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My six-cylinder 2008 Acura TL specifies "premium" fuel. My wife often fills it with "regular" (i.e., 87 octane).

As I understand it, under certain operating conditions and loads, the lower-octane fuel might begin to knock. The engine is equipped with knock sensors, and the presence of knock will be noted in one or more OBD parameters. Question 1: What OBD parameters would indicate knocking (and via what values/units)?

In response to knock, the ECU will retard the spark timing. The degree of spark delay will also be noted in OBD. Question 2: Which parameters and values would indicate the timing and other changes implemented to avoid knocking?

I want to analyze the effects of using low-octane fuel over the course of a tank of gas. Question 3: What is the most economical way of collecting and storing the data involved in the first two questions over an extended period? E.g., are there plug-in OBD loggers with large memory buffers? Or is there PC software designed to interface with the OBD for this sort of analysis?

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  • With the money you will spend on equipment to collect and analyze this data, you could just buy the fuel specified by the manufacturer and be done with it. – cory Mar 30 '17 at 15:11
  • @cory - True, but I want to do real-world research to answer two questions that have not been quantified anywhere I can find: (1) How often during routine driving does an engine tuned for premium knock on regular octane gasoline, and (2) How much fuel is wasted compensating for the lower octane. This is the beginning of that research. – feetwet Mar 30 '17 at 15:29
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I think this question has 2 sets of answers.

If you can get your hands on a Honda/Acura specific scan tool, they probably have some sort of output of how many knocks the knock sensor detects. These manufacturer specific systems are typically very expensive (X,XXX USD or more), but would probably tell you everything you want to know. Keeping this hooked up while driving around could be a problem depending on the design.

Doing this with a regular 1 size fits all OBD-II scanner would be more problematic. There are plenty of codes related to the knock sensor no longer functioning, but I don't know of any output that indicates "number of knocks" or "knock sensor has activated" or anything like that. Large spark advance spikes usually, but not always, indicate the knock sensor has fired.

TL, DR;

  1. none, as far as I know
  2. mainly spark advance. STFT and O2 sensor values would potentially be affected as well
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  • Interesting: So you're saying that manufacturers are known to "hide" data in such a way that no standardized OBD-II scanner can find or read it? – feetwet Mar 30 '17 at 18:04
  • This is only speaking for the United States, but I wouldn't say they are "hiding" it. Only 7 or 8 of the 16 OBD-II pins are regulated by the government. The other half are manufacturer discretion. They all exercise this discretion differently. So it is hard/impossible to make a sensor that can read all the different manufacturer discretion fields correctly across every manufacturer. This wikipedia would really be a good read en.wikipedia.org/wiki/On-board_diagnostics#OBD-II – Zshoulders Mar 30 '17 at 18:22
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Re Question 3, there are OBDII systems that run on pc's which do download and store data - you will need to research and find the ones suitable for your car : I know where to get the ones for my car and I have an old laptop ready (have to reset the date etc), not had the time to complete that project yet...

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