Does anybody have a better idea of how the ecu determines when to switch on the secondary injectors?

I know there is the staging rpm that is set with the rtek but what other conditions need to be met to turn on the secondaries. When tuning for 1600cc injectors its hard to eliminate the stumble around the transition point without leaning it out.

Perhaps maybe in the next update there could be a choice between an intelligent logic for the transition or just a flat logic of if above this rpm then secondaries on.

 

This is why we don't recommend running pri/sec injectors that are very different. It's nearly impossible to get the transition right because of the way the ECU controls the injectors. The fact that you are running only 550 primaries with 1600 secondaries is the worst case scenario. It's going to be basically impossible to tune the stumble out.

The problem doens't have to do with the transition logic, but a hardware limitation of the ECU. The S4 ECU controls the injectors using only one pulsewidth signal. That signal goes to just the primaries or both the primaries and secondaries. It cannot send different pulsewidths to the primary and secondaries. So since the stock injectors are both 550, when the secondaries turn on, doubling the fuel delivery, the ECU cuts the pulsewidth in half which effectively does not change the fuel delivery but increases headroom 100%.

Now you have 550/1600cc. The problem here is that halving the pulsewidth when the secondaries come on, even with halving the pulsewidth, the secondaries are flowing over 3 times the amount of fuel that you need. Running the 550/720 injector preset will help a little, but theres no way around this without bringing the pri and sec injectors closer to the same size.

 

That makes sense. I tweaked it a bit more and when rolling into the boost from below the transition is just kicks for a second versus just sticking at that rpm and dropping boost. I am getting 720cc/1000cc injectors next week so this won't be much of an issue anymore