Ram air - Fitted standard on most modern superbikes ram-air uses the bike's speed to pressurise the air box forcing more air into the cylinders at high speed. After market ram air boxes can be fitted to fuel-injected as well as bikes with carbs, such as the R1, but re-jetting and carb adjustments will be required.
Cost e.g. R1 kit £500.00

Hyperchargers - Although not strictly ram air boxes, they can give good results on cruisers due to a much less restrictive and free-flowing design. When used with a good pipe, e.g. Vance and Hines, they can give considerable torque and hp gains.
Cost around £180 plus fitting.

Air Filtration - Ram air bikes ingest a considerable amount of muck due to their forward facing air intakes, (can Suzuki's SRAD really stand for Suzuki Rain And Dirt?), a high flow serviceable filter such as a K&N or DNA can be a sound investment. Alarmingly, many filters on ram air Kawasakis seem to stay clean, as they can lift at high speed and allow dirt straight into the engine! Kawasaki owners check in you airbox, a K&N could save your motor.

There's More…

Turbocharging - Can give huge power gains, but engines require a lower a compression ratio and stronger pistons when used at higher boost levels. A turbo is driven by waste exhaust gases and can be used with carbs, although fuel injection is a much better option. Older installations had the turbo mounted behind the head, drawing through a large single carb. The modern way is to use a blow-through set-up with the turbo mounted in front of the engine often using an intercooler to cool the charge on its way to the throttle bodies or a set of modified carbs. Cost e.g. GSXR1300 around £3-4,000.

Supercharging - This is an alternative way of gaining considerable increases in power from a large motorcycle engine. It is a mechanically driven pump that forces air at pressure into the cylinders. This method has become much more popular recently with the arrival of modern compact compressors. It has a number of advantages over turbos, such as no lag, more bottom and mid range power and no need for a complex custom exhaust system. Usually driven from the right side of the crankshaft via a chain or belt and mounted behind the cylinders they work especially well with modern fuel injected motors. Cost e.g. GSXR1000 around £3,000.