Discussion and questions related to the course EFI Tuning Fundamentals
Can I get a recap as to why ignition timing must be retarded as airflow increased?
The increase in airflow would necessitate an increase in fuel. A densely packed compression stroke doesn't need as much advance to achieve the 16-18* After TDC "push" from the combustion event for best torque. As RPM increases there is less valve open time to fill cylinders so more advance will be needed to start the combustion process to finish at the desired 16-18* ATDC. Hope that answers your question.
@chris thanks for the answer buddy. okay so at high RPM, when the air is very densely packed, youd be running rich for the high oxygen content and also to cool the engine (so as you said, not much advance is needed). However, there is also less valve open time to fill the cylinder so you mentioned as a result, more advance would be needed to start the combustion process earlier? Abit confused as to how these go against each other.
There are two trends that we see in the ignition table. Broadly speaking, as rpm increases we need to advance the timing, and as airflow/load increases we need to retard the timing. I'll break down each situation:
As engine rpm increases each engine cycle happens faster. The combustion process takes a finite amount of time to complete so as the rpm increases we need to start the combustion process earlier (advance the timing) in order to get peak cylinder pressure at the optimum point.
With regard to load or airflow, at very low airflow we have a limited number of fuel and air molecules in the combustion chamber and as a result the combustion process is slower than what we see at high load (when there are a lot of fuel and air molecules tightly packed together). When the combustion process takes longer we need to start it earlier but as we increase airflow into the cylinder the combustion speeds up so we need to retard the timing in order to maintain peak cylinder pressure at the optimal point in the engine cycle.
Hi Andre, thanks for the answer. Okay so I have understood that at low load, there is a less concentrated mixture of air and fuel so since the flame travels slower, we would have to advance the timing. At higher RPM, the engine cycles are faster, so there is less time for a combustion process, hence, we would need to start it earlier.
However, at high RPM(where the engine cycles are faster), the load would be high too. So following the logic of retarding timing at high VE(high airflow/load) this is where you would be at high RPM, where it is advised to advance timing due to faster engine cycles. So at this point, is there a net advance or retard in timing?
Im assuming all the 'advances' and 'retards' in timing are with respect to the original ignition timings.
That's a good question. I imagine if you extrapolate the map past the 100% load for the same high rpm, you would see that the theory still holds true. Correct me if I'm missing something.