If you're going to use the filter on an e36 - 325 or 328, you should also look at taking out the small flat plastic tube that's in the standard air box. It's a bit restrictive and is there only to lower the noise levels. And once you do do it, it sounds so much better. Some guys also recommend taking off the splash guard thats right infront of the standard intake, but in Malaysia, with our heavy rains, not a good idea me thinks.
Having said that i use the Venom version of the Viper filter. Sounds positively mad since i run it without the cold air intake. And before you all start flaming me about not using the CAI, if you look at the piece supplied, it's smaller than the OEM one and without it, my car sounds so much better. Having said that, i belong to the club that feels at speed, there is so much air coming into the engine you don't really need a CAI. Also, when you are reving 6k+ your engine needs so much air that any CAI will be restrictive.
Don't believe me? Anybody have a standard 328 they wanna "test" against mine? :nyehehe:
Lastly, i usually only put on the Venom when i have to go up to Penang. When i am doing my usual daily rounds through traffic, i am on the standard box with a K&N. ( which mind you, you should not over oil as the oil will bugger up your MAF, just a thin coat and let it seep in)
My next upgrade will be a Dave F heat shield for an open Pod K&N filter with 2 CAI's! That i feel will sound super, not have any heat soak problems and not be restrictive at high revs.