Master Fuel after changing injectors

Hi guys. I am little confused as a stock ca18det injector is 370cc low impedance and its on 14ms master fuel on link ecu

which means if i change to 700cc injectors high impedance, it would be around 7.4ms, am i right?

But if i were to change it to 7.4ms, my afr reading is too lean.

may i know whats happening?

@hachoo,

Mathematically yes you're correct, with the increase in injector size and decrease in master fuel values.

However, unfortunately it doesn't quite work like this and the engine will need to be retuned.

Things like Short pulse width adder, min effective pulse width, flow rate vs pressure, fuel system type etc aren't taken into account during "traditional" mode in the Link which will be why the engine is not running right.

Injectors are not linear, so with only having a dead time value, a cc rating and tuned in traditional mode you won't be to just change the master fuel value without retuning the engine.

I have attached below the core courses below, the EFI tuning fundamentals course has a great section on injector characterization.

https://www.hpacademy.com/courses/efi-tuning/#core-courses

does it mean that i should actually stick at 14ms and adjust the fuel table accordingly?

Yuan,

You want to set the master fuel value at a point which gives best resolution in the fuel table.

There is a great webinar covering this below

https://www.hpacademy.com/previous-webinars/011-master-injector-pulsewidth-fuel-table-resolution/

You have to choose a value of master fuel that in the middle of the fuel table you will have 50% of fuel.

my car was tuned by some other tuner last time. it is on a ca18det. based on the map, it is at 14ms with a 690cc low impedance denso injector previously. but, i realised my injector resistor box is not there anymore, so i changed the injectors to a unknown brand 700cc high impedance injector. may i know what i must do?

do i change the pulse width? or based on 14ms, i change the fuel table again? because after changing the injectors, afr is around 21

If you have changed to another injector brand etc you will need to re-tune the fuel table. Because you're tuning in traditional mode which doesn't work on creating a fuel model based on fuel mass, just changing the injector data will not work.

You basically need to re-tune your whole fuel table and set your master fuel number at a value which will give you good resolution and values of around 50 in the middle of the fuel table.

Thanks chris. Will go ahead to retune the fuel table

after changing from denso 690cc low impedance injector (peak and hold) without resistor pack

to a 700cc high impedance (saturated) injectors, apparently at 14ms which is stock in the base map, i am getting 21 afr.

what should i do? I can say from a 690cc injector to a 700cc, it usually doesnt change that much. Am I right?

Unfortunately it doesn't work like that.

Although the flow rate is similar, it doesn't mean the injectors operate the same throughout its range. The biggest difference will be at low injector pulse widths.

As mentioned earlier, unless you need to go from a tune file which had the accurate data such as short pulse width adders etc then swapping injectors with another set and the same accurate data.

Basically what I am is saying, unless you have either:

A: Tuned the car in fuel model with ACCURATE and complete injector data (dead times, short pulse width, flow vs pressure, pulse width limit etc)

or

B: Replacing the injector with the EXACT same injector, same brand, same part number, same flow.

If you haven't done either of these then you need to retune the fuel table.

but what happened is i am on Link G4 storm without injector resistor pack. previously it was on a 690cc low impedance injector. i am afraid it might fry the ecu, and i have changed the injectors.

If you're on a G4 Strom and running low impedance injectors you need to fit resistor packs or you can burn out the injector drivers in the ecu.

Or change to a high impedance injector and re-tune the engine.