The K&N 57S-4000 is a 57i-series cold-air induction kit that replaces the restrictive factory air filter and air box lid. The manufacturer says its non-metallic rotationally molded tube reduces air temperature for improved performance. The kit includes a red cotton-gauze E-9289 replacement filter with a round outlet, so you get a washable, reusable element in place of the stock paper filter.
Compared with stock, the manufacturer guarantees a horsepower increase and claims improved torque and acceleration. Compared with a cheaper universal filter, the case rests on durability and serviceability: the manufacturer says the filter can run up to 100,000 miles before service is required (depending on driving conditions). Owners leaving reviews on the manufacturer's site report a straightforward fit, and one Focus diesel owner paired the intake with a remap and reported stronger pull. The warranty term is described differently across sources, so confirm current coverage with the manufacturer before buying.
Fitment is the first check to make. Application lists vary between sources, so verify your exact model, year, engine and market against the manufacturer's vehicle application list, which the kit references through its European TUV certification. Note that the kit is not legal for sale or use in California on any pollution controlled motor vehicle. For the install, the manufacturer notes that this performance air box is relatively easy to install into the air box base with commonly available tools.
The manufacturer states that the kit is not legal for sale or use in California on any pollution-controlled motor vehicle, so it is not an option for California-registered vehicles.
The manufacturer's specifications list no clamps and no couplers included, while mounting hardware is included. Plan to reuse the factory clamps or source your own before installing.
The manufacturer says the washable, reusable filter can last up to 100,000 miles before service is required, depending on driving conditions.
The manufacturer describes the install as relatively easy, with the new housing fitting into the existing air box base using commonly available tools, and it is typically completed in about 90 minutes. Mounting hardware is included, but clamps and couplers are not, so check your existing clamps for condition and fit before you begin. Confirm your vehicle is on the manufacturer's application list first, since a mismatch is the most likely reason for a fitment problem, and avoid using the kit on a California-registered vehicle.
