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I can endlessly describe why a template-based approach is rubbish in car aero modifications - and I have done so previously - but here's one straightforward example.
1. You want flow to stay attached to the sides/top/bottom of a boat-tail extension.
2. Flow attachment is heavily dependent on boundary layer thickness - the thicker it is, the more easily separation will occur.
3. Boundary layer thickness varies, depending on the car and the surface roughness / change of shape / length of body ahead of the boat-tail.
4. So when shaping a boat-tail, how can a predetermined shape (a template) take into account the variation in boundary layer thicknesses between different cars?
Answer: it can't.
A simple low drag shape (incidentally, one of many that have been produced) has been extrapolated into purported car modification uses which are quite absurd.
Unfortunately, believing that a template can be used for all sorts of car modifications like....
Guide the shape of extensions to cars, to guide the heights of spoilers, to assess the 'aerodynamic purity' of shapes, to determine where there will be attached and separated flow etc, etc
... then has flow-on impacts on a whole range of aero understandings. That is, get the basics wrong and then so much that follows is also wrong.
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