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Calculated Load (g/rev) vs. Ignition Timing

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Good Afternoon,

I am transitioning to tuning Subaru's from tuning Hondas via Hondata, where most of my experience is based on tuning fuel and ignition based on injection time and boost pressure for load. Subaru ECU's (via Cobb) use a calculated load (g/rev) vs. ignition timing and I am interested in learning more about that relationship. This would help me make more informed estimates as I am preparing initial timing maps before tuning. Are there classes HPA offers or possibly some educational books that would help educate me on the subject?


Well, g/s or g/c or g/rev is just another way of describing how much mass of air is getting inside of a cylinder per cycle. The other way of doing it is using the LOAD function which is operated in percents. Load represents actual mass of the air trapped in the cylinder vs theoretical mass of the air that can occupy the same cylinder displacement under normal conditions -0 humidity, 0 C temperature, 1 Bar absolute pressure. For instance, under normal conditions a cylinder with 1 liter displacement would be getting 1.168 gram of air which would represent 100 percent load. If the air gets hotter to 30 C it becomes lighter and less dense so in the same 1 liter cylinder you would be getting 10 percent less air mass because of heat - it would be only 1.055 gram of air getting inside the same 1 L displacement cylinder. ECU would devide 1.055 by 1.168 getting 90 percent. So calculated load became 90 percent and therefore ECU should start using data for ignition timing and fueling from 90 percent Load cells. And of course it is vise versa - if the air temperature decreased (or you intruduced forced induction to the engine) the actual mass of air trapped in the same cylinder increased as well as the engine load. How much - depends on air temperature and boost if engine is blown.

The next thing is very easy to understand - the more mass of air gets inside the cylinder the more dense it becomes effecting on flame propagation speed. Therefore the higher the load the less ignition timing you need -that should be corresponding to increasing load values. It is absolutely identical strategy to working with SD based ECUs.

Hi Costas, g/rev is just derived from the mass airflow meter. The general rend we see is no different than a speed density system using MAP as the load axis - As the load (MAP or G/REV) increases, the ignition timing is retarded. G/rev is really a more sensible load axis if anything as it is directly reporting the mass of air in the cylinder. Initially it might not seem very intuitive but we can log the required parameters so we know exactly where we are accessing in the relevant tables. If you're interested in Subaru reflashing then the Subaru V11 STi worked example in the Practical Reflashing course will be very helpful for you - https://www.hpacademy.com/dashboard/courses/practical-reflash-tuning

Good Morning,

Sorry for the late reply. Thank you both for your input, it is much appreciated.


Costas M.

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