725 injector dynamics

[evilmiata]

hey Matt, Ben! im taking that suggestion on the injector dynamics 725cc's i know ill need to change the Req' fuel setting, but what else needs to be changed switching from low to high impedance, and is this a good time to switch to high res?

[ben91]

When you change the injector size, MegaSquirt can handle most of the adjustments with just one variable, REQ_FUEL, which sets the base pulse width. MegaTune can calculate this value automatically if you click the Required Fuel button on the Engine Constants page, where you just need to enter your engine size and injector size, as well as 12.7 for the air/fuel ratio. Using that A/F ratio will reduce the amount of tuning needed when you change injectors significantly as that’s what was used for the req_fuel calculation on the base map with stock injectors. Yes we know the MegaManual advises uses 14.7 for gasoline as the stoich ratio, please note that, within reason, it doesn't matter and there was a reason we chose to use a different AFR here. If you're wanting to scale for different injectors with minimal re-tuning then use the AFR we recommend. If you are going to fully retune anyways then feel free to use 14.7 as the AFR. It won't effect your ability to tune your car properly at all. It will effectively scale your entire VE table a bit.


Here are the settings we used on the base map to come up with a REQ_FUEL of 13.4:
http://www.megasquirtpnp.com/models/mm9093/mspnp_mm9093_manual/images/mm9093_req_fuel.jpg

There are a few settings which do not adjust when you change REQ_FUEL as they are static pulsewidth (PW) settings, so you will need to adjust these by hand. The acceleration enrichment settings will probably need to be retuned for the new injectors. Also, larger injectors require shorter cranking pulse widths. If you double your injector size, you will need approximately half the cranking pulse width. As a general rule, it’s easier to fine tune the cranking pulse widths by setting them to a lower value than expected, and then increase this in 0.1-0.2 millisecond increments until the engine starts. This will make it less likely that you flood the engine.


For both the cranking PW and Accel Enrichment PW you can do much of the adjustments using math—then do the rest via fine tuning.
For example—you need to first subtract the injector opening time from your current numbers. Injector opening time by default is set to 1.0ms. So if your Cranking PW at 100degF is 4ms, subtract 1.0ms from that and you have 3ms. Now, that that 3ms and divide it by the percentage of the size difference of the new injectors compared to the old. If you had 20lb stock injectors, and you’re going to 42lb injectors, you’ll need about ½ the cranking PW minus the opening time. So in this case, ½ that 3.0ms is 1.5ms. Add that 1.0ms injector opening time back and you’ve got 2.5ms. Your old cranking PW at 100degF was 4ms, your new time at the same temp is 2.5ms. This should be pretty close to what’s needed. Do this for all cells in your cranking PW and Accel Enrichments and you’ll be close with only fine tuning remaining.

[evilmiata]

hey Matt, Ben! im taking that suggestion on the injector dynamics 725cc's , but what else needs to be changed switchingi know ill need to change the Req' fuel setting from low to high impedance, and is this a good time to switch to high res?

quit speed reading & cut & pasting .

[Matt Cramer]

Right now the only way we recommend using low impedance injectors with the stock wiring is to splice in an injector resistor pack. This requires no settings change.

Hi res certainly wouldn't hurt with those monsters...

[evilmiata]

Thanks Matt, I have a 90-93 MSPNP, I thought it was low imp. Ive always been running 2nd gen DSM's which are low imp. so your saying i shouldn't have to change anything but the settings for the size(Req.Fuel)?

[ben91]

You will need to adjust the req fuel as well as the other parameters outlined in the bottom 2 paragraphs of my original reply.