OK. Time to give all you F/I guys something to ponder over...
I've been doing little more this rainy Saturday than thinking cars, and I ended up getting onto the whole PCV and catch can system kick. Actually, somebody brought a related issue up in the N/A forum, it got me to thinking about both my current and soon to be new setups (not to mention, my low compression issue), and 6 hours later... LOL!!
So I did a little reading. Did a little research. Did a little illustration. And am posting a little thread.
Let's start off with the illustration (so we all know what the hell I'm talking about).
Alright. Diagram "A"; a positive displacement blower setup (as I wanted us ALL to be included). This is currently the "recommended" PCV and/or catch can setup for most of us boosted guys. However, after doing the little bit of "reading" and "research", I found a few "drawbacks" to this setup configuration. First - the entire PCV system is now entirely dependent upon the crankcase pressures pushing themselves out (under both idle and boost) as there is no vacuum acting on the system. As such...
All air is metered. Rear valve cover is under crankcase vacuum... possibly drawing in fresh ventilation air (and potentially some recirculated fuel and/or oil vapors). PCV has been removed and crankcase pressure is being pushed
out into the environment as it increases. Idle ventilation suffers slightly due to a lack of manifold vacuum.
All air is metered. Boost pressures do not affect crankcase pressure due to the manifold connection to the crankcase being eliminated. Crankcase pressure ventilation is still pushed
out into the environment as it increases. There is still no vacuum acting on the system.
While this is "adequate" for the most boosted engines... so is a drag tube in some people's opinions. However, neither of these options are considered by most as an optimal
one since there is never
a positive vacuum placed on the crankcase. Keep in mind this is important for several reasons, including (but not limited to) increased ventilation, better ring seating, and minimal HP differences.
However, there is also a great deal of debate about what is the "optimal" setup, so I decided rather than going with the general consensus, I would sit down and attempt to analyze our particular engine to see what I could come with. Diagrams "B", "C", and "D" are what I came up with...
On to Diagram "B"; a draw-thru turbo and/or centrifugal blower setup.
In this diagram, I have "re-installed" the PCV valve into the front valve cover, utilized an inline one-way check valve between it and the upper manifold, relocated the crankcase breather hose pre-blower (and post-MAF), and placed a catch can (which helps to catch the oil vapors) inline. Here's what happens...
All air is metered. Rear valve cover is under crankcase vacuum, drawing in fresh ventilation air from before the turbo and/or blower. PCV is present and manifold vacuum is aiding in proper crankcase ventilation via the PCV. Slight risk of minimal fuel and/or oil vapors entering the intake charge, but no contamination of the environment.
All air is metered. Boost pressures do not affect crankcase pressure due to the one-way check valve being placed between the manifold and PCV. Since the boost pressures are keeping the check valve closed, crankcase ventilation cannot be vented through the front valve cover, and must be totally vented through the rear one. However, the vacuum produced by the blower helps to suck the increased pressures out of the crankcase. The majority of the fuel and/or oil vapors are caught by the catch can, and the rest are recycled into the intake charge instead of the environment.
My only concern with this setup is whether or not the nipple on the rear valve cover is actually large enough to vent all
the crankcase pressures out of a single outlet. It could be modified and enlarged, of course... but I'm trying to also keep things as simple as they are effective. As such, I kept looking for another option.
That option is Diagrams "C" and "D".
Both of these diagrams illustrate the exact
same PCV system on both F/I setup types... which is what I currently believe to be the MOST effective PCV setup possible (both at a very reasonable cost and with a simple installation).
All air is metered. Rear valve cover is under crankcase vacuum, drawing in fresh ventilation air from before the turbo and/or blower. PCV is present and manifold vacuum is aiding in proper crankcase ventilation via the PCV. Very little risk of fuel and/or oil vapors entering the intake charge with a "proper" catch can inline, and no contamination whatsoever of the environment at the same time.
All air is metered. Boost pressures do not affect crankcase pressure due to the one-way check valve being placed between the manifold and PCV. The vacuum produced by the blower helps to suck the increased pressures out of the crankcase, but at the same time, practically all
the fuel and/or oil vapors being vented from the valve cover are caught with the "proper" catch can, and whatever may
be left are recycled into the intake charge instead of the environment.
So the next question is... what is a "proper" catch can? Obviously, you can see from the diagrams that it is chambered to basically combine 2 catch cans into one (2 separate catch cans being the key to this system), but it goes a bit deeper than that. Anyway - I found one that would be IDEAL... and believe it or not, it's quite
a bit different from some of the over-advertised (and poorly designed) "vented overflow tanks" quite a few of you guys have purchased from various vendors.
Now I'm not trying to say anyone is doing it technically "wrong" (even though at this point I seriously believe that you've been "misguided" - LOL), for as I said before... your current setups seem to be "adequate". I'm simply offering what I personally consider (and intend to prove) to be a better option than anything
else currently available for the ventilation system on these cars.
Now... let's see who's interested.