Quote:
Originally Posted by jamesm
what's idiotic about it? and to answer the question no, of course a catch can of any sort isn't 'required'. recommended, not required.
i would argue that it's idiotic to go to all the trouble of installing a catch can then just plumb the junk back into the intake tract, but who am i to argue...
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The idiotic part of a vent to atmosphere system is that it doesn't actively pull the blowby gasses from the crank case. This will allow the oil to be contaminated MUCH faster and will allow the crankcase to pressurize, at least to a higher pressure then original. Having the crankcase pressurized will cause the rings to not seal as well which will increase blowby even farther.
The positive crankcase ventilation system exists for a purpose, to remove blowby gasses from the crankcase and maintain a vacuum so as to increase ring sealing. When modifying the system to better suit a boosted engine the goal should be to make the system better without removing it's main function.
There are a few way to accomplish the main purposes of the PCV system: The best way would be a vacuum pump as they work at all times and under all engine conditions. The discharge can be routed to the intake if you're concerned about the environment, or to atmosphere if you don't care. The down side is they are expensive.
An engine based vacuum system is cheap and reliable, but only works at partial throttle or below, when the intake plenum is under vacuum, or if the intake to the forced induction compressor is pulling sufficient vacuum. These allow the blowby gases, including unburnt fuel, to be burnt in the engine which is better for the environment, but often route oil vapor and even liquid droplets back into the engines intake.
Another way is with an exhaust venturi, but these only work when the engine is at wide open throttle, are horrible for the environment, are illegal in most places, and will destroy a catalytic converter if it's downstream of the venturi.
So, most people stick with the engine based vacuum system, but why on earth would you just completely disable the system in an effort to modify it for boosted use. Why not maintain the system, but upgrade it for boosted use. A decent air/oil separator will allow the engine to maintain a vacuum in the crankcase but will remove any liquids from the air stream. You'll have to drain the aos, but that's a small price to pay for not having to change your oil every couple hundred miles. Or worse, letting your oil go, getting contaminated with exhaust gases so that it turns acidic & eats away at the bearings, all the while getting diluted with fuel so that the oil losses viscosity.