I recently logged for a short period using datamaster connected to my OBD1 coversion PCM in a 97 TA. When I looked at the log I noticed that the Duty cycle of the injectors on both banks reached 103.5% at 6200 RPM-6400 RPM @ WOT. The car made 380 at the wheels on the last dyno pull, I have a stock fuel system, with stock injectors with 106,000 miles on them.

What does this mean?
and do I need bigger injectors? My PCM was programmed by MTI using LT1 Edit.

What is the maximum reccomended Duty cycle? How does this effect performance?

What will larger injectors do for me other than ensure that a lean situation is avoided?


Reccomendations please...

 

If your BLM's are ok, (128), then you should be fine. My friends 93 Formula has his running on the NOS dry kit and his injectors run up as high as %120.

My 2 cents
Brian

 

Yes, my BLM's all seem to be 128 from what I have seen. Just wanted to make sure I wasn't looking at leaning out on the top end.

 

BLM's are simply a measure of how well the PCM is making fuel calculations at part load/part throttle (closed loop). There is no way BLM's can be used to judge the adequacy of injectors at max fuel demand/WOT. There is simply no feedback from the O2 sensors to provide any correction.

If the PCM is calculating injector pulse widths above the time available in two crank rotations, the injectors are way too small.
Duty cycle is the % of the available time in a 4-stroke cycle (2 crank rev's) that is being used to spray fuel. In practice, it is never a good idea to go over 80-90% duty cycle. As the injectors exceed 80-85% duty cycle, they are opening and closing so fast that they start to "flutter", reducing fuel flow dramatically. A very good way to lean out the engine, and do some serious damage.

I would first check the DataMaster duty cycle calculations. I have seen some data logs where the calculations made by DataMaster looked suspicious. I am not convinced it produces the correct numbers. As noted, the only way to tell if you are getting enough fuel is with a wide-band O2 sensor. Anything else is pure guessing.

My recommendation is always to calculate your required injector size at 80% duty cycle, and a reasonable BSFC (e.g. - 0.46 for N/A, 0.53 for power adders). That provides an injector that will not lean out on you at WOT/max fuel demand. Also keep in mind that a typical dyno pull only puts your injectors under max load for a second or two.... perhaps not enough for overheating and flow disruptions to become apparent. But run that same engine for a longer period of time with excessive duty cycle, and the injectors will die a premature death, and the engine may follow.

Remember... you can also increase injector flow rating, and reduce duty cycle by increasing fuel pressure.... you just need to be sure to change the flow constant in the PCM.