[mid_life_crisis]

Originally Posted by Kodename47

As @moto-mike said, its not so much the percentage as the IPW or injection time. On my SC car I run a lower percent PI than stock yet my PI injection time is higher than stock. Think of it like this, 20% of the fuel at 12:1 AFR will be using the injectors less than 20% for 11:1 AFR. Now, let's put aside boost and needing both sets of injectors to fulfill the actual fuelling requirements, if we assume that a certain amount of PI may be required to give a homogeneous mixture, does that sit as a percentage of the mix or as a certain volume of fuel? Again, I understand that this may be over simplifying.
Having gone an re-read all the old posts on here, there must be a reason why running a 20:80 split was chosen. I have seen little benefit of leaning heavily on the DI on a NA car. I have taken Mike's advice though on my car, running up to 6ms DI and then having it pegged there until redline seems to.work nicely.

I can see that. Increasing the PI pulse length would increase the amount of fuel per injector cycle. I wish I had a better understanding of how the ECU tries to meet the percentage target. With all those other tables to reference, what exactly is it doing by way of reading what table and then physically activating injectors, cycling pump valves, etc. What does it take to satisfy it that the target percentage was met? If you change a pulse length, does it take that into account? Times like this you really want the programming code.
From what I've read, you want some fuel hitting the back of the intake valves. This not only helps keep them clean, but I bet it has a cooling effect as well. In a high rpm, high compression motor, you need to consider the health of the valves. This could explain the PI in the upper rpms when the DI can handle it.
As far as the benefit of DI in an NA motor, again, high revs, high compression = high heat. The DI has a definite cooling effect, which not only allows for more aggressive tuning, but could also contribute to the long term health of things like valves and piston tops. They don't spray the tops of the pistons just to create a particular combustion pattern. Although that is extremely important, it also has to have a cooling effect on the piston top. It may be small, but every little bit helps.