Quote:
Originally Posted by Calum
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.
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i see your point, and understand how pcv systems work pretty well, i just don't think there is anything materially different between running vacuum in the crankcase and not doing so. just in my experience, owning lots of cars with lots of different setups and knowing lots of people who do the same, it's not an issue.
the only issues i think i've ever seen on boosted cars were either from someone having nothing at all or a catch can that wasn't catching everything. the only way to guarantee there is no oil in your intake is to not give it a path to get there. not having vacuum in the crankcase isn't ideal, but it's better than pumping some amount of junk back through your turbo plumbing. i've never actually heard of or seen a real person have issues with crankcase pressure with a full vta setup, period. it's a theoretical argument, with no real material impact. the only thing that *does* have a material impact is how effective the extraction mechanism is, which in the case of most catch cans isn't very great. vta is 100% effective at keeping shit out of your intake, and that's kinda the whole point.
i'll change my mind when i see someone actually have a material issue arise from using a vta setup. until then i'll keep my shiny clean intercooler pipes.