Medium Intake Manifold The Accord Euro R and '06+ Civic Si gets a manifold (called 'RBC') that flows much better than the factory K series manifold, another great OEM upgrade. For people that want a bit more air and a simpler install, there's units from Skunk2 and Xcessive. Bang: estimated +10hp Bucks: $250+
ECU Hondata's K-Pro programmable ECU is one up the best upgrades you can do to a K engine. Even on stock engines, people are reporting significant gains. Bang: estimated +15hp Bucks: $1000+
Camshafts There's quite a bit more of a selection of cams these days and your choice will depend on the engine size, purpose and goal. Popular choices include Blueprint Manufacturing, Brian Crower, Drag Cartel, IPS, R Crew Racing and Skunk2. Bang: estimated +10hp Bucks: $600+
K Series Wild Individual Throttle-Bodies Available kits for the K series include Clockwise Motion, Kinsler, Hayward Performance, Jenvey, Toda Racing and TWM. Clockwise Motion's kit is an interesting roller design, which is compact in design for cars with little hood clearance. Bang: estimated +10hp Bucks: $2000+
Turbo If you want a complete turbo kit, check out companies like Full Race, GReddy, HKS, PeakBoost and Rev Hard. Piecing together a kit might not save you money, but it will allow you to pick out your own parts, such as a custom manifold not offered in a kit. Bang: estimated +100hp Bucks: $3500+
Supercharger Depending on what chassis and which K series engine you have, these companies offer supercharger kits for various K-series applications; CT Engineering (Comptech), Jackson Racing and KraftWerks. Bang: estimated +100hp Bucks: $3000+