From what i gather from spending some time on the Turn 10 forums and looking through the Forza 3 forums scanning for information.

The PI (Power Index System) has been changed.

The PI is now worked out on a power,Speed,Handling and breaking ratio.
Every time the car is upgraded the game "test drives" it round a track,
This happens in a split second and tests the top end speed,Acceleration,Breaking and handling.
The power index will be worked out from these results.

This is to stop the "rocket cars" in Forza2 which were a low PI when bought,
But with upgrading and tuning could become devilishly fast whilst keeping within a A-S rating.

Faster cars with a High PI when bought,Would hit a U999 rating much quicker.
Taking them out of the PI class limited races.

Also added to the new PI system is a new set of ranks,The new confirmed ranks are as follows:

F Class ~ - 200
E Class 201 - 275
D Class 276 - 350
C Class 351 - 425
B Class 426 - 500
A Class 501 - 575
S Class 576 - 650

Which should allow for cars to be balanced against each other with ease.
the new PI ratings spaced 75 Pi points apart for F-S don't take into account the Racing class cars (R4,R3,R2,R1)
which are now rumoured to be achievable for any car.
After the S ranking is broken the car would enter Racing classes and then finally the U class.

These figures aren't official but logic would denote that they will be something like this:

R4 Class 651 - 725
R3 Class 726 - 800
R2 Class 801 - 875
R1 Class 876 - 950
U Class 951 - 999

making the full power index list as follows all classes separated by 75 PI points.


F Class ~ - 200
E Class 201 - 275
D Class 276 - 350
C Class 351 - 425
B Class 426 - 500
A Class 501 - 575
S Class 576 - 650
R4 Class 651 - 725
R3 Class 726 - 800
R2 Class 801 - 875
R1 Class 876 - 950
U Class 951 - 999

I'm looking forward to see what changes the New PI system and ratings bring to the racing and gameplay.
another 7 weeks and a few days and we'll all find out