VE Means Very Exciting
Once you’ve tuned a vehicle or two on a Volumetric Efficiency (VE)-based engine management system, you kind of get angry when you have to tune a non-VE-based system. VE-based systems allow faster tuning and do not require remapping for injector changes or fuel pump upgrades. Considering that insufficient fuel supply is the number one cause of incomplete tuning session, it’s nice to know that you do not have to remap the entire fuel table when you upgrade the injectors or fuel pumps. Instead, you just put the new injector data into the appropriate tables. If you change your fuel from gasoline to E85, you simply change the Fuel Density Reference, Fuel Density Temperature Coefficient and Fuel Stoichiometric ratio and all of the VE-based fuel tuning tables are left alone. VE-based engine management systems are the future. Essentially, VE marks the same leap in technology and tuner interface that was found between between DOS and Windows.
Most functions that an ECU makes to optimize engine operation are based on engine speed (RPM) and engine load. While engine speed is simply measured with the factory crankshaft position sensor, there are a number of ways to measure the load on the engine. The factory ECU relies mainly upon the Mass Air Flow (MAF) sensors to relay load information. On the contrary, most aftermarket ECUs use a Manifold Air Pressure (MAP) sensor plumbed in the intake manifold for the load measurement. In some applications, aftermarket ECUs can also be set up to measure load based on the readings from the Throttle Position Sensor (TPS). Each load measuring strategy has its advantages and disadvantages.
[pullquote]PROPERLY CALIBRATED, A MAF LOAD INPUT CAN PROVIDE A VERY ACCURATE PICTURE OF THE ACTUAL AMOUNT OF AIRFLOW[/pullquote]If the MAF sensor is properly calibrated, a MAF load input can provide a very accurate picture of the actual amount of airflow going into the engine. When modifications are made to the engine that result in small changes to airflow, the MAF load input still provides correct data to the ECU (provided the sensor isn’t maxed out). Weather changes are also well handled by MAF-based systems. However, MAF sensors can foul and they need to be recalibrated when the location of the sensor is moved or placed in a charge pipe or intake pipe of a different choke area. MAF sensors also have a limited range. Once this range is exceeded, the MAF is not able to tell the ECU that additional airflow/load is going into the engine. When larger camshafts are used on the vehicle and intake reversion is present, MAF sensors will provide inaccurate data to the ECU. In the case of a charge pipe blowing off, a MAF-based system will enter a limp mode or kill the engine as the airflow into the engine is no longer properly metered.
With a MAP sensor, you do not have to worry about high-horsepower levels going beyond the range of the sensor (unless you are running boost pressures beyond its preset range). High intake reversion from long duration camshafts doesn’t affect the MAP-based load measurement either. Charge pipes and the like can be designed without consideration of accurate sensor readings, like on a MAF based system. However, MAP-based load calculations always assume that the engine is making the same power at the same boost level. On the VR38DETT engine, there is also an issue with signal noise as the MAP sensor is only measuring three cylinders. Changes to atmospheric conditions (weather or altitude) are simply not handled well by MAP-based systems. Of course, a dedicated barometric pressure sensor can reduce the calculated error significantly.
As for TPS-based load measurement, the input to the ECU is extremely repeatable and predictable. On vehicles with individual throttle bodies, TPS-based load measurement is often the only way to get a consistent load value as MAF and MAP values go all over the place. When the engine is not in boost, throttle-based load measurements are generally more repeatable than MAP signals. In boost, TPS-based load measurements need to be augmented with additional information (i.e. boost pressure) in order to accurately predict engine load by way of modeling the manifold pressure. The result is a load signal without the signal noise of a MAP