RallyX and WRC are very different. If you compare FIA appendix J art. 279 (RX) and art. 255 (WRC) then you have your good night reading in store for a good while.

They do share some things (except for the looks).
- Both categories must use a standard engine block as base (in WRC they use standard head too).
- The WRC cars are homologated while the RX supercars have to be recognizable with a standard car regularly on sale (can of course use a FIA homologated car).
- Both WRC and RX supercars have to be based on a 4 seat, closed non-convertible type road legal car.
- Both use front and rear differentials all which have to be mechanical (Supercars can have hydraulic central diff of a non-adjustable type)

A few major differences:

WRC
The WRC cars have much less freedom on the technical side. The regulations states that:

- Up to 1.6 litre 4 cylindre turbocharged petrol engine with minumum weight
- Direct injection
- Max boost of 2.5 bar absoulte
- Max rev limit 8500 rpm
- 33mm restrictor (meaning the engines will produce 'around' 300 BHP)
- 1200 kg minimum weight
- 1820mm maximum width
- A regulated position for the centre of gravity
- Road legal with lights, number plates and so on (ask Gronholm in 2003 Rally GB if all that was necessary)

and the list goes on forever.

RALLYX supercars
- Up to 2.058 litre supercharged 4 cylinder petrol engine (2.33 litres for diesel).
-The block must derive from the same reg. trademarks original block as the cars original bodywork (If car looks like Citroen then block needs to be Citroen)
- 45mm restrictor (power around 600 BHP)
- Minimum car weight depending on engine size
- Bigger brakes than WRC cars if one wants, adjustable brake balance in-car
- Free rim size (max 18")
- Cars must have sprung suspension, other than that it's free (you can use hydropneumatic suspension if you like - cool!)
- Free positioning of water radiator(s) (eg. mounted in the rear)

The RALLYX cars have much less regulations stipulating what can and can't be done. And you have to bear in mind the fact that RX have to withstand a certain amount of bashing. So many differences between the two categories comes from that

Note when I say "free" I mean the regulations are free

Feel free to fill in here and of course expressing your fury over my so called knowledge.