Datalogging
Datalogging is a great tool for measuring the performance of your vehicle. It can be carried out in a number of different ways, either logging your existing ECU sensors, adding better sensors with higher resolution, or adding new sensors to measure parameters previously not covered. Most commonly, a wide band lambda is added to check the air/fuel mix, and accelerometers, tied in with throttle position and road speed can measure performance.
Many vehicles can output data direct from the ECU via the OBD2 protocol. An OBD2 interface can be added into the Log-chain to give calculated parameters used internally by the ECU so you can follow and record changes made according to the ECU image of the engine situation.