91 octane vs 87 octane


If a car is asking for 91 octane and you use 87 octane can it cause problems, or can the computer on the car just adjust the leaness and be just fine?


That depends on the car. Most newer cars will make some adjustment for it, but that means they will reduce power and mileage. On the others you will have that and the possibility of engine damage.

The answer to your question should be in the owner’s manual. If it recommends premium it usually means it does the adjustment.


If someone bought a performance car, why would they want to chince out on the gas for it?
The price is usually 20 cents more per gallon for 91 over 87. So, even if they filled up a 20 gallon tank on empty, they’d only spend $4 more than if they had put in 87. Now, looking at it that way, does it really seem so bad?


Octane rating doesn’t mean how much burnable fuel is in the gasoline, only the burn characteristics. 87 octane fuel will burn a tad quicker than 91, and will combust at a lower compression. The reason to use 91 octane is to prevent pinging and knocking caused by pre-ignition. Modern cars have knock sensors that allow any car to use what ever grade of gas you pump into it. If your car calls for 91 octane, and you put in 87, it will run, but the knock sensor will tell the computer to back off the ignition timing, and your performance will drop off considerably. Also, you’ll probably end up using more gas, since your developing less power with the timing adjustment.

However, if your car calls for 87 octane, using 91 octane does nothing for you. The computer has no reason to make any adjustments, and the fuel doen’t burn any better.