• Universal kits are just kits with the basic components to get you started on your own custom project. Universal kits are usually purchased by mechanics or engineers who want to piece together their own kit, or who already have other components required to complete a kit. Just like engine specific kits, the universal kits use the EXTREME X1, X2, X3, and X4 filters. • Engine specific kits are complete, or mostly complete, engine specific kits. They also include the Extreme X1, X2, X3, and X4 filter family. • We can build you an engine specific kit too. Use the "build me a kit" service to have our teach gather all components required to build your kit.