Advance Multipler and Dynamic Advance (in newer Subaru vehicles) are an important function of ignition timing that is present on most modern vehicles in some fashion or another. In it’s most basic form it adjusts the ignition timing (and thus how much power the vehicle produces) in response to knock.

On EJ Subaru vehicles this function is a simple multipler that normally gives 100% (mutliples by 1) of the ignition timing when everything is happy. It also wasn’t very busy, keeping the multiplier consistently at a value of 1.0 unless the car was knocking a lot due to inadequate fuel octane, vehicle temperature or other factors.

On the 2015+ WRX with the newer FA20 and FA24 engines however this started to change. Over the many years the EJ was around, engine building and control have grown both in quality and complexity. The engine tolerances are tighter, the overall strength of components and their materials are a bit stronger, and emissions have become a more important concern globally.

This newer engine generation follows the trend of many other manufacturers (VW, Porsche, Ford) of keeping the engine operating more optimally for the given conditions. (Rather than stacking a ton of vacuum lines on like with the EJ engines). As a result there are many more compensations and tables running to keep everything running within the band where the engines are the happiest, rather than picking one value and deciding that’s always the best.

Getting back to the main point, what this means for Dynamic Advance is that it tends to move around a lot more. Rather than using this adjustment to ignition timing in response to problems, it’s being used to compensate for driving conditions, or to allow you to run lower fuel octane normally, and gain extra power when you put a higher octane in.

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In the graph above is one drive cycle of more than 40 minutes.  You can see engine RPM in yellow, and on the bottom vehicle speed.  The 2^nd and 3^rd panels are feedback knock and fine knock learning.  These instantaneous ignition adjustments are reflected by changes in DAM(orange) over the drive.  Here DAM goes from 1 to 0.7 and back up to 1.0

Most people have all had that friend that “found” some 100 octane gas and put it into his old honda and swears they gained 40hp. In the case of something like the 20-22 Outback/Legacy XT it sort of ends up being true (to a point). When running the 87 octane this car is rated at on the stock map, we were seeing DAM of 0.32 instead of the “normal” 1. This value does tend to move around a bit depending on how hard the engine is working and the driving conditions, but that’s the average of what the computer throught was appropriate for the knock resistance of the fuel it had available. We tried a few different fuel octanes and didn’t find find the DAM up to 1.0 until 93 octane. And even with 93 octane we found that it fluctuated over the course of any normal driving. This goes to show that the computer is set up to optimize the way it drives, in response to the conditions the car is under.

Looking back, by deciding a value of 1 was always supposed to be present on older cars it meant that it was no longer a compensation, and just an indicator of a problem like a low oil light (which subaru owners know nothing about). On these newer cars it’s important to know that while we have maps that are optimized for each octane, DAM will still change. Rather than DAM being an immediate indication of a problem, it really needs to be looked at in context with things like feedback and fine knock to actually know how the car is running.