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Fuel flow rate through an injector is affected by a number of different things. This page focuses on the fact that even with nicely matched sets down to 2ms (0.002 seconds) pulsewidths, there can still be significant differences in your typical operating range.
Most injector testing places will not typically test below 4ms or if they do, certainly not below 2ms. For the most part, that's usually fine. Testing from 4ms up to 16ms (with a "typical" total time of 20ms) provides a pretty good idea of the general behavior of the injector. As the chart below illustrates, injectors are almost always very linear in this area.
The problem is that you may very well be spending a lot of time below that range. And having injectors that are matched well in that range means little about the behavior outside it.
For example, on a set of 1450 cc/min injectors running on gasoline with a 2.0L 4-cylinder engine, you'd expect to idle in the 1.6-1.7 ms range. And as you can see in the next section, the behavior of any given injector down in that range can be very different from the behavior in the 2ms range. It doesn't sound like much of a difference, but it can be as much as a 30-50% difference in some areas. That's huge! That's like having one cylinder running at 14.7:1 and another running at 9:1 with the same pulsewidth.
As an example, here's a chart collected from a set of FIC BM1450s. Notice how the set is very well matched above 2ms. But also notice how different one injector behaves below that point. The problem is that your typical idle (assuming gasoline and a 2.0L, 4-cylinder engine) starts around 30 cc/min on this chart. Cruise would be around 60 cc/min.
To be clear, this behavior isn't specific to the FIC BM1450s. Every injector set out there can have this problem to varying degrees. We're forcing these injectors to operate in an area where they were really not intended to produce precise flow. Basically, we're using the injectors in an application where they weren't designed to be used. So we have to pay a little extra attention to the details.