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Minimum Injector Pulsewidth Calibration

Josh Burt

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The injector dead time table is for just that, injector dead time. Even with that calibrated properly injectors still have an area at low pulsewidths where they can become extremely non linear in flow. http://www.injectordynamics.com/LowPulseTech.html For example, in the link above, the first graph shows flow vs. actual PW (which accounts for deadtime) with those injectors I wouldn't want to idle below say 1.2ms so I stay out of the "knee" area of the plot.

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Having a low pulse width lockout may not be very practical as far as tuning goes.  It would mean that there would be a minimum amount of fuel that you could give the engine and hence may not be able to get it lean enough when needed.  It is possible to operate the injectors in the non linear part to get the mixtures where you need to but it is usually best not to go to pulse widths that short (eg between 1 to 2 ms).

It is up to the tuner to know what the pulse width of their injectors are at idle.  Here are some points:

  1. If your pulse widths are too short at idle then your injectors are too big (or fuel pressure too high etc..)
  2. The pulse width is always the same for a given number in the fuel table.  So, it is not like your pulse width will mysteriously get too small if was OK before.  Of course you need to work out what the minimum pulse width will be after all fuel trims are applied. ie IAT and ECT corrections.
  3. Injector dead time is not related to the non linear part of the flow.  It is a voltage/mechanical time correction to compensate for the physical time it takes to move the solenoid valve.  Don't use it as a minimum pulse width setting or voltage compensation will be wrong.
  4. The opening (and closing) time and flow characteristics depend a little on if the injector is wired with ballast resistors.

Note that that link you refer to shows the effect of injector dead time.  Also note that the injection is close enough to linear to tune with between 1 to 2 ms.  Also, the figures shown for that injector may be completely different to those used in your car.

Just do a check and make sure that you know what your minimum pulse width will actually be and then confirm with your injector characteristics that you are well in the usable operating range.

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  • 4 weeks later...

Ashley, Thanks for the response. There are many solutions to the problem, I figured a min pulsewidth cal is an easy implementation. The problem was I didn't have the flow vs. pw data for the injectors that I was running. I ended up purchasing the injectors shown in the link which are linear down to 1.2ms. Idle is silky smooth now at a 1.7ms pw. Yes you can go through the math and make sure your fuel map and comps don't ever get you below a certain pw, but that is a bit of a pain. And yes like you said, if you get into a condition say a heat soaked idle, you might run real rich with a min pw cal. In some cases I would rather have a pig rich idle than a stumbly lean misfire idle. Also something I don't need at the moment, but is a nice feature a competitor's ecu has is the ability to take your target afr table and put zeros in cells that you want to be open loop in. Thanks staff for your replies, I don't want to come off like an ungrateful cry baby in these threads :) So far this ecu has gone above and beyond my expectations. Bang for the buck, it doesn't get any better.

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Thanks Josh. I noticed the forums have stopped allowing lines between the text. It got upgraded the other day... I think you can put a zero in the table to disable the closed loop AFR anyways but I would have to check as I cant quite remember.

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