The newer DIT car’s (direct injection turbo – 2015+ WRX and 2014+ FXT) strategy is a bit more reactive. This is intentional and is expected since “premium” fuel is not required on the DIT vehicles. The strategy has changed and is comparable to what may have been seen as Fine Knock Learning in the earlier non-DIT cars may now result in a drop in DAM in the DIT cars. It shouldn’t be surprising to see a drop in DAM on the DIT vehicles (as it is on early cars where DAM being less than ideal is an indicator there is an issue) especially running lower octane fuels. How long the DAM takes to return to 1 is not indicative of how serious the knock event was and does not mean the car is still knocking. In order to advance the DAM on DIT vehicles, you have to drive in specific load/RPM ranges without knock. Determining whether there is a a potential issue on a DIT vehicle require considering how low the DAM has dropped and what other knock responses ( Knock Feedback Correction and Fine Knock Learning) look like overall.
Hopefully this post can help ease some concerns when you see those values under the knock parameters. If you think your vehicle might be having some negative issue mentioned above, feel free to contact us and we can review datalogs, help troubleshoot, and answer any questions you might have. Also check out our knowledge base for additional technical information on all things COBB. If you’re interested in learning more about Subaru Knock Monitoring, check out this FAQ.
Subaru knock detection is like using a large net to catch fish. You’ll definitely catch what you want to catch (actual knock) but you will also catch what you don’t want (false knock). If you used a small net, you would get less false knock but might actually miss actual knock. It’s a compromise. Be sure to look at the entire picture when trying to determine whether there is an issue.