Tuners: Test Your Injectors!


Opening Time (OT) or latency is measured as the difference of the start of the current rise on channel #1 to the “knee” and coinciding 1st oscillation on channel #2. The Closing Time (CT) or latency is the difference of the Current off line on channel #1 to the 1st downward closing oscillation on channel #2. It is easiest to hit the hold button on the scope once you have a clean sample to measure these 4 events from. Once you have the OT and CT times, subtract CT from OT to get an overall or net latency time for the conditions you are currently testing under, which can vary with changes in voltage or pressure, etc. It must be noted that the actual flow during OT and CT can’t be assessed from the physical latency times alone, but basically there will be a deficit of flow during the OT that is partially offset by a small amount of additional flow gained during the CT. The actual flow deficit during OT and CT is better assessed by the dynamic flow latency test described later. Net Latency is typically calibrated in the ECU as a set of pulse width offsets vs. injector voltage, to be added (it’s always an offset, not a coefficient) to the final pulse width.


Tuners: Test Your Injectors!
I am speechless that this sort of stuff can be sold!


For the actual flow tests, there are many ways to run these tests and J1832 only makes suggestions for the sake of accuracy, but is vague on prescribing exact finite test cycles, so you will need to find a work path that fits best for accuracy in the attributes you are interested in. That said, for static flow tests, simply turn on the timer for 60 seconds of static flow, that’s about as fancy as it gets. The dynamic flow testing is where things get a bit more muddled. My preference for the dynamic flow test is to do a 50% duty cycle of 5ms of on time and 5ms of off time for a total run time of 120 seconds (which equals 60sec.of total on time). This requires you to set the signal generator to 100Hz (confirm this on your scope as 10ms of period) and run it thru the timer set to 120 seconds of test time.


Tuners: Test Your Injectors!
Opening Time Latency (OT) can be seen in both waveforms as the  “Knee” on channel #1 and as the 1st up cycle on channel #2. The Closing Time Latency (CT) can only be seen on channel #2 as the 1st down cycle after power is shut off.


For consistency, run this test twice per injector, the 1st run is to normalize the injector; the 2nd run will be used to get the Dynamic flow value. Now if you want to make a simple correlation between the physical latency and the flow latency as a percentage of the 5ms dynamic flow time, first divide the total physical latency (in milliseconds) by .05 (Example: if total latency is .96ms then .96/.05=19.2%). To get the dynamic flow latency as a percentage of static flow (so it can be compared to the physical latency as a %), subtract from 1 the dynamic flow quantity (5ms PW at 50%D.C. for 120sec.), divided by the static flow quantity (60sec at 100%D.C.) and multiply the total by 100 or: Dynamic Flow Latency %=100(1-(dynamic flow / static flow)).

Again, there are many ways to set up your testing, but this is a step I like to do that normalizes latency to a percent that can be compared quickly as a datum value. Flow latency is typically a few % higher than Physical latency, differences higher than 5% may be associated with injector flow non-linearity issues at other pulse widths, warranting further testing.

The rig as described up to this point, will at least get you a good idea of how much fuel is flowing and when it is flowing, but won’t tell you anything about where it is flowing. That is the subject of SAE J2715 (Port Injector Spray Measurement), so if you want to do more than spraying into a “patternator” (it’s like a glorified ice cube tray) for spray pattern measurement, you need this paper. If not, then at least inspect every injector spray head under a microscope. Late model multi-hole injectors incorporate angle cut holes for very precise vectoring to avoid wall wetting on the ports. If this type of injector has been modified by drilling out the spray holes, chances are it’s no longer vectored to the optimized spray pattern. This isn’t much of a problem at WOT, but can make tuning for sharp throttle response a whole bunch trickier. Spray vectoring is used to control cone angle, cone separation angle, off axis angle, and cone shape.


Tuners: Test Your Injectors!
Modern Spray plates are precision vectored to optimize atomization and avoid port wall wetting. Drilling out this spray plate would destroy this beautiful  “Mirrored D” shaped vectoring! (Nissan VK56 injector by Denso.)


So that’s the basics, other than finding a coefficient you will use to convert gram weight into a value that accurately relates to a particular flow rating system used to identify OEM type injectors you are involved with and to use the same on injectors you are testing.  As an example, 1.333 * Grams of Mineral Spirits flowed at 68F is a good number that correlates to the middle of a large sampling of OEM injectors rated in cc/min. Start with 1.333 for a coefficient, keeping a box of “calibration injectors” locked away, retest them on a regular basis, and write down the date and flow data each time you flow them for consistency in your testing over time. A good practice is to flow a “calibration injector” at the beginning of daily testing and at the end of testing. If you are working with other groups that are also flow testing injectors, it’s a good idea to send them a few of your “calibration injectors” to flow on their rig, so you both are at least on the same page.

Safe Sex! Ok, bad metaphor, but if you have a good reputation as a tuner and you continue to just assume what’s up with the injectors when you tune, well then, not such a bad metaphor after all!


Order SAE J1832:http://standards.sae.org/j1832_200102/


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