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Given the number of variables involved in whether an engine will knock on a given fuel in a given environment: How can an end user determine whether the engine is retarding timing due to fuel octane?

My understanding is that the relevant ECU parameters should be readable on the OBD, so researching the codes and running a continuous scanner would do it. Is there anything easier or more obvious?

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Having a live scan running is by far the easiest way to tell if the ECU is pulling timing while the engine is running.

Beyond that, you need to be aware of what the manufacturer's requirement for an engine is, such as it's performance level. Several situations which call for higher octane:

  • Turbo/Super charging
  • High static compression ratio (10+:1)

Both of these situations are a lot more common today and require the higher octane to support the engine and keep it from pulling timing. Without the specific knowledge provided by the manufacturer (or known due to specific engine build requirements), the individual is flying blind. The engine can pull timing to prevent knock without the driver ever knowing it is happening ... that is, to a point. The ECU will only pull so much timing. Once it gets to a point, knock will occur (that point is going to be different for every setup). Once the ECU has as much timing as it can, knock will occur and the driver can then start hearing it. By that time, things can get ugly, though.

Another thought here is that if a combustion chamber becomes encrusted with carbon deposits, the engine can start knocking. While higher octane fuel can mask this situation, it really isn't helping anything. It's by far better to have the combustion chambers cleaned and to then utilize the lower octane fuel than it is to just run the higher octane. A clean engine is a happy engine (both inside and out).

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    Strange...my 2014 Honda Accord has a compression ratio of 10.5 but is supposed to run on regular. I'm guessing that's just about the borderline between regular and premium. – BillDOe Nov 10 '15 at 0:12
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    @BillOer - There is no hard and fast numbers, what I put out there is an approximate. Some engine technologies do not require higher octane fuel with higher CRs for reasons like shape of the combustion chamber and pistons, direct injection, etc. Good of you to point this out, though. – Pᴀᴜʟsᴛᴇʀ2 Nov 19 '15 at 16:07

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