Here is my easily reversed air box mod on a stock 240 B230F non-turbo...
(modify as needed for B21, etc)
Materials: New or used good exhaust manifold pre-heat flex DUCT/HOSE just like on the car.
1. Remove and save: fresh air box thermo and flap (snaps out/in), mass air meter screens (2) and stock air filter.
2. Disconnect the pre-heat duct from the heat collector at the exhaust manifold. Re-route this end under the fan shroud to a point just above and behind the front air dam near the RF corner of the car - secure.
3. Connect one end of the new/used duct to the heat collector and follow the stock route to the fresh air box but do not connect - just dummy under the fan shroud and secure.
4. Re-install stock air filter or aftermarket as desired (if re-usable aftermarket filter do not over-oil).
5. Inspect the ENTIRE intake system for leaks and repair as needed (including the flex hose between the mass air meter and throttle body).
Now the fresh air box has TWO sources of cool air - the original snorkel and the duct at the air dam.
Next would be a free-flow exhaust and a test of the cat for no flow restrictions and proper Ox readings at the tailpipe.