Just to second what Dave said, its re-oiling them that causes the problems. Its not an exact science of how much to put on, just common sense. I had a K&N 57i on my 146 which was tucked well out of the way of the engine and with a decent cold air feed. Along with some other mods it makes sense. No point sucking in all that air if its going to struggle getting gasses out at the other end. After modding my 146 it maybe contributed to no more than 3/5 extra bhp (and that's being optimistic). Induction kits make more sense on bigger/high tuned engines which are packing more ponies. A decent panel filter in my opinion would be more practical. Ive just fitted a BMC filter to my 147......although I do miss the induction roar.
SO MANY ROADS