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Originally Posted by 8URSTI
Also if you run anything through your PCV valve you are just dumping bad crap into your motor. The 2.0 guys have it easier as they dont have the extra CCV that the 2.5 guys do. Therfore that is why most people claim that 2 catch cans is best to handle the other CCV. So what is another source of vaccum on our motors other than the intake pre turbo and intake manifold(pcv) ????? The EXHAUST!
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Except using the exhaust as a vacuum source is illegal in most (all?) SCCA classes, IIRC. Fine for the dragstrip, but not workable for the majority of people on here that need to comply with SCCA regs.
And the 2.0L motor's oil breather system sucks... it's prone to pumping oil out the valve breathers under high lateral Gs. My hybrid motor (STi block WRX heads) could fill up two Perrin catch cans in 2 laps at RFR. 'Course some of that may have been due to the excessive blow-by from my pistons, but without the secondary breathers there's no way to keep crank pressure from forcing oil out the heads.
Here are some drawings (as best as I can remember w/o a car or service manuals in front of me) that diagram how the whole thing works.
First the layout of the WRX and STi systems:
The only important difference is that the STi has dual breathers on the heads and block, but since they don't plumb into the intake at any point, they don't matter at all in terms of how to hook up catch cans.
Here's the normal operation of the breather lines:
You can see that while the car's under vacuum, the PCV is open and clearing the crank-case of the nasty fumes in there by sucking them into the intake to be burned in the cylinders. To replenish the CC with clean air, the valve breathers suck from the intake.
But when the car is under boost, things reverse a little bit. The CC is still being ventilated by the crank-case breather, but now the draw is from the intake's high-velocity flow, because the PCV is closed due to boost pressure. Additionally, if there's blow-by past the piston rings, the CC is being pressurized by that exhaust gas. So the primary flow should be out of the CCV and into the intake. If that flow can't ventilate enough pressure, then the valve breathers reverse their flow, also bleeding off CC pressure through the oil galleries. What's bad about this, is that on cars with big turbos that are seeing lots of lateral G's, oil can crawl up the side of the valve covers and get blown out the breathers. My WRX had a huge problem with this, partially because of the single breathers in my WRX heads, plus the high CC pressure due to the big turbo on the STi block I was running.
Here are the popular layouts for single and dual catch cans:
The first is the normal way to T a single can into the system. What's crucially important when doing this is to ensure the hose size for the different breathers. You really need to make sure that the CCV lines are significantly larger than the valve breather lines because of the situation where the valve breathers are drawing air from the intake through the catch can. I don't know all the "whys" to how this is supposed to work... but it's what's generally accepted as "okay". Frankly, I wouldn't run a single can like this.
The second picture is the "normal" way to hook up dual cans. This I'm fine with. Basically everything should work just like stock, except if there's ever any oil in the system it's caught before it ends up in the intake.
The third picture is an "ideal" way to put the CCV can in before the PCV so even under idle you're not sucking directly from the crank-case into the intake manifold w/o catching the oil, but doing this requires quite a bit of extra plumbing because of the way the outlet on the CCV is designed... it's really not worth all the extra effort, but it's "ideal".
Now then, while dual catch cans seem to be the right solution, some of us (i.e. road racers) are still blowing an awful lot of oil around the breather lines due to lateral Gs. In this situation, I don't think a catch can is the right solution. The right solution is to avoid any chance of getting oil into the intake by never routing the breathers back to the intake at all.
The first diagram is a basic breather can. The two sets of breathers are simply ventilated to atmosphere with a can to catch the oil/water in the gases. This means the CCV and valve breathers can suck/blow/whatever as they need without the threat of octane lowering oil in the intake.
The second diagram goes one step further by returning any caught oil back to the block, so if you're pumping oil out the valve breathers around long hard turns, it goes back into the block instead of just sitting around in a catch can.
The drawbacks to the breather cans is twofold... 1) They're messy and stinky... you have blow-by exhaust directly blowing into the engine bay. Not so nice on a street car. 2) They're not street legal... there's a reason we have CCV's and PCV's... it's to meet emissions regulations. And finally, on the drain-back can, you end up polluting your oil with water (exhaust condensation) and all the rest of the crap that's in blow-by gases, meaning you have to change the oil more frequently. None of these things are a big deal on a race car, but IMO make breather cans on street cars "too much".
Anyway, that's what I can remember off hand about all this junk... I've tried pretty much everyone of those configurations above and about 20 other ones that didn't work. I'm still not all that confident that I know what I'm talking about on this, since I never got it all working on my car. My current motor is getting built with a drain-back breather can designed by GST Mike, so we'll see how that works... should be good, especially since I'm getting my blown ring-lands repaired as well, meaning a lot less CC pressure to deal with.