Why do you convert the P38 air box to the Classic air box?

We recommend changing to the Classic Range Rover style air filter box as it is made from metal and far more robust than the plastic air filter box. Therefore if you were to encounter a backfire (say due to a deteriorated spark plug) then the metal air filter box will cope with this far better than the plastic air filter box which is prone to cracking in this instance.

The modification that you can do to the plastic air filter box (if you can not find a metal classic box) is to leave the metal clips off of the box and to install springs between the top and bottom section so as to allow the top to lift up if pressure was to build up in the induction pipe.