Getting the most out of stock fuel injectors:

The flow rating of a fuel injector is at a given base fuel pressure. You can increase or decrease fuel pressure, and hence injector flow rating. Obviously, you can’t expect a 260cc/min injector o be bale to flow 800cc/min with of fuel. The shortcoming ends up being the fuel pump’s ability to keep a certain pressure at a certain flow; as pressure increases the flow decreases. You can increase your fuel pressure as much your pump is able to handle under boost. Keep in mind that boost pressure is added on top of base fuel pressure, so 60PSI base at 18PSI boost will be 78PSI of total pressure. With stock injectors, the Walbro255 HP pump I had was able to handle almost 85PSI when being pushed to the limit. Your results may vary.

When in do the pressure increase, you will have to recalculate the K (as shown below) based on the injectors “new” flow rating. First, you have to calculate the increase in flow rate from the new pressure:

The square root of (new pressure/old pressure) * stock flow rating = new flow rating

Ö(60/37) * 260 = 331

New Flow rating = 331cc/min

Now that we have are new flow number, we can recalculate the K as if we had installed larger injectors. You can also play with increasing the TTPmax setting in order to raise the injector duty cycle limitation. You are also able to get 15-20% more flow by running the injectors up to 100%, but it is not recommended as injectors will get VERY hot when operating at that level. Proceed at your own risk.

Upgrading Fuel injectors:

The injector trim value, also known as the K value, has a big effect on how your cars engine operates. Finding the correct K value for a given injector/AFM/ECU combination can only be done accurately with a wideband oxygen sensor. You can closely estimate the K value after an injector change by using the following equation:

Stock K value = 025F Hex

025F Hex = 607 Dec

260cc * 607 K Value = 157820.

440cc (new injector size) * X (unknown K value) = 157820.

X = 358.68 Dec

In this case X is ~359 Dec or 0167 Hex; that is our new K Value for 440cc injectors.

The Void/Latency/blast-off time is just the correction factor for how long the injector takes to fully open it’s pintle. It also has an effect on AFR because it is directly added to the TP being used at any given time. Void time for larger injectors can be estimated by this example (taken from this article):

“75dec / 370cc = X / 555cc, X equates to 112dec or 70hex.

This formula for adjusting injector latency seems to give a bit too much fuel.

Through some experimentation I have found that taking the equated value (112), subtracting it by the stock value (112-75 = 37), dividing that value in half (37 / 2 = 18.5), adding that value to the stock value (18.5 + 75 = 93), usually gives a more accurate Latency value. In this case it would be 93dec or 5Dhex”

I installed 440cc/min low-impedance direct-fit fuel injectors. After setting the K value and void time using Live Edit (which I’m sure you’ve noticed is a nice feature), the AFR’s were off the chart rich under boost (I was reading mid 9’s). Undoubtedly the equation used to calculate the K value in Live Edit is very conservative in the rich direction, for safety. I re-calculated for a 480cc/min injector and started from there, yet still well on the rich side. After some time I was able to continue to reduce the K value until I had the fuel maps reading fairly accurate to the actual AFR. My K value was around 0120 Hex and void time of 5B Hex, which might or might not do you any good at all, even if you have the same size fuel injectors. Just follow this simple procedure of reducing your K value based on the flow rates of your injectors, from there the only way is to collect data and make changes accordingly.

These are third gear pulls showing AFR’s at 16PSI boost pressure with the stock MAF.

With K value tuned by live edit for 480cc injectors (note that actual injector size is 440cc):

With K value further reduced, maps altered to reflect 12.0 AFR across the board, after much trial and error:

Notice how the AFR heads towards lean from about 11.7 to 12.2? This is not so great. The AFR should really look about like that, only flipped and headed in the other direction; richening up instead of leaning out.

The problem lies in the fact that at 16PSI boost pressure, these fuel values are the very right hand column on the map because of the limitations of the stock AFM setup (it's well over what people would consider "maxed out"). The AFM can meter more air (output higher voltage), but the ECU won’t see anything over ~5.12V.

Unfortunately when your AFM/ECU combination is over it’s maximum metering ability, you will lose precision in tuning because you have to set all the values for more airflow than the ECU actually thinks it is getting. Because the ECU cannot see the difference in airflow between 13PSI and 16PSI (with my turbo, in both cases it only sees the maximum readable voltage), you have to change only the very highest edge of the map to reflect the changes you would have made to a column representing your highest boost pressure; in this case 16PSI. Then you adjust injector duty cycle in the end column, except entirely based on RPM because you have no more map to work with. In theory this is not a sound method, but in practice it does in fact work quite well, up to a certain point.

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