Power Flow Kit Backfire Replacement Filter Twin Air Replaces the stock air-restricting backfire screen and comes standard with a backfire filter. 2-Stroke Kits: Include a rigid, lightweight billet aluminum cage that provides a tight seal to the air box; Replaces flimsy plastic stock cage. 4-Stroke Kits: Include a rigid, lightweight, billet aluminum cage with a special backfire-resistant layer of foam laminated to the inside of the filter