The maximum dutycycle of an injector is important. When it becomes too high, heat problems might occur but more importantly the accuracy of the fueling will diminish A usual upper limit is around 80 to 85%.

The ECU has a possibility to up the maximum dutycycle of the injectors, to keep the fueling in order. If this change isn’t made, the injectors won’t be able to deliver enough fuel for a tuned engine (2.4 intake + good exhaust).

To test the actual dutycycle of the injectors, we connected a scope to my car.

The scope is connected to a notebook where the software shows the information. With special clamps the scope is connected to one of the injectors.

With a low engine load, the scope shows the following picture.

With high load and high RPM’s we got the results below.

According to the scope the dutycycle is around 93%. Repeating this test gave values between 90% and 93%. These values are only seen at high RPM, so the injectors aren’t run very long at these dutycycles.

In the piece I wrote on injectors there is information on the relation between dutycycle, injector size and maximum power.

Based on thie information the following formula can be used:

Where:

• SF == static injector flow, in this case 252cc/min
• D == duty cycle, in this case an average of 91%
• K == contant, this is 4.6 for NA and 5.6 for turbo and superchargers

Filling in the formula you get the following values: