87 octane in an engine that requires 93–what happens?
Octane has nothing to do with power. Gasoline manufacturers add more octane to prevent the premature ignition of the gasoline. In a high performance engine with higher compression, 87 octane will ignite itself from the heat of compression during the compression stroke or from any carbon buildup inside the combustion chamber. The computer will see that as a misfire and the incomplete combustion gases will flow down to the catalytic converter. There, any unburned gases will be burned off. 
Unfortunately, all that extra unburned fuel raises the temperatures inside the converter—sometimes to the point of melting the precious metals inside.
The pre-ignition of the lower octane fuel will cause the engine to “ping” and that pinging will be detected by the computer’s knock sensor. The computer will retard the timing to try and compensate and more closely match the spark event to the actual combustion. But the computer will be chasing an unknown. You will lose power and fuel economy will suffer at the very least. You might also see a Check Engine light with misfire codes. It’s a bad idea all around. You gain nothing and you risk damaging the catalytic converter.
For more information on this repair or any others for your vehicle, buy an online subscription to either Alldatadiy.com or eautorepair.net. Click on this link to compare the two services: Compare Alldata and Eautorepair.
If you just need information for a single repair and want to save money,eautorepair offers a lower price 1-week subscription for only $11.99. Or, if you'll be working on this vehicle in the future, you can buy a 1-year subscription (Alldatadiy.com for $26.99, or eautorepair.net $29.99)
© 2007 Rick Muscoplat
Alldatadiy.com
eautorepair.net
Both Alldata and Eautorepair offer professional grade shop manuals at a price you can afford. Check them out!.
Hover your mouse over the REPAIR ARTICLES tab at the top of the page. The click on DIAGNOSTICS to search by SYMPTOM, TROUBLE CODE, or read TECH TIPS on how to diagnose a problem
Click on MAKE an MODEL to see all the articles I've written for your vehicle
Click on REPAIR TOPICS to read individual articles written for specific automotive systems
If you've had your car scanned and know the trouble code go to TROUBLE CODES
Here are some quick links to speed up your research
Click on the link below for speedy navigation:
ABS issues, how to bleed ABS brakes
Alternator issues or how to test
Belt diagram or serpentine belt issues
Brakes, brake issues, what kind of brake parts to buy, ABS issues
Car Alarms or Antitheft systems
Cooling system, Water pump, What type of coolant to use
Firing order, cylinder layout, finding #1 cylinder
Oxygen sensors, How they work, How to change, How to test
Window issues, power window motor, replace window regulator, glass adhesive
Sorry, your answer only applies to very few ultra-high performance engines. In most modern engines the computer will not be ‘chasing an unknown’, it will find the timing that eliminates pinging. There is no damage risk to the cat. You’ll be gaining money, the only question is whether the loss in economy (if any!) is greater. If you use regular, hear no pinging, and lose little/no mileage, then there’s no problem.
Sorry Randy, We’ll have to agree to disagree. I have a friend who put 87 in her 2000 Acura TL, hardly an “ulta high performance” engine. The PCM lit up the check engine light like a Christmas tree. A scan showed misfires on all cylinders. All in all, it’s a bad idea. You can try this with your car and your friends, but I’ll advise my readers to avoid this where ever possible.
I will agree that a car thats suppose to use 91 or higher should be using higher octance as labeled on the gas cap, owners manual or gauge cluster.
Few people might not agree but I have a 2.0L Turbo DOHC Mitsubishi 4G63 motor in my car. Today I made the mistake of using 87 octane, only since I am runing lower boost 9lbs vs. 25lbs. Well as I drove I started to get misfire codes. Car would take off, hail butt then at a cruizing speed would misfire. I pulled out my scanner to see that I had code P0300 and my P0400(motor swap removes EGR). I now have a misfire till I refuel with 91 or above. How this helps!
you rock ! keep up all of the good work we really appreciate your expertise.
sincerely, a fellow car DYIer