Adding an intercooler to the engine should give an increase in power, the simple theory being a cooler air is denser so more can be forced into the cylinders, this is especially applicable to a turboed engine as the air is heated by the turbo as well as compressed. The intercooler is basically a radiator but has hot air flowing through it instead of water, it is plumbed in between the outlet on the turbo and the plenum chamber.

The plenum chamber will need modifying to fit the necessary pipework as its inlet neck normally points downwards. You can either do this the crude way by sawing the neck off at the elbow, remember the plenum is made of alloy so welding is difficult. Or you can get hold of a plenum chamber from a MG Montego or Maestro Turbo, both used S.U. carb and will bolt right on, the difference being the neck points horizontally enabling the intercooler fitment.

The other option is to make or have made a custom plenum where you can have the neck point anyway you like.

For the intercooler itself you need to find a small one as there isn't much room under the Mini bonnet. Two popular choices are from Renault 5 Turbos or Ford Escort Cosworths. The picture below is a Ford one that we are using.

The Renault intercooler has both the inlet an the outlet on the top so maybe easier to install depending on where you want to put it. Of course the ultimate is a custom made alloy one if your budget will allow.

Then its just a case of connecting the pipes. Its best to use metal pipe for as much as possible as it wont expand, use silicone hoses for the corners- remember the air is under pressure!