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Old 07-28-2021, 07:00 AM
Luce Luce is offline
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Join Date: Aug 2010
Cobra Make, Engine: Lone Star with IRS, 427W with megasquirt, T56 magnum
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Good read Ed.

I have mine running well beyond acceptable, so didn't bother going this deep, but I remember one method of calculating "dead time" or all of the dynamic variable you mentioned, involved running the engine in different pulse patterns where you found the correct milliseconds for idle with 1 squirt per cycle and milliseconds with 2 or 4 squirts per cycle. Of course, the engine should need the same amount of fuel, so if the sum of the 4 squirts is different from the 1 squirt you can back into a theoretical dead time.

Dead time is the injector constant megasquirt uses to assume the time needed to open the injector minus time needed to close the injector plus the time needed for the fuel to actually start flowing. It ends up being pretty close to a constant number if the injections are not at a resonant frequency (spring/mass oscillation) of the fluctuations in fuel pressure.

Different ECU's allow different back EMF bleed voltages that effect pintle closing speeds, as might the size and geometry of the fuel rail effect just after open flow rates, more so with batch fire, so published data is of limited use. (If you want to get into the weeds about this, consider the injector electrically is a coil, and takes volts over time to build the field that opens it, and also has to dissipate this energy to collapse the field to allow the spring to close it. If you just disconnect the power without a controlled bleed, you'll have a spark, and that's not OK.)

Your second link is a great illustration of the "dead time" the non linear kink for the most part can be baked into the fuel table cake if you're willing to take the backyard hack rout and tune it in. But the closer your characteristics are modeled into the fueling equations the more accurate and predictable your adjustments are across the board.

I've seen quite a few useful megasquirt videos of pro tuners going through driveability issues like accel enrichment and low speed timing. It's really interesting how you can use data logging and see what causes things.

Tuning for WOT top end is easy. It's the idle, off idle and transitions that take understanding of what's going on.

My real world experience has been limited to the Cobra and a 383 SBC with 8 stack where a little bit of idle lope is accepted and maybe even expected, but stalling or stumbling certainly is not.

Last edited by Luce; 07-28-2021 at 07:02 AM.. Reason: spelling
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