I have the pipercross viper induction kit for my cooper.
Felt a good difference with it. Its not loud either until you hit the hammer and a few revs count up and then you can hear it.
The best filter that was ever made was the JCW Open Cone Filter as used on the Challenge cars. It was made by Pipercross. If you can get hold of one of them somehow your laughing. Other than that it's probably the viper.
After several filters, K&N replacement, Pipercross, K&N typhoon, it's defo the AFE CAI, which I've had the last year now. Open air with heat shield and gives me extra HP's (confirmed on dyno)
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