Subaru EJ vs. DIT engines: Critical Changes in Knock Control
Subaru’s new DIT (direct injected turbocharged) engines have replaced Subaru’s aged EJ series which have been found in Impreza, Forester and Legacy models since the 1990’s. While many things have changed, some appear the same. The knock control related functions work in similar fashion and monitor names are the same, but there are critical changes that have caught some enthusiasts and tuners by surprise. These changes in calibration require a change in expected readings.
The knock strategy on previous generation EJ series engines was relatively conservative. When premium fuel is used, advance is generally at the maximum allowed by the calibration. The range of FKL control was great, but the advance was learned very slowly, in small increments. In the rare case a knock response was deemed necessary while on premium fuel, the reaction generally started in the form of feedback knock correction. This is a temporary response to an unacceptable level of knock detected at that time. When knock levels subside below the acceptable threshold, the knock response tapers off and operation returns to normal. If unacceptable persistent and/or rather significant knock is detected in a given RPM and load range, fine knock learning occurs. Fine knock learning reduces ignition timing advance in the area(s) of incidence only. The DAM (dynamic advance multiplier) provides the primary “universal” or omnipotent layer of knock control. While feedback and fine knock response are more finely targeted, DAM changes affect engine operation more broadly. A reduction in DAM can also elicit reduction in boost level and/or a change to failsafe ignition, fuel, and cam phasing if severe enough. On DIT engines, DAM reduction also results in richer AFR targets prior to entering failsafe mode.
Subaru has kept this structure on the DIT engines, but changed their calibration choices significantly. Fine knock advance is now on a very short leash with 1/8 the potential response of many EJ engines (1 degree vs. 8). Instead DAM is used to increase advance from a base level each time the engine is started. The default value at startup is .6875 unless modified in the calibration, or unless DAM was below the default at time of engine shut down. Rather than a failsafe to be used in a worst case scenario, DAM now changes quite frequently while the engine is running.
To allow the DIT ECU to make quick and broad changes in timing advance with reduced risk, knock sensitivity has been increased. In addition to increased knock sensitivity, fine knock learning retard of ignition timing regularly occurs the first time knock is detected, even if it is minimal. While FKL is only allowed to add 1 degree max, it can still pull many degrees as needed. These changes in FKL also no longer necessarily reset with a DAM change. Subaru’s factory calibration is far more aggressive in that it is set up to “ride” the knock threshold as many German cars have for years. This strategy provides maximum global advance as conditions change to improve efficiency. Fine knock learning and reduction in DAM are now expected, even when using premium fuel. Those responses are by design and should not necessarily be cause for concern. All things in moderation, so excessive negative learning or DAM reduction to the point of entering failsafe tables should be investigated. It’s important to be aware of what’s expected and what’s unacceptable.
Aftermarket calibrations vary greatly. Some calibrators have retained and embraced this new strategy, while others have manipulated the system to make it behave more like a factory EJ calibration. The new style is more likely to maximize timing advance, while the more conservative EJ strategy is more likely to be consistent. Calibrators familiar with EJ engines have learned they are not knock tolerant, and many are calibrating the new DIT engines as such. Subaru’s choices certainly indicate they feel the new DIT engines can handle being run to the limit of their available octane. Time will tell what these new engines can handle, but either strategy or the many possible variations of each can be successful. It’s important that car owners and calibrators are on the same page regarding expected readings to avoid confusion, and unnecessary concern.