ECMTuning - wiki

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<h1>Estimated A/F ratio</h1> <div class="level1"> <p> The A/F Ratio display item causes more confusion than any other. But it's really very simple. It's basically taking <u>mass</u> airflow data reported from the datalog and dividing it by <u>mass</u> fuel flow data. The calculate <u>mass</u> fuel flow, the display item uses injector pulsewidth data from the datalog and the flowrate of your injectors along with the specific gravity of the fuel you're running. </p> <p> What's confusing many times is that you have to <b>TELL</b> 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. </p> </div> <h2>IMPORTANT 1G NOTE</h2> <div class="level2"> <p> From the factory, manual transmission 1G DSMs came with a base fuel pressure of 37psi. The ECU, of course, knew this and calculated fuel delivery based on the assumption that you were running 450cc injectors at 37psi. Most injectors are rated at 43.5psi, however. Including those 450cc injectors installed on the 1G DSMs. So that 450cc/min rating is not the actual flowrate into the engine. The actual flowrate is closer to 415cc/min: </p> <pre class="code">450 * sqrt( 37 / 43.5 ) = 415 </pre><p> </p> <p> 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. </p> <p> 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. </p> <p> A few examples should help to make sure this is clear. </p> <pre class="code">GFS = global fuel slider value ISP = A/F Ratio injector size property value </pre><p> </p> <p> <b>Example 1:</b> 1G MT, 37psi base fuel pressure, 450 injectors </p> <pre class="code">GFS = 450 / 450 - 1.0 = 0% ISP = 450 * sqrt (37/43.5) = 415 </pre><p> </p> <p> <b>Example 2:</b> 1G MT, 37psi base fuel pressure, 660 injectors </p> <pre class="code">GFS = 450 / 660 - 1.0 = -32% ISP = 660 * sqrt (37/43.5) = 609 </pre><p> </p> <p> <b>Example 3:</b> 1G MT, 50psi base fuel pressure, 660 injectors </p> <pre class="code">GFS = 450 / 660 / sqrt(50/37) - 1.0 = -41% ISP = 660 * sqrt (50/43.5) = 708 </pre><p> </p> </div>