concentrate you want to use one that that is tested and certified the same way commercial racing fuel is. In that way you can be sure that you are getting the octane you need.


To be safe, only use fuel products where the octane rating is in AKI. If it isn’t, you can’t be sure what you are putting in your car. Better safe than sorry! 


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