So, this has been wracking my brain for a while.
As the title says, this toy line specifically involved race cars, I believe mostly of the rally car variety, that all could turn into a flight mode.
The doors flipped out to allow for the wings to be attached and the hood also changed shape somewhat.
I think they had flip out weapons when the hood changed as well.

The car I remember having was primarily purple.

I'd provide a pic for reference, but I can't find any online anywhere.
Hance this thread.

They also included mini comics.

 

Hot Wheels Key Force?

 

I definitely don't remember those. They look kinda cool tho.

 

I remember these. Might still have that motorbike somewhere. The transformations we're done with a key. Pretty neat toys.

 

Sadly that is not it.

 

You

I have a feeling you're not thinking about M.A.S.K, but it's still worth a shot  


EDIT: It appears there's a ripoff version called Vor-Tech, see if this hits your target:

 

Negative.
The line in question was strictly race cars.

 

Nascar racers?

 

What scale / size? Any die-cast parts?

 

Can't speak for scale but they weren't small.
No die-cast that I know of.

 

So..... a 10" car or a 5" car?

 

I don't know. It was the early 2000s. I wasn't really concerned with measuring it.
As I said, not small. Exact size I can't tell you.

 

Vortech isn't a ripoff btw, it was made by kenner, the retoold a bunch of mask molds for it.

 

Maybe Hot Wheels Shogun Racers?

 

Man that has to be it  

2001 black holed this toy line and Nascar Racers at the time for obvious reasons...

 

Oh my god! That's it!
There's that very same purple car!

Seems I was way off with the "all had flight modes bit".
But I only ever had the purple one.
Also completely forgot it was Hot Wheels.

Anyway, thank you!