K&N in 2004 Prius
Took a look inside the air box of my Prius and I don't think the K&N filter would be of any use. There are 2 filters involved. The primary filter is in the lower half, left side of the box. It is a normal pleated type filter. However, there is also a finer meshed fabric type filter in the top half of the box. This secondary filter appears designed to trap even finer particles of dust than the pleated filter. Because the physical size of both filters is the same, replacing the pleated filter alone with the K&N would not improve air flow at all unless the top filter was also replaced with a less restrictive filter or eliminated altogether. I think Toyota engineered a little filter over-kill here in the interest of engine longevity. Everything about the design of the car is starting to look like they want this to last a VERY long time, probably to reinforce the durability of the hybrid system. I think it best not to modify anything involved with the HSD system...Toyota engineers have done an excellent job already. Wash the car, wax it, change the oil and filter, drive it, and LOVE IT!