Forum » General Tuning Discussion » Idle AFR too rich but can't reduce injector pw

Idle AFR too rich but can't reduce injector pw

General Tuning Discussion

Discuss all things tuning in this section. News, products, problems and results. 

Page 1


I'm curious how you guys deal with situations where large injectors are fitted to an engine and the minimum pw that you are capable of running produces an afr that is far to rich.

For example I am tuning an sr20det fitted with older style 1000cc injectors on an aem v1. Fairly old equipment but this what the customer's budget allows. At idle with the final injector pw at around 1.3ms the engine still runs at 10.9-11 afr. Obviously this is not ideal and the spark plugs are having trouble coping with this. Below 1.3ms the injectors become erratic and afr is uncontrollable.

I'm contemplating lowering the base fuel pressure down to around 30 psi from 43.5. Any suggestions would be great.

Out of interest are the injectors being controlled sequentially or batch fire? Batch fire can be problematic with larger injectors on a small engine like this - not that 1000 cc could be considered large these days.

I guess the most obvious option would be to fit a better set of injectors that offer better control and linearity at short pulse widths. If that isn't an option then you could try dropping the fuel pressure but often that will cause it's own problems such as poor atomisation of the fuel. Another alternative would be to try a higher idle speed - Not ideal but it's an option.


The issue that you will be having is that you are trying to drive the injectors at a pulse width that is most likely within the dead band of the injectors. This is the point where the latency of the coils to the pulse injection from the ECU has been used up, and the coil is just starting to the drive the injector pintle off of it's seat and open the injector to allow fuel flow. If this is not correctly modelled in the ECU, then the ECU will be requesting a pulse width time that has a portion of latency in the time, meaning that the injection of fuel is inconsistent.

I have attached a screen grab of a injector calibration that shows the volume injected (ul) (x axis) vs. pulse width (ms) for varying battery voltages (8, 10, 12, 14, 15 top to bottom) at 400kPa fuel rail pressure. The injector is a fairly commonly used 1000cc unit, and as can be seen, has a considerable variance in the volume of fuel delivered.

Attached Files