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Fuel injector static flow test variations

An explaination of why fuel injection service shops report different, (sometimes very different) static flow test results when testing the same injector.
At first glance one would think that it should be a non-issue since you are just turning on the injector for a certain length time and measuring the volume. Everyone should get the close to the same results... Right? Things are not always as easy as they seem.
There are quite a few things that can cause both large and not so large errors when testing. If more that one error is occurring and if they are in the same direction, the total error can be substantial. It is not uncommon to see a 10% difference between one shop's readings and another.

In this discussion, we will only concern ourselves with the static volume measurements of fuel injectors and the error sources encountered.
Dynamic testing has even more things to contend with.

All the information here are the professional opinion of WitchHunter Performance and is believed to be accurate. The owner of WitchHunter Performance has 20 years experience as an Electronic Design Engineer and over 5 years experience running a Metrology Lab in the past before starting this endeavor.

Listed below are the some main reasons for flow variations in order of error. There are other things that affect accuracy, but lets concern ourselves with the top 6.


Test fluid type

The differences in the flow test fluid is where the largest variations can occur, ideally we want a fluid that will flow the same as gasoline and is safe to use.

It appears that a majority of injector shops use Heptane N since is is available at any chemical supply house and is low cost, in fact, one of the injector flow testing machine manufacturer recommends to use it.
The problem is that Heptane N flows about 5% higher than gasoline and is nonlinear with injector size.

Our fluid is much closer to that of gasoline when kept at our target temperature.


Machine error

A little know fact is that one of the major fuel injector test machines has been sold with a firmware version that has a 3.5% timing error.
That means that all flow test results of these particular machines are 3.5% high.

We have electronic test equipment in house and evaluate all of our injector test equipment before purchasing. Most other shops have no way of knowing if their equipment is accurate or not.

Test fluid temperature

Variation of the the test fluid temperature can cause up to 2 % change in readings, depending on what fluid is being used. Even gasoline flow is affected by temperature changes.

WitchHunter continuously pumps the fluid out of their machines, filters it through a 40 micron and then a 2 micron filter, the fluid is then heated or cooled as needed to within 1 degree C of our target temperature.


Test fluid contamination

The test fluid gets contaminated with gasoline, cleaning fluid, and whatever contaminates that used injectors have in them.

Our fluid is continuously filtered and the specific gravity tested twice a week. Our test fluid is changed on a regular schedule regardless of testing.

Poor measuring techniques

The title speaks for itself... Example: measuring a 1600 cc injector with the standard test 130 cc graduated cylinder.
The above example would require a very short test times (less than 5 seconds) and any errors would be exaggerated by the large multiplier needed to compute the cc's per minute value. Also using these short of test times, there isn't enough time to perform fine tuning the test pressure during the actual test.

WitchHunter Performance has various sized graduated cylinders for each task.

Test machine limitations

Some of the flow test machines only have a limited choice of flow test times in the static test mode. Some only have a 30 second test available.

The stock liquid filled pressure gauges can't be zeroed for changing barometric pressures.

Our PC controlled machines have the capability of testing at any time inputted, The addition of digital pressure gauges are more accurate and can be zeroed at any time.


It's not rocket science.... But some shops can't get it right.