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Estimated A/F ratio (AFRatioEst)

NOTE: This page is a little dated since the release of ECMLink (V3). Much of this information is still accurate, of course, but the specifics of what you enter into the screens is different because this page assumes a V2 application. A better reference for the V3 application can be found in the AFRatioEst section of this page.

The A/F Ratio display item (AFRatioEst) causes more confusion than any other. But it's really very simple. It's basically taking mass airflow data reported from the datalog and dividing it by mass fuel flow data to arrive at an estimated A/F ratio. To calculate mass fuel flow, the display item uses injector pulsewidth data from the datalog and the true (more on this below) flowrate of your injectors along with the specific gravity of the fuel you're running.

What's confusing many times is that you have to TELL the A/F Ratio display item the flowrate of your injectors. It doesn't know and can't assume that the global fuel scalar is correct. You configure the A/F Ratio display item the same way you configure other display items. Right click on the A/F Ratio label in the graph legend and select “Props”. Fuel specific gravity data will be available from your fuel supplier. C16 race fuel, for example, will have a very different specific gravity than regular Exxon pumpgas.


From the factory, manual transmission 1G DSMs came with a base fuel pressure of 37psi. However, most injectors are rated at 43.5psi, including those 450cc injectors installed on the 1G DSMs. So that 450cc/min rating is not the real flowrate into the engine. The actual flowrate is closer to 415cc/min:

450 * sqrt( 37 / 43.5 ) = 415

So, what does that mean? Well, because the estimated A/F Ratio display item must know the REAL flowrate of fuel into the engine, you have to provide this adjusted value. If you're not running a base pressure of 43.5psi, you can't use the “standard” published flowrate of your injectors (because that flowrate assumed 43.5psi). So on a 1G DSM, that means you would use 415 cc/min in the A/F Ratio display item's configuration if you were really running 37psi with “450” injectors.

You can still use 450 when calculating the global fuel slider with upgraded injectors at factory fuel pressure because the flowrate adjustment term just cancels out. But of course, if you change fuel pressure from stock, then you have to adjust the global fuel slider as well according to the instructions in the user's manual.

A few examples should help to make sure this is clear.

GFS = global fuel slider value
ISP = A/F Ratio injector size property value

Example 1: 1G MT, 37psi base fuel pressure, 450 injectors

GFS = 450 / 450 - 1.0 = 0%
ISP = 450 * sqrt (37/43.5) = 415

Example 2: 1G MT, 37psi base fuel pressure, 660 injectors

GFS = 450 / 660 - 1.0 = -32%
ISP = 660 * sqrt (37/43.5) = 609

Example 3: 1G MT, 50psi base fuel pressure, 660 injectors

GFS = 450 / 660 / sqrt(50/37) - 1.0 = -41%
ISP = 660 * sqrt (50/43.5) = 708
afratioitem.1236344639.txt.gz · Last modified: 2009/03/06 08:03 by twdorris