Jesus!...not another of Forchetto's long-winded and tedious write-ups...yes lads, get the popcorn out...
Removing the blowby gases from the crankcase is especially important on 360 degree engines as the simultaneous rise and fall of both pistons moves a huge amount of air within the crankcase. 180 degree engines on the other hand, tend to just move the air around the crankcase. Older bike engines had all sorts of weird and wonderful timed breather valves and oil traps, some so inefficient that makers directed the flow of oily gases to the drive chain and claimed it was "an automatic chain oiler".
Most would just let the gases spew towards the rear of the bike through a long hose pipe.
In the 70's emission laws came in banning open breathers so makers re-directed all the crap into the carburettors to be re-cycled and burned.
These blow-by gasses effectively lower the octane rating of the incoming mixture by diluting the charge, displacing oxygen and, being hot, also lower the charge density. Both efects tend to harm Torque/power output.
Oil mist carried in the gasses can also deteriorate the O2 sensors in the EFI bikes, and does no favours to things like the Air Intake Temperature and MAP sensors and other components in the throttle bodies.
Redirecting blow-by gasses away from the air box also helps keep carburettors or throttle bodies and other internals cleaner. Paper filter elements soon become contaminated with oil and moisture as a cursory examination inside the airbox will confirm. They're often found to be oily and/or damp inside.
Some systems are worse than others, ask the owner of an old Honda CB400F how often his air filter needs changing due to choking with oily damp gunge. On those the breather used to discharge straight onto the filter housing.
Most of today's airboxes incorporate a more or less effective coalescing filter arrangement to separate the bulk of oil mist and moisture from the gases, this muck collects in a blocked-off tube under the airbox. This has to be drained periodically by removing a bung. Have a look under your airbox to see it.
Doing away with this recycling of gases from the intake gives a consistent and measurable power gain, typically 1 to 1.5%. As an example on a 82-odd BHP Buell it has been found to give a nice and consistent 1 to 2 hp gain over a wide rpm range. The gain on a lower power Bonnie will be smaller but as a percentage it might be just over 0.5 bhp. On large-engined powerful cars as much as 5 hp has been seen.
Drag racers and other max power maniacs go all the way and even use a vacuum pump to evacuate the crankcase and reduce internal losses. Some people think that the suction inside the airbox does the same but the vacuum level at most speeds is far too low to have this effect.
Recycling the blow-by, the way the factory does, cost a little power over a wide range. A little bit of the power gains noted by our posters on removal of airbox are to be credited to the enforced elimination of the breather arrangements by removing the airbox.
I've done mine by replacing the pre-formed hose with a suitable 12 mm car hose formed into a "U" shape, some hose clips and a rubber-lined "P" clip to hold the filter on to one of the manifold bolts, have placed the small filter to stop muck getting in on full view, it's a nice chromed thing and adds a bit more bling as well as prompting questions from the general public: "What's that thing mister?". The hole that's left in the airbox can be blocked by a suitable rubber bung or screwing an old M8 bolt in there.
The easiest way would be to use the existing hose, disconnected from the airbox and fitted with a suitable filter on the end. This can be tucked in close to the airbox, on top of the crankcase.