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injdata-lowpwvariance

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Injector Data - Low PW Variance

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.

Overview

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 total time of 20ms, for example) 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.

Click to enlarge

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 10:1 with the same pulsewidth.

Data

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 2.0L, 4-cylinder engine) starts around 30 cc/min on this chart. Cruise would be around 60 cc/min.

Low PW Variance
Click to enlarge

This behavior isn't specific to the FIC BM1450s. I'm sure every injector set out there can have this problem. We're operating the injector down in an area where it's really not expected to produce precise flow.

injdata-lowpwvariance.1323869883.txt.gz · Last modified: 2011/12/14 08:38 by twdorris