There are many stories and anecdotes and what fuel octane is best and what effects it has on a vehicle. One of the most common discussions is whether higher octane will give you a benefit. There are many myths about fuel octane and power and gas mileage and I will try to break them down using real world data.
Octane and Knock
The octane of a specific fuel is a number that states how likely the gas is to ignite under pressure. The scale works so that the higher the number, the lower the chance of igniting under pressure and causing detonation inside an engine. The marketers that sell gas would love to lead you to believe the higher numbers are cleaner or more efficient but that is simply not the case unless the higher octane is mixed with special detergents or other additives (Thanks for the tip ddavidn).
. Higher octane may be slightly more efficient when combined with modern engine computers but it does not save money overall.
The main question people ask in regards to this explanation of octane is "Why wouldn't I want the fuel to ignite in an easier fashion? "The answer for that is that, higher octane provides for less of a chance of pre-ignition of fuel which causes knock. In a normal scenario, fuel is sprayed into an engine and only ignites when the spark plug lights up and therefore causes proper movement for the engine. In a pre-ignition or knock condition, the fuel ignites before a spark plug lights up and causes the engine to run less efficiently or to get damaged.
Premium Fuel Required
In most modern engines, the combination of low compression ratio and advanced engine management prevents knock from happening. The compression ratio is the pressure that air and fuel is put under inside a cylinder. Engine management works by monitoring knock sensors to see if there are pings and adjusts fuel and timing accordingly to manage it. In this case, if the vehicle requires 87 Octane then the engine computer has been tuned for that specific octane and will run efficiently with it.
Many of the modern high compression engines or engine that use forced induction require premium fuel at 91 octane or higher. The reason for this is as the compression ratio of an engine increases the chance of knock increases as well so higher octane prevents the knock from happening. In this case, putting a lower grade of gas will cause knock to become apparent and the engine computer will make changes to prevent the level of knock.In testing, we have seen lower octane to cause efficiency to go down as much as 20%.
Cheaping Out but Losing Money
To put our theory into real world terms let's say "Car A" usually averages 25 mpg, requires premium gas and has a 10 gallon tank. The price for 93 Octane is $3.50 and the price for 87 Octane is $3.10 for our example. A full tank of 93 Octane will cost $35.00 while a tank of 87 Octane will cost $31.00.Driving on the 93 Octane yields 25 mpg so the driver is able to pass 250 miles for $35.00 dollars and the cost per mile will $0.14. With the 87 Octane gas in the tank the engine computer scales back and the car only averages 20 mpg. Driving on the 87 Octane yields 20 mpg so the driver is able to pass 200 miles for $31.00 dollars and the cost per mile will be $0.16.
The example above is simplified but shows that even though the entry cost is lower and in the end, the gas will end up costing more. What's even more important is that the detonation caused by running lower octane will reduce the health of an engine over time and if strong enough can cause permanent engine failure at once.
The Placebo Effect
Many people live by the fact that even though their car requires 87 Octane, they choose to fill up with 91 or 93. There are a few reasons for this action and one of them is people stating that it keeps their engine clean. This is a myth as all of the fuels from a single source will run just as clean in any engine. The biggest statement that is put out is that running higher Octane will cause the car to run more efficiently. We did some testing and found that in certain situations, the engine computer will adjust and run in a more aggressive manner but we never saw increases in fuel efficiency of more than 3%.
Using an example similar to the one above, we will show the economics of these changes. Let's say "Car B" usually averages 30 mpg, requires 87 Octane and has a 10 gallon tank. The price for 93 Octane is $3.50 and the price for 87 Octane is $3.10 for our example. A full tank of 93 Octane will cost $35.00 while a tank of 87 Octane will cost $31.00. Driving on the 87 Octane yields 30 mpg so the driver is able to pass 300 miles for $31.00 dollars and the cost per mile will $0.103. Driving on the 93 Octane yields 31 mpg so the driver is able to pass 310 miles for $35.00 dollars and the cost per mile will $0.113.
The example above is simplified but shows that running higher octane in the best possible engine management scenario still yields a higher cost than running the lower octane. On the economics side of this argument, running the higher octane is a waste of money. On the engine health side, if a car has been designed and tuned for 87 Octane, it should not have any issues if it is properly maintained.
In the end, the best advice is to run what you manufacturer recommends.
Bozi is the founder of Hoonable.com and creates articles on everything from engine swaps to late model car restorations. You can follow him on Twitter and Facebook or drop him a line at Hoonable!