Sale ends todayGet 30% off any course (excluding packages)

Ends in --- --- ---

Trying to understand how I hit max duty cycle

General Tuning Discussion

Forum Posts



Tech Articles

Discuss all things tuning in this section. News, products, problems and results. 

= Resolved threads


Howdy, I'm just trying to build my intuition about fuelling and trying to wrap my head around the interplay of these items:

* Master Fuel

* Injector Dead Time

* Fuel Table Cell Values

* Actual Duty Cycle and how it's calculated

I've been tuning my RB25DET on a Link G4X, following the initial setup guide I trimmed master fuel until it was idling correctly, which landed me up at 14ms (felt higher than I expected, and online people seem to think between 8-12ms was the range). I also set the injector deadtimes based on the data I had on the injectors.

I have been tuning the values in the fuel table (MGP/RPM) and the max fuel input at current max load of 5500rpm/145kpa is 50. I got up to 6500rpm at 145kpa MGP, and hit max duty cycle. At that cell in the fuel table, the values are only 0.5 higher than the 5500rpm 145kpa cell, and I was expecting a duty cycle of around 60-70% at that point.

What could cause the duty cycle to spike to max given the same inputs from Master Fuel, Dead Time, and the fuel table's requested fuel? Does RPM factor into the equation for the injectors total open time somehow? As I understood it, the RPM just told the ECU where in the table it should look for the fuelling amount.

Thanks for any advice!

What fuel equation are you using? I'm guessing you're using Modeled Fuel Equation? Are you targeting a richer Lambda target in that particular cell?

And what capacity are your injectors? Are your injector size entered correctly in the Link G4x?


Duty Cycle is the percent of time the injector is open. The maximum available time decreases with RPM, so even if the injector open pulse width stayed the same, the duty cycle would increase with RPM. To convert from Engine Speed in Revolutions Per Minute, to time per cycle (ie, two revolutions of the engine), you divide 120 by the Engine Speed -- so for 6000 RPM, 120/6000 = 0.020 (20 milliseconds). If your injectors were open for 14ms at 6000 RPM, then you would be 14/20 = 0.70 or 70% duty cycle.

At 8000 RPM: 120 / 8000 = 0.015 (15 ms). If the injectors were open for the same 14ms, that would be 14 / 15 = 0.93 or 93.3% duty cycle!

Roughly the calculation for PW is Master x (MAP/100) x fuel table number. So for your 5500RPM example, PW would be 14ms x 1.45 x 50% = 10.15ms. This excludes small compensations such as IAT, target lambda & dead times etc. Cycle time at 5500RPM is 22ms so that is roughly 45% duty cycle.

At 6000 cycle time is 20ms so duty cycle would be a bit over 50%.

What does the log show? I have seen before a trigger error cause a short spike in RPM that will give you an artificially high duty cycle at that instant.