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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.

Pᴀᴜʟsᴛᴇʀ2
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Sidney
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2 Answers2

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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.

Pᴀᴜʟsᴛᴇʀ2
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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).

blacksmith37
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