Falling Injector Duty - Experts Please Help!

[Hiflow Tony]

Hi everyone,

As the title suggests I have a strange issue with injector duty cycle.

I use an M84 on my Mazda RX7 Turbo with great success. I recently ran in a new engine and decided to up the boost and revs and see how she’d go!

It felt great except when I pulled the log I was stunned to see the injector duty cycle start to plummet at 7500rpm!! The resulting lambda is way to lean for my combo.

I cannot see why this is happening. My tune looks good. I can’t find any strange compensations that would cause this to happen. Help!!

I am running 2 x 750cc low impedance Siemmens injectors for primaries and 4 x 870cc low impedance Siemmens for secondaries. I am running two injectors per secondary output.

I have attached my log file only as I cannot seem to attach the tune file???

I emailed Motec directly a couple of times but have had no response?!

Any help would be much appreciated.

Cheers,

Tony - Brisbane, Australia

[David Ferguson]

I don't think your duty cycle is actually going down starting at 7500, more likely it starts dropping after you close the throttle. You are only logging the "Fuel Inj Duty Prim" and "Fuel Inj Duty Sec" at 1 hz. If you toggle the display trace style (press the S key while in the time/distance graph) you will see the actual data points. The graph is interpolating between 7500 RPM/100% throttle, and 6500 RPM/2% throttle.

I think the reason your mixture goes lean is due to the loss of fuel pressure. It's 397 kPa at 5500 and only 319 kPa at 7500 (2 sec later). The Fuel Pres channel is also only being logged at 1 hz, you might want to bump that up to 20hz while diagnosing the problem.

BTW - your ECU config is contained in the log file (use Tools->View Device Config... to launch M84 ECU manager with your file open). I see that you have fuel pressure comp trim of about +15 % over the pressure change noted above -- I think you need to increase that (or fix your fuel system to maintain the flow/pressure required!)

One last thought -- you have alternator control turning off the alternator at full throttle, but you aren't logging ECU battery voltage -- are you sure that the battery voltage isn't dropping rapidly causing both the fuel pump and the injectors to work harder.... What happens if you leave the alternator charging?

[Hiflow Tony]

Hi David,

Thanks heaps for the reply. I can now see how my logging at different rates could give this result. I was confused as to why the lambda had not leaned out even worse when looking at the duty cycles alone and had already noticed the fall in fuel pressure.
I did not think that I had any fuel compensations active? Where do I find that in the software?
Also I was not aware of the alternator control. Again, where do I find it in the software?

Thanks again,

Tony.

[David Ferguson]

I may have made a mistake with your ECU config. Normally this is contained in the logged data file (but if you exported only subset, then perhaps not), but the more I look at it, it seems like it's actually one of my configs (unless you have a Rover K-series in your RX-7!).

So my comments about your compensations & Alternator control is probably incorrect. Sorry.

You will find the fuel pressure compensation in "Adjust->Fuel->Compensations->Fuel Pres Comp".

The Alternator control (if you're doing this) is found in "Adjust->Auxillary Output Functions->Aux Out X - Alternator"

[Hiflow Tony]

Hi David,

Phew. That's what I thought!!

Now back to sorting my fuel pump.

Thanks again.

Cheers,

Tony.