So before you can test the modifications you have made, you need to decide what modifications to make, and how to make them.
So that I don't get people's noses out of joint (because whatever aero mod I pick, they'll think it's criticism directed at them), let me give an example from a different area of car modification.
When I decided to turbo my Gen 1 Honda Insight, I first looked at what others had done. There were then only a handful of Gen 1 Insights that had been turbo'd (there probably still are), and there were photos around of all the installations.
I remember one in particular, because as I looked at it, it became my role model in how
not to do it. Nearly every single aspect that I could see was poorly done, expect for one. (That one was the turbo choice, that looked fine.)
It was intercooled, but the intercooler was
behind the radiator. The intake to the turbo was picking up hot air - definitely not good on a turbo car - and the engine management mod was, even then, decades out of date. (It used an extra injector.) It had also, not surprisingly, experienced an engine failure through a melted piston. (Oddly, this rather important fact was almost always brushed under the carpet.)
Now the person who had done the mods must have done them that way for a reason - and as far as I could see, it was because those ways were easy. So why hadn't he mounted the intercooler in front of the radiator? As I looked at my car in more and more detail, it became obvious that it was darn near impossible to fit an intercooler in front of the radiator - and also get the plumbing to and from it. So I needed to go water/air for my intercooling. (It's much easier to run small diameter hoses to a front extra radiator.)
Using an extra injector is a really poor way of doing things, so I needed bigger injectors. Now, what injectors fitted? Someone on the same discussion group described some Toyota ones that dropped straight in - and their sizing looked fine to my eyes, so I took that information and used it. (And it proved to be good information.)
To make the piping runs neat, and not pick up hot engine bay air, a new airbox would be needed. I looked and looked for a replacement from another car, but in the end gave up and made my own.
So I was looking at what others had done, but with a critical and assessing eye.
(Incidentally, the person who had done what I thought was a really poor job in turbocharging his Insight was very popular on the group, and in fact was regarded by many as an expert.)
Here's what the engine bay looked like when I was finished (the looped EGT cable on the strut tower was just temporary). In the 6 years since I finished it, there has been only one issue - a leaking fuel hose clamp. And it has done plenty of hard driving, including about 100 full load runs to the redline on my home dyno (I bought the dyno after I'd finished the Honda). Other than the new fuel hose clamp, I have done literally nothing to the engine other than oil changes.
Now there are literally hundreds and hundreds of neat aftermarket turbo installations that are of decent quality - mine is nothing outstanding in that context. That is not the point I am trying to make.
What I am saying is that in any car modification, look at what others have done
but also ask questions.
Questions like:
- Why did you take this particular approach?
- What other options did you consider?
- Was the modification successful? (Or if you want to be more tactful: how successful has the modification been?)
- How do you know that?
...and then ask yourself,
how relevant is this person's car to mine? Is it identical, similar, or in fact nothing like my car?
This last question is critical in all car modification (including aerodynamic modification).
The turbo that works on a 1 litre, low power Insight engine will not be the right turbo for a 1 litre, high performance motorcycle.
The rear diffuser angle that is best for a sedan is likely to not be best for a wagon. (Happy to cite evidence for that statement, if anyone wants it.)
Covering the wheels on your car may make a significant drag difference, or basically none at all. It depends on the car and wheels.
For drag reduction, fitting a front air dam may be better than a full undertray - or , it may be significantly worse. Your choice depends on the car (and also the facilities you have, your budget and time).
In no area of car modification - none at all - are rules of thumb of much use. Cars vary too much in their dynamic behaviour to be so neatly categorised. It doesn't matter if it's suspension, camshaft design, intake modification or aerodynamics, if that 'rule' has not been tested on the car you are driving, it's very likely incorrect to a greater or lesser degree.
So look around with a skeptical, questioning eye that seeks evidence rather than pat panaceas. And if that evidence isn't proven on your car, look even harder at what people are saying.