Setting the PWM Criteria

To tune the PWM [pulse width modulation] values for your engine, you need to know what kind of injectors you have: low impedance or high impedance.

* If you are running high-impedance injectors (greater than 10 Ohms), then set the:
o PWM Time Threshold to 25.4 msec, and the
o PWM Current Limit (%) to 100%.

In essence you are disabling the PWM mode. This allows full voltage to the injectors throughout the pulse widths.

* For low-impedance injectors (less than 3 Ohms), you need to limit the current to avoid over heating the injectors. To do this, there is a period of time that you apply full battery voltage [peak] current, then switch over to a lower current-averaged [hold] current, i.e. peak and hold. Alternatively, you can add resistors in series with the injectors. See the Injectors and Fuel Supply section of this manual for more details.

To run low-impedance injectors with the PWM current limit mode, you need to set two parameters - the "PWM Current Limit (%)" and the "PWM Time Threshold (ms)" - both are on the “Constants” page. The current limit % is the percent duty cycle when the current limit is invoked. The time threshold is the amount of time from when the injector is first opened until the current limit is activated.

On the Settings/Constants page:

If you are running high-impedance injectors (greater than 10 Ohms), then set the:

* PWM Time Threshold to 25.5 msec, and the
* PWM Current Limit (%) to 100%.

If you have low impedance injectors (less than 4 Ohms), set the:

* PWM Time Threshold is the time required to open the injector, this is when full power is used to open the injector. Start at 1.0 msec, and
* PWM Current Limit is the Pulse Width thats used after the Time Threshold, this actually pulses the injectors on and off, keeping the injector from over heating at high duty cycles. Try starting at around 75% (30% if you have installed the 'Flyback Board' daughter card or if using a V3.0 pcb with the Flyback components installed).

You will tune these after getting the engine running. See “Setting the PWM Criteria” in this section. Failure to perform these steps can result in damage to your injectors. If you have high-impedance injectors, set these values to 25.5 ms and 100%, and you do not need to tune them further.

Required Fuel – (Req_Fuel) this is top field of the Constants window. It has a calculation dialog to help you find an appropriate value. It should contain the injector pulse width, in milliseconds, required to supply the fuel for a single injection event at stoichiometric combustion and 100% volumetric efficiency.
Ensure that the NUMBER of CYLINDERS has been set before using the wizard!

In order to come up with this value, MegaTune provides a calculator that will suffice for 99% of applications (those for which it will not work generally require changes to the MegaSquirt controller code itself, and that is beyond the scope of this manual). To use the wizard, click on the Required Fuel button, and fill in the fields (Engine Displacement, Number of cylinders, Injector flow, and Air:Fuel ratio(14.7), then click 'Okay').

For a 4-stroke, a complete stroke cycle is 720 degrees of crankshaft rotation (i.e. two revolutions); for a 2-stroke, it is 360 degrees (this is also factored in the REQ_FUEL value down loaded to MegaSquirt).

In the tuning software, the upper REQ_FUEL box is the amount per cylinder, as noted above. The lower REQ_FUEL box is the value down loaded to MegaSquirt. It is the REQ_FUEL number on top, but scaled by your selected injection mode (number of squirts and alternate/simultaneous), this can be ignored as it is the upper number thats the important value.

For example, if you inject simultaneous and one injection, and have the same number of injectors as cylinders [i.e. port injection], then REQ_FUEL on the bottom is the same as REQ_FUEL on top. Same with alternate and two squirts. If you put in simultaneous and two squirts, then REQ_FUEL is divided in half - because you squirt twice, you need to inject 1/2 the fuel on each shot.

Note: if you choose alternating for port injection, make sure your number of squirts is an even number (2,4,...) and evenly divisible into the number of cylinders. For example, with an eight cylinder engine, you could use alternating and 2, 4, or 8 squirts/cycle. With a six cylinder, if you choose alternating, you MUST use 2 or 6 squirts/cycle. Also, the only possible combinations for an odd-cylinder count engine are either 1 squirt/simultaneous or N squirt/simultaneous combination, where N is the number of cylinders."