| The purpose of an engine's crankcase ventilation system is to remove combustion gases that make it past the piston rings in order to prevent a build-up of pressure in the engine crankcase. In particular, a small number of (US only?) Ducati 998's build up so much crankcase pressure at high revs that excessive entrained oil is forced/sucked into the airbox through the crankcase vent system, resulting in excessive oil consumption.
The way the crankcase ventilation system works is that a one-way reed valve opens to allow positive-pressure crankcase gases to be drawn toward the airbox vacuum to be then sent through the engine along with combustion air and fuel to be burned. Since the crankcase gases contain suspended oil droplets, the system contains an in-line oil separator intended to capture the droplets before they enter the airbox. Any oil droplets that make it past the separator are combined with intake air and burned. The separator is essentially a maze that causes the suspended oil drops to impinge on the separator's walls and then be drained back to the engine oil sump by gravity.
When you install high-compression pistons, the piston ring pressure differential, and consequently blow-by will increase, and if excessive, can cause higher than normal crankcase pressures.
The K&N breather simply vents the crankcase to the atmosphere instead of the airbox. Its filter prevents dirt from entering the crankcase under a negative crankcase pressure differential (pulse) when a piston is on an upstroke.
Under positive crankcase pressures the K&N device vents oil droplets direct to the filter medium and (when it becomes saturated) then straight to the rear tire and the environment (both not good.)
The theory for the best crankcase ventilation says that you loose less horsepower through pumping losses if you vent to a large volume. That's why the K&N setup initially looks attractive. But if you vent to an airbox vacuum there are fewer losses that if you vent to a larger volume at atmospheric pressure.
Jon Nichols makes an excellent aftermarket crankcase ventilation valve for Ducati's. But listen to what he has to say ...
"We firmly believe the best system is the stock system. Let me expand on this. The only part worth replacing is the breather itself, and only if it is leaking or if you want to dress-up your bike. We see many Ducati's with the K&N filter attached to the breather hose. This is simply not a satisfactory replacement or alteration to replace the stock system. The stock system is less restrictive because there is more volume making it less restrictive.
Part of the problem with this modification is that the only efficient replacement systems made were made for race circumstances. This was accomplished by increasing the tank volume into which the engine pumps. On the early bikes, 851, 888, 955 in the 851/888 chassis Ducati had a very large carbon fiber tank which was connected to the engine crankcase without any breather. The tank was mounted in the tail section of the bike. This effectively doubled the crankcase volume and was responsible for an additional 3 to 5 HP at high RPMs.
This type of system is also available for the 748/916/996 chassis. This type of system is not available for Monsters or SS models, unfortunately. Therefore, the system that allows you the largest volume to pump into is the stock system. This system pumps into the airbox and has the added benefit of a collection/separation tank between the crankcase and the airbox."  |