First I must say I am a long time Uni filter user as they were the only filter that didn't kill my horsepower on monster power hill climbs. However I believe they are about the biggest POS ever made in respect to the cheap glue they use on the outer foam. It simply falls apart as in gets De-laminated for no good reason. I have had almost new filter after filter thrown in the trash. I even tried cleaning it in cold water with dawn and the glue failed. They make other dirt bike filters that do not use the worthless glue that comes on the WR450.
I even called them and asked if they would please use other glue that works. Well they sent me a couple of filters with their cleaner and oil and said to use it. They still fell apart too soon.
So I have been on a mission not to loose the power of my built 450 to a lessor filter but I knew the Uni had to go. So I guinea pigged about all the air filters that I found on the internet. Unfortunately for me it was the last filter ordered that did the job. However I am still happy due to my findings. Bigger filter = more power.
The filters we use are shared with the WR250's. Now does this mean they put a 450 air filter in a 250. Ah no. It appears we end up with a power robbing 250 filter in a 450.
Oh and by the way remove the power robbing back fire screen from the cage. I use the billet ones with no back fire screens or carefully cut out the screen from the stock cage. There was also a 250 cage that came without the screen.
On to what works: PC Racing Flo air filter Mod. PCF1X. It is Hugh at first I thought that the larger filter might not breath too well as it could be too close to the air box. Well that was not the case. More power everywhere down low mid range and especially top end. So I should also say my bike is built and the air box is opened everywhere I could, well it could be opened way further on the sides which I don't do due to water issues. The top is opened everywhere I could and four 1" uni vents service on both sides so eight total. I also run the smaller Li po battery for more air room on top.
Below are pictures of the filters some are quite puny.