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Are you suggesting that the VBox Sport cannot, or is not very good at, estimating wheel power? The iPhone app Can calculate wheel power and estimate crank power from a coast down. The crank power will therefore clearly be an estimate. Interestingly, the PC software that reads the card cannot calculated power directly.
Wheel power estimate should be quite good. The device has a 3 axis MEMS accelerometer coupled to a 10Hz GPS. Using something like a Kahlman filter the inherent drift of the accelerometer is therefore cancelled. In this way accurate distance and velocity are obtained. In the app, a vehicle weight is entered (curb weight plus driver weight).
Since power is just the time derivative of kinetic energy and kinetic energy is half the mass multiplied by velocity squared, the absolute accuracy depends on the absolute mass and velocity but the relative error is just dependent on velocity errors. Unless the velocity error is large, which it is not, then the power vs speed should be quite relativity accurate.
Compared with a rolling dynamometer, there is an error associated with real world air drag, however, unless the drag changes dramatically during a run it itself is proportional to velocity squared and so easy to cancel out.
All this argument is also a bit moot. The absolute peak power from the estimate varies a bit from run to run, but the power from 2000-4500 rpm where everyone actually drives does not vary much at all. Most importantly it has not varied with any changes to the car. Finally, and least significantly in the absence of firm numerical data, the car feels only similar to two similar aged boxster base models I have driven at the same elevation (not the S as I have) and does not feel to have changed with modifications.
We shall see what the numbers and feel have to say after it gets back from the shop. The annoying thing is that I wanted to plot wheel power as a function of minor and major teaks. If there is indeed a limitation based on, for instance, fuel delivery then all improvements will instantly summed after that bottle neck is removed.
Philip
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