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We probably all know that using premium grade gasoline in a low compression engine (less than 9.5:1) is nothing but a waste of money, but what about medium compression engines with a compression ratio between 10:1 and 11:1? Before computer controlled ignition systems anything greater than 10:1 would quite likely require premium, but today an engine with a compression ratio of 10.5:1 can run on 87 octane (or at least my '14 Honda Accord can).

I figure the ignition system, which would be getting the octane data from the octane sensor, would advance the timing appropriately to accommodate the higher octane fuel. This would then result in combustion being further along as the piston passed top dead center as compared to 87 octane fuel.

My owner's manual recommends 87 octane or higher, and it does seem like my car has somewhat better acceleration with the 91 octane fuel. With gas being so relatively inexpensive and my not driving as much as when I was commuting to work, I don't mind paying extra for the premium. But I wonder if the increased performance is all in my head.

(I don't think this is the same as other questions regarding premium vs. regular, as I'm specifically limiting the question to medium compression engines or engines that without computer-controlled ignitions would have required premium grade fuel.)

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There is no such thing as an octane sensor. Octane is analyzed in two ways. Analysis of the chemical composition, they call this Research. They place the fuel into a test engine and run it until it pings, they call this Motor. If you look at a gas pump it will usually show for example, 87 octane (R + M)/2. This is an average of Research and Motor.

A modern ignition system has some type of sensor to detect pinging or knocking, what ever you want to call it. The computer then calculates the base timing for all the cylinders. The computer then optimizes the timing for each cylinder individually. The computer keeps track of cylinders similar to how sequential fuel injection does. The computer then pushes the ignition advance on a cylinder until it detects a knock. When a knock is detected the timing is backed off slightly. This is the optimal timing spot for that cylinder at that specific time. After a couple of ignition events the timing is pushed again to find the next optimal spot. This process happens in each cylinder individually.

There are two different methods for knock detection that i'm familiar with. A piezo knock sensor is the simplest and most common. When the sensor is vibrated at specific frequencies it generates a voltage. That voltage constitutes a knock. The other involves sensing the dynamic resistance of the spark plug gap. The resistance of the secondary windings is reflected in the primary windings. By sensing current in the primary windings the resistance of the secondary windings can be inferred.

You may actually be seeing a slight increase in power. With a higher octane the ignition can push the timing further. This can allow faster and more complete combustion resulting in more power.

  • I didn't think there was such a thing as an octane sensor ... I know there's a flex fuel sensor (detection of ethanol in your fuel), but not octane. Thanks for confirming that. It's been my experience you run an engine at the lowest octane it's rated for (in the OPs case - 87) as anything above that and you are basically wasting money. I don't think a butt dyno will detect the difference between 87/89/91 octane fuel, especially if your are buying up (if you go down from the higher to use lower you can probably tell a difference). – Pᴀᴜʟsᴛᴇʀ2 Jan 5 '16 at 21:53
  • @Paulster2 Theoretically by igniting the fuel sooner a higher combustion chamber pressure can be achieved at the moment of the power stroke. A higher octane fuel can allow this. There is a law of diminishing returns at work here. You will gain a little power from regular to plus and less from plus to premium. Unless you on the race track and want to squeeze out every last ounce of power you are wasting you money with anything higher than what your vehicle calls for. – vini_i Jan 5 '16 at 22:00
  • Agreed. My understanding is a regular performance vehicle has a tune (including timing) which will only allow for so much advance. That advance is based on the octane rating the manufacturer specifies. The tune will not allow advance beyond that. The only way to get more of an advance is with an aftermarket tune ... if you're going there, your tuner knows how much advance to allow (or the dyno will tell them). – Pᴀᴜʟsᴛᴇʀ2 Jan 5 '16 at 22:05
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    @Paulster2 The advance is not arbitrarily limited. The knock sensors detecting knock is what limits the upper level of advance. When a vehicle is retuned the base or open loop timing curve is changed. This curve pushes the pre knock sensor optimized timing. After the tune the knock sensors are still used to push the timing to the optimized limit. – vini_i Jan 5 '16 at 22:28
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    A lot of time, the spark advance is taken directly from the map and the knock sensor (if the car even has one) is only there as a safety measure. I'm not too hot on American stuff but I can only think of one management system that uses a knock sensor as part of a closed loop system for advance control, from Saab turbos. I would suggest that the OP's modern 10.5:1 CR engine can cope with lower octane fuel because of advances in combustion chamber design. The shape of the bowl in the head face, cylinder face, spark plug design and position can help reduce the occurrence of detonation. – Sam Jan 5 '16 at 23:41

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