The Fuji Racing Coolant modification for the Subaru EJ Series Engines was designed to reduce the coolant passage ‘dead spot’ which is located over cylinder 4. Basically, this kit allows the coolant to travel through the cylinder 4 passage and recirculate around the cooling system. What this does is balance out the cylinder temperatures which in turn allows for a more equal fuel and timing map on the car So now that all 4 cylinders are running at similar temperatures this will help reduce cylinder knock & detonation. This kit is Dyno Tested and proven to work !!