My manual recommends I use 91 octane, especially when at altitude or towing, but says I can get by on 85. To my understanding, the octane rating is a measurement of fuels tendency towards preignition, which is caused by compression heat. I doubt my car detects lower octane and somehow magically lengthens the cylinders, show how can it run on either octane?

Ford reference, I have a 2017 ford escape with a turbo 2.0l ecoboost, but this is really more of a theoretical question on how any car could do this.

2 Answers 2


In current fuel injected vehicles, the computer (ECU) adjusts for it. There are sensors in your engine called "knock sensors". When the sensors start picking up a knock, it will pull timing. Usually, timing is advanced where it sparks to fire the engine before the piston reaches top dead center of the compression stroke. This is the start of the power stroke. If the ECU picks up any knock or pinging, it will "pull timing", which means it won't fire the spark as soon, putting the piston closer to top dead center. The overall effect of this is, your engine will lose power. The more reduction in advance, the more power is lost.

The ECU does all of this automagically, but it can only pull so much timing before the engine will stop running correctly. The ECU will pull as much timing as it can prior to causing run issues, which then allows more knock to occur. If you are noticing knock while it's running, there's some seriously bad fuel in your vehicle, or there's other issues going on.

If your vehicle is in question, it is really important not to run the lower octane fuels. In boosted applications, knock can occur very quickly and very harshly under severe loads. It will get beyond the point for which the ECU can pull timing. Under these types of situations, damage can occur to your engine. Something which regularly occurs under these situations like this is for the ring lands of the pistons to bust due to the harsh conditions provided by the boost and knock together. This is an "engine rebuilding event", meaning, your engine will suffer catastrophic failure, and will most likely incur the need for replacement.

  • I certainly don't recommend it but I ran regular gas in a 10.5 compression engine. An '62 Olds 394 ( 6,5 L) ; it was a second car and was going to the junk yard because of rust , so I commuted over 10.000 miles with regular. Starting from a stop it made a terrible rattling noise with any more than about 1/3 throttle, but it cruised fine at 70 mph. When I finally drove it to the junk yard , the guy came out to verify serial numbers and couldn't believe it was running because it was so quiet. PS; Wikipedia has this wrong as they list the compression as 9.75 to 10.1. Commented May 1, 2019 at 22:16
  • If, in a pinch, I were forced to use a lower octane than recommended, then would I want to reset the ECU when I refilled with 91?
    – Sidney
    Commented May 2, 2019 at 13:58
  • @Sidney - With the knock sensors, the ECU is quick to pick up on it while the engine is running. It also picks up on it rather quickly when there isn't any knock, so will adjust accordingly. I wouldn't worry about resetting the ECU when you go back to the higher octane fuel ... the system will adjust itself rather quickly. Commented May 2, 2019 at 14:49
  • @blacksmith37 - While I'm not disagreeing with your assessment of the CR on a '62 Olds, that ratio would be quite large in comparison to what I know of the engines of the day, which usually ran in the 8.5-9:1 ratio range. 10.5:1 would have all kinds of pinging going on with the fuel of that era, though the leaded gasoline of the day might have helped with that. You could be right, though, as you may be describing the "Oldsmobile Rocket" engine of the era, which was a particularly stout engine (for the time). Commented May 2, 2019 at 14:53
  • It was intended for premium gas which wasn't so expensive then; roughly $0.34 for regular and $ 0,38 for premium . All were leaded except some Amoco. Commented May 3, 2019 at 14:23

There is a difference between "running" and producing maximum power. Modern cars have knock sensors; when knock is detected , the computer retards the spark timing. It keeps running but produces a little less power. These sensors can detect knock before a very skilled professional can. And at higher elevations and higher temperature , lower octane is needed because there is less pressure in the cylinders due to "thinner" air. Oil companies blend high altitude and hot weather gasolines with slightly lower octane ( like one to three octane lower).

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .