Empirical Injector Timing?

[quan3165]

I am trying to make sense of the injector timing tables I see in the MS PNP Pro Base Maps. These injector timing maps seem to be well thought out judging from the range of values in the table, but don't jive with any of the explanations I have read, which make sense, i.e. squirt complete before inlet valve opens plus a few degrees. These values seem to have the injector open while the inlet valve is also open (probably most fuel squirted while valve is on the way down/closing). I imagine these values were found on a dyno or some other controlled environment. Can anyone help me understand why this works at these values?

Attached is a screenshot from the 99-00 Miata base map (no VVT).

[Matt Cramer]

The tuner who worked on that table no longer works here, so I'm not entirely sure what settings were tried and found not to work. Remember that the injector timing table works exactly the same way as the ignition timing table. So at idle, the injection event is finishing almost at the same time the ignition is firing. This table ended up being for the most part the same as our 1994-1995 Miata timing table, but it needed earlier injection timing at idle - here's the '94-'95 timing for comparison:

1995 Miata injector timing.png

The reason appears to relate to the changes in the cylinder head design. Here's a cross section with the '94-'96 head at the top and the '99 and later head on the bottom:



Neither one has injector targeting that's exactly pointed straight at the intake valve; the fuel is hitting the side of the runner in both cases. However, the later head has much better targeting.

So, what's going on? I'm not 100% sure without sticking a high speed camera into an intake port. My guess is that the turbulence that happens when the valve snaps shut and the incoming charge bounces off the valve helps atomize the fuel. But as is often the case when one is tuning something empirically, sometimes you have to set the numbers first and figure out what's going on later.

[quan3165]

Very helpful, thank you. It appears trying to use any sort of calculated values won't work well for all but engines with injectors pointed directly at the valve, and in close proximity.