1 Wrench: Very easy installation either requiring no tools or minimal tools. Easy access to components. Can usually be completed within 15 minutes. (AP, shift knobs, post maf intakes, oil caps, battery tie downs, air filters)
2 Wrench: Relatively easy installation requiring basic hand tools. Moderate access to components with slightly more effort involved. Can usually be completed within 30 minutes. (short shifters, intakes, airbox, crank pulley, heat shield, catback, exhaust hangers)
3 Wrench: Moderate installation requiring more hand tools and more steps. Access to components can be more difficult and may involve requiring the entire car to be raised. Can usually be completed within 60 to 90 minutes. (downpipe, turboback, shifter bushings, uppipe)
4 Wrench: Significant installation effort. Special tools may be required. Modifications to stock components may be required. Installation time exceeds two hours. (MS3 FMIC, EVO X turbo kit).