2001-2005 Audi R8

Images copyright Patrick Michl, Michael J. Fuller, and Audi Sport Press
Text copyright Michael J. Fuller

The most substantial modification to the 2001 R8 was the revised air management of the rear fender/diffuser/radiator inlet area.  Note the large fender louvers and double set of dive planes. 
The fender forms a pontoon connected to the sidepod.  Air exiting the underbody can travel freely into the radiator intake or down the side of the car.
Looking back into the diffuser.
Modifications for the 2004 season were in response to rules changes to the LMP900 class in light of the regulations for the new LMP1/2 category.  At the rear of the car is a rear wing span reduction, from approximately 2000 mm to a maximum of 1800 mm.  This reduction in rear downforce would need to be rebalanced by modifications made to the front end of the car.  In that light, the '04 Audi R8 appeared with a redesign of the trailing edge of the front fender.  The front diffuser exit dimension is critical to its function and moving the trailing edge of the fender rearward would effectively reduce the size of the diffuser's exit and therefore rebalance the loss of downforce caused by shortening the span of the rear wing. 
2003/2004 ALMS:
2004 LMES/Le Mans:
The trailing edge of the new front fender comes much further reward closing the gap between the fender itself and the leading edge of the radiator inlet.  Note how the fender is slab sided, nearly flush to the side of the car, and with a very sharp trailing edge.
Looking forward into the front diffuser, note the bodywork offset created by the differing planform shape of the new pontoon fender compared to the old fender.
The bodywork offset is even more apparent looking down.

ęCopyright 2005, Michael J. Fuller