Japspec
Japspec Reader
4/16/21 7:48 a.m.

Hi everyone. My NB2 Miata turbo build is (hopefully) soon ready to go to the tuner. I'm currently buttoning things up, and realized that my exhaust side valve cover vent hose now does not have a way to get to the Intake tract, so I decided why not get a catch can. So, I did some research, and this is where my confusion starts.

I have been researching catch cans and catch can routing extensively over the last few days. I have come up with a few different conclusions from my research. A few threads talk about routing the port on the Intake Manifold and the PCV Valve to the catch can, then throwing a filter on the exhaust side to vent to atmosphere.

Then, other threads are talking about routing the PCV Valve side and exhaust side to a vent to atmosphere catch can, and plug the port on the intake manifold. They also talk about deleting the PCV Valve if going this route?

I also found a few threads talking about just throwing a filter on the exhaust side, and just leaving the intake side PCV system as is.

I am just confused. What I thought should be something simple has me stumped? What would you guys say is the best way to use a catch can? I made a few diagrams below to illustrate what I've read from my research:

Configuration 1:

Configuration 2:

Configuration 3:

wae
wae UberDork
4/16/21 8:13 a.m.

I remember boggling on this for a while.  Eventually, I decided to run with the configuration that the factory put on the SRT-4 motor that I'm using.  I'm using the stock PCV valve because apparently there's something different about PCV valves on turbo engines but I can't quite recall what that was.  I believe that it is to prevent the system from going from suck to blow.  From the PCV, I go in to a catch can and then from the catch can I attach to the intake manifold so that remains a closed system.  On the other side of the valve cover, the hose connects to the intake pipe that feeds the turbo.  I ran a second catch can there for a while but it was always completely clean and empty, so I reduced my collection by 50% and just went with the one. 

Someone smarter than I can explain exactly how the PCV system is supposed to function, but the take away that I got was that you can't really plug up either side of the system and expect it to work right and that you've got to have the venturi effect from the intake as well as the engine vacuum.  I could have that totally wrong, but my car doesn't generate any puffs of oil smoke and it doesn't have any boost leaks, so it's at least moderately functional.

Gimp (Forum Supporter)
Gimp (Forum Supporter) GRM+ Memberand UltraDork
4/16/21 8:35 a.m.

Not sure if it helps, but here is a diagram I made up when I had a SRT4 many moons ago.

Japspec
Japspec Reader
4/16/21 9:49 a.m.

Thank you! So from what I'm gathering, I should have the exhaust side port running to the airbox, just like it did in stock configuration? Are there downsides (besides emissions) to just throwing a small filter on the exhaust side port?

gearheadE30
gearheadE30 Dork
4/16/21 11:06 a.m.

In reply to Japspec :

If your catch can doesn't have some kind of coalescing filter in it, you may end up with aerosolized oil all over your engine bay. My solution to this happening was to run a road draft tube like HD diesel stuff does down the firewall under the middle of the car so airflow can take it away. You may still end up with a light oil mist, but under the car it's rust proofing in a small area instead of turbulent air under the hood making the oil go everywhere.

Japspec
Japspec Reader
4/16/21 11:42 a.m.

In reply to gearheadE30 :

I see what you mean, that makes sense. I would use a filter if I was to vent to atmosphere for any reason. What configuration is your catch can in, if you don't mind explaining your setup?

asphalt_gundam
asphalt_gundam Reader
4/16/21 12:19 p.m.

Least amount of problems is going be Config 2.

First off no intake side hookup means you can't pump your intake full of oil (or any other crap). I've seen an intake manifold pumped FULL of oil and hydrolock an engine.

Secondly I HIGHLY recommend ditching the use of any stock pcv or vent hose nipple. When the engine starts building crank case pressure they're not big enough to handle it. Especially in turbo, poor ring seal, and track/high rpm use. Get some -12 weld on fittings, hose, the biggest catch can you can fit, baffled catch can even better and make sure that the engine has an easy to breath system. A baffle to block direct oil splash to the opening in the valve cover or where ever is a great help too. If you're getting enough crankcase pressure to push oil out then its not large enough connections because any oil getting to the opening is being pushed out. My Olds engine in stock form has a tendency to push oil out the valve covers in drag race distance at WOT. Crankcase pressure would hold up the oil trying to return to the pan and then splatter it all over the headers....not good. So before I started tracking the my car I built a big baffled catch can (mounts in one of the battery trays) with two 12AN lines from the intake manifold (Not the runners or plenum. I drilled through to the valley area because there is less oil splashing around there) and two vent filters. In one entire year it collected more condensation than oil. The dipstick doesn't blow out and leak all over, gaskets are all leak free, and at most the filters would see a little dampness of oil but not getting the fender liner messy. I see way too many "catch cans" the size of 20oz soda bottle that guys have plumbed to stock pcv ports fill with oil then make a mess every session or worse...every hard pull. Then they add a quart of oil and say that's just how it is......Its not, its just not knowing any better.

gearheadE30
gearheadE30 Dork
4/16/21 12:48 p.m.

In reply to Japspec :

asphalt_gundam's advice is good.

My setup doesn't really match any of the ones listed here just because the design of the engine is a bit different. There isn't a PCV valve in it as such; there is no "crankcase air inlet". Just an outlet in the valve cover that has a baffled oil separator built into it. That crankcase vent runs to the catch can, which has a coalescing filter in it and a drain on the bottom, and the catch can has an outlet hose that runs down the firewall to below the front floor pan. I usually check the catch can every time I drain the oil, but the engine's separator is pretty effective so there is rarely much in there.

iansane
iansane Reader
4/16/21 1:19 p.m.
asphalt_gundam said:

Least amount of problems is going be Config 2.

First off no intake side hookup means you can't pump your intake full of oil (or any other crap). I've seen an intake manifold pumped FULL of oil and hydrolock an engine.

Secondly I HIGHLY recommend ditching the use of any stock pcv or vent hose nipple. When the engine starts building crank case pressure they're not big enough to handle it. Especially in turbo, poor ring seal, and track/high rpm use. Get some -12 weld on fittings, hose, the biggest catch can you can fit, baffled catch can even better and make sure that the engine has an easy to breath system. A baffle to block direct oil splash to the opening in the valve cover or where ever is a great help too. If you're getting enough crankcase pressure to push oil out then its not large enough connections because any oil getting to the opening is being pushed out. My Olds engine in stock form has a tendency to push oil out the valve covers in drag race distance at WOT. Crankcase pressure would hold up the oil trying to return to the pan and then splatter it all over the headers....not good. So before I started tracking the my car I built a big baffled catch can (mounts in one of the battery trays) with two 12AN lines from the intake manifold (Not the runners or plenum. I drilled through to the valley area because there is less oil splashing around there) and two vent filters. In one entire year it collected more condensation than oil. The dipstick doesn't blow out and leak all over, gaskets are all leak free, and at most the filters would see a little dampness of oil but not getting the fender liner messy. I see way too many "catch cans" the size of 20oz soda bottle that guys have plumbed to stock pcv ports fill with oil then make a mess every session or worse...every hard pull. Then they add a quart of oil and say that's just how it is......Its not, its just not knowing any better.

Wow. I was always under the impression you needed to have the vacuum from the intake to pull vapor into the catch can. It's functional without? I just bought two of those mann/hummel assemblies in prep for adding catch cans to two of my cars but I may have to rethink the design.

bentwrench
bentwrench SuperDork
4/16/21 1:45 p.m.

Ideally you need to draw some vacuum on the crankcase (in high vacuum situations) to pull the nasties out of the oil. So configuration 1, except there is little need for the external vent. Unless you are boosting it to the brink or setting land speed records. You want to keep the oil as clean as possible without hurting performance.

If you run without suction I would expect a dramatically shortened oil change interval.

Factory PCV's from boosted models are metal and made to block boost.

Any external venting engineering should take into account any affect on the measured air if using a MAF.

asphalt_gundam
asphalt_gundam Reader
4/16/21 1:54 p.m.

In reply to iansane :

The crank case is pressurized by the pistons going up and down which moves significant volume of air around very fast plus the leak past the piston rings. Boost heavily increases the ring leak because of the higher cylinder pressures. That pressure gets higher than atmospheric pressure which will make it try to escape to atmosphere all on its own. Intake vacuum will assist that movement from high to low pressure area by being below atmospheric. Kinda like having a push pump on one end and pull pump on the other vs just a push pump....there is flow either way.

Honsch
Honsch Reader
4/16/21 2:58 p.m.

It's important that the crankcase never see boost pressure.

The catch can outlet cannot see boost or the crankcase will be pressurized. 

wvumtnbkr
wvumtnbkr PowerDork
4/16/21 3:38 p.m.

One point I learned the hard way...

 

If you vent to the intake, pre turbo, it can destroy your impeller.

 

I destroyed a stock rx7 impeller from having liquid oil hit it by venting the engine in front of the turbo.  No bueno.  It looked like erosion on a weird scale.

 

Japspec
Japspec Reader
4/16/21 11:38 p.m.

 Thanks for the info guys! I didn't realize that catch cans were as in depth as they are. So now I'm thinking, would I have any issues with running configuration 1? I researched some more and came across a thread on a miata forum where that was the preferred method (not sure if I'm allowed to link other forums? If so, I can provide a link to the source). My thought is that I have a catch can between the PCV (I will be purchasing a Mazda 323 GTX PCV) and the intake manifold, then vent the breather port to atmosphere with a small filter. Just like this picture I found on a forum:

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