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An engine torque curve is available for most vehicles, but it does not contain enough information to completely describe the engine in Vortex Studio. For example, most curves do not include torque and throttle positions other than 100%. The other necessary information can be extrapolated based on some reasonable assumptions.
The torque curve provided for the vehicle generally only includes the torque at full throttle between idle and maximum RPM.
Since a vehicle can also be operated at lower throttle positions, those torques must be provided. Every engine has different throttle performance, and the throttle system itself might have a particular scaling. However, the general shape of the 100% curves below is usually similar for all engines. At low RPM, the low throttle curves will follow the torque of the full throttle curve, but at higher RPM will decrease and approach 0 torque. This behavior, captured on a spreadsheet, can be used to generate a more complete engine torque table.
A vehicle must also function correctly below idle speed, since it can be in this range when starting or when the torque converter pushes it below idle. Little is known about behavior in this region, so the curve is just linearly extended to 0 RPM. In a real engine, the torque would reach 0 torque (and stall) at some RPM above 0 but since the starter system is not modeled in Vortex Studio, the engine torque has to always be positive to allow the engine to spin up when initially started.
Finally, a curve is needed for 0 throttle. It is expected that there is no positive torque when the throttle is at 0, but an engine does generate a positive torque below idle speed. A default value for this part of the curve is also provided by the spreadsheet, but it can be tuned to change the behavior of the vehicle creeping. When the transmission is engaged, the vehicle will generally roll forward slowly, which puts some load on the engine and pushes the RPM below idle.
An important concept relates to where the throttle curve reaches 0 torque; for example, at 25% throttle, the engine will not produce any torque above 1700 RPM. This is the point where the engine will not try to spin faster at a given throttle position. If the engine speed is raised above this speed (which can only happen when decelerating or driving down a slope), it is overspeeding, and will produce a negative torque. This engine braking amount is usually not known for a vehicle, but the general shape can be derived based on the size of the engine, and is calculated in a table.