Speed density operation is an alternative to using a mass airflow (MAF) sensor to determine airflow. In speed density operation, airflow is estimated primarily from engine displacement, rpm and intake manifold pressure.
The key to an accurate estimate is knowing the engine's volumetric efficiency (VE) at each combination of manifold pressure and rpm. VE varies from engine to engine depending on the various modifications present, but most will follow the same general pattern, so the default SD VE table settings will usually be close enough to allow the engine to run reasonably well.
VE is the proportion of air volume that engine actually flows relative to the engine's displacement. Although some engines have a VE slightly greater than 100% under some conditions where intake manifold resonance is used to pump air into the cylinders, DSMs generally peak at about 100% at around 5000 rpm, or perhaps at a bit higher rpm when using a sheet-metal intake manifold. VE will generally drop at lower rpm and lower intake manifold pressure.
In addition to displacement, intake manifold pressure and rpm, air temperature and coolant temperature also have some effect, but the effect is limited somewhat because the engine operates in a rather limited range of absolute temperature compared to the other variables.
MORE READING: More details on all this can be found on the ECMTuning wiki:
http://www.ecmtuning.com/wiki/sd101
Make sure one of the supported MAP sensors is installed and ECMLink is configured on the ECU Inputs tab to use that sensor on whichever ECU input the sensor is installed.
If the car is presently configured with a GM MAF sensor (as opposed to a Mitsubish MAF sensor), a GM or AEM IAT sensor can also be installed on the ECU's intake temperature input. If no IAT sensor is installed, speed density can still be dialed in and used reasonably well as long as the intercooler is keeping the air entering the intake manifold somewhere around 50-90 degrees F. To dial speed density in without an IAT sensor installed, adjust the SDTempWeighting table to ignore the IAT sensor.
MORE READING: More information on how to setup ECMLink for speed density operation can be found on the ECMTuning wiki:
http://www.ecmtuning.com/wiki/sdsetup
If the car is presently running off of a MAF sensor that is producing accurate airflow readings, a log can be captured that will help with dialing in the VE table prior to switching to speed density operation. By capturing SDRatio in logs with the engine operating over a wide range of operating conditions, the ECMLink app's SD VE Adjust (SDRatio) function will provide VE table element-value suggestions that should produce airflow readings close to those produced by the MAF sensor. Make sure that SDRatio's engine displacement property is set correctly.
If the car is not presently running off of a MAF sensor, skip the SD VE Adjust (SDRatio) step and just use the default VE table settings. They can be tweaked later by observing CombinedFT.
Once an initial VE table has been established, switch to speed density mode by selecting Speed Density as the base MAF on the MAF Comp tab (obviously the car won't actually be running on a MAF sensor at this point; ECMLink will be using speed density to simulate a MAF sensor's output). Make sure that Engine Displacement on the Speed Density tab is set correctly. Zero the MAF compensation sliders and the global MAF compensation value. Double check that the MAP sensor is installed and configured properly, and make sure the IAT sensor (if present) is installed and connected to the ECU's intake temperature input. At this point the engine should be able to run quite well without the MAF sensor.
Further automated tweaking of the VE table can now be done based on CombinedFT instead of SDRatio. Simply select SD VE Adjust (CombinedFT) instead of SD VE Adjust (SDRatio). Final tweaking can be done by hand to get the most stable CombinedFT and to get measured wideband A/F ratio to match the values dictated by the ECU's open-loop fuel table.
MORE READING: More details on the SD VE Adjust tools can be found on the ECMTuning wiki:
http://www.ecmtuning.com/wiki/sdveadjsdratio
http://www.ecmtuning.com/wiki/sdveadjcomft
As airflow increases, more weight is typically given to intake temperature and less to coolant temperature when determining air density for speed density operation. Higher values mean more weight given to intake temperature. A value of 0 means intake temperature will be ignored (air density will be based solely on coolant temps). Likewise, a value of 1 means coolant temperature will be ignored.
In order to use a stock MAF sensor in conjunction with the SD VE Adjust (SDRatio) tool, you must basically disable speed density's use of the air temperature sensor for airflow calculation. You do that by setting the entire SDTempWeighting table (found on the Direct Access tab) to zero across the board
If you notice any hesitation on rapid throttle blips once speed density operation has been activated, you may want to adjust the values found in the BaseTipInTPSAdj table on the Direct Access tab. To properly use this table, you'll need to add TPSDelta to your datalog. This value will indicate which index in the BaseTipInTPSAdj table is being used at any given point in time. Typically, you'll want to slightly increase the values in this table.
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