Catch can summary

[RileyR]

So I spent far too long researching catch cans while (almost literally) watching paint dry at work. After several hours here are my findings.

I certainly do not proclaim to be any sort of expert, but this might help others cut down on other research.

Seems to be a hot topic - and some serious conjecture, but these are the summaries I have made. Correct me if I have got something seriously wrong.

Diagrams are taken from multiple sources who beleive their system is superior, so disregard "correct way to..." at the bottom of diagrams - they are just used to explain.

The PCV valve is a 1 way valve to prevent the positive pressure from the intake manifold from entering the valve cover, which would create pressure in the crank case and possibly blow seals. The PCV valve also allows the crank case to vent and release that pressure - hence the 1 way valve.

The pcv valve is CLOSED when: there is positive pressure in the intake manifold (eg a F/I car on boost).

The pcv valve is OPEN when: there is negative pressure in the intake manifold. Eg an N/A car with Wide Open Throttle (WOT), or a F/I car when shifting gears or not on boost.

Here is a diagram of stock setup:



When the pcv opens, the air and gunk is SUCKED into the intake manifold (vacuum), then through intake valves and into the combustion chamber to be burned (bad). This is an active system. Ie being actively sucked/vacuumed out of crank case (engine) and into the intake manifold. making this side more important than non-pcv vent for a single catch can setup. This is confirmed by observations of
When running 2 catch cans (both sides) the pcv side fills up much faster than non-pcv side.

Here is a diagram of pcv side single catch can



Here is a diagram of a dual catch can setup




The non-pcv side (aka hot side, exhaust side, on the right when looking at engine) is a passive vent, therefore not a lot of gunk comes out. A catch can in-line here is only useful if turbo'd due to the pcv spending a lot of time shut (as above).

If deleting the pcv valve and running a catch can with both out ports (pcv side and non pcv side, both on cam/valve cover) leading to the catch can, then there is no ACTIVE scavenging and only passive venting which is an old and innefective system used in the 60's. Active venting also creates vacuum in the crank case which is said to be good for the seals and the engine.

Here is diagram of both outlets going into catch can (passive venting)




Other things to be noted:
Catch can should be located somewhere away from heat to improve the condensing of the gunk, therefore keeping it in the catch can. Steel wool simply put in the catch can is also said to help with this.

On high cornering g forces oil can leak out of both the pcv side and non-pcv side. If not collected properly (ie only having an air filter attached to non-pcv side) it can make a mess. For this reason some opt to block off pcv side completely which MAY (possibly) cause oil contamination with fuel and gunk not getting taken out.

Conclusion:
If engine is naturally aspirated run the catch can on the pcv side, keeping pcv in place and dont vent to atmosphere, but plumb back into intake manifold so that the system can work actively, not passively.

If running forced induction run a dual catch can setup with pcv side as above and non-pcv valve either vented to air or back to stock air intake nipple.



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[bruce]

Waste of time for a street car.
Added weight if u r genuine in reducing weight in your car.

[RileyR]

Waste of time for a street car.
Added weight if u r genuine in reducing weight in your car.

Would like to know why you say that Bruce

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[bruce]

If it was needed they would have fitted it from factory or everybody would have added one aftermarket.
Also.is your intake an oily mess? If so you've got problems elsewhere.
Plus it would weigh an extra kilo.

[ManiacLachy]

Another common configuration option is to route the exhaust side gas back into the intake after the MAF, before the Throttle Body, with a catch can in between rather than venting to atmosphere through a breather.

There's a great thread on MiataTurbo.net that covers the subject, you might have read it as part of your research, if not go do a search, it's Miata/MX-5 specific, and those guys focus on the performance end of things.

I like the config with the intake side going PCV -> Catch Can -> intake manifold. and the exhaust going straight to a breather. If you notice the exhaust breather getting oil residue, then maybe plumb in a new can and vent that to a breather.

Investigate if you need it first, if you're turbo, or spending long periods at high RPM you might need a setup. Pop the top off your intake manifold and take a look. Note, you're looking for oil, not carbon. The carbon will be due to the EGR, and that's another topic. But if your car is mostly street dríven and the engine is mostly stock, you'll probably find you don't need one, or at most you just need a new PCV valve.

[RileyR]

If it was needed they would have fitted it from factory or everybody would have added one aftermarket.
Also.is your intake an oily mess? If so you've got problems elsewhere.
Plus it would weigh an extra kilo.

Agreed if it was NEEDED they would have added from factory, though a moot point.
My na8 ran for 170,000km before getting a rebuild. Coolant reroute? Bigger radiator? Oil catch can? All things that might help but needed? Depends on opinion.
There are a myriad of aftermarket parts which make the car better that didnt come from factory and arent NEEDED for the car to be dríven. Good to have? Open to opinion.

In this specific case, for me, I reckon that yes the car will run fine without a catch can. However I think that it could run cleaner and better with a catch can so that any fluid is hoepfully stopped in the catch can and not put through the intake system and in to be burned. When im leaning on the engine i want it to be as happy as it can be so i can be too
Why wasnt one fitted from factory? Many reasons I would guess.

Intake has never been an oily mess. Though would be very interested to see what comes through there!

I will let you know the total weight of the catch can, and ill test to see if it catches some gunk and re-assess its value

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[RileyR]

Thanks maniaclachy,
I read a whole bunch of threads on a few forums and may have come across the one you are refencing, im not sure.
The car gets leaned on at the track so I feel its worth testing it out and see what it collects.
Yes that config of intake side pcv>catch can> intake and having exhaust side vented to air with a little filter is a common and logical setup from what i can tell. Im not sure in the legality of venting to air though. From what i can tell the exhaust side is less important that intake side for a N/A car.
I have wondered if you could plumb exhaust side into the same catch can as the intake side and still keep the active scavenging by retaining pcv and plumbing back to intake? This is possibly going too far against what mazda designed.

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[greenMachine]

Do the tiny hole mod, if you need a catch can. You may then find you don't need a catch can. Or, need to empty it as often.

I have done the mod, used strainer mesh, and at 140rwkw am not getting anything in my can after either a dyno session or a track day. I mean, nothing.

YMMV of course, especially at higher boost/power levels. But make sure you do the basics first.

[Okibi]

If it was needed they would have fitted it from factory ...

The SE does have one from factory, Mazda understands turbo BPs have enough blow by they put a bracket to hold the PCV in.

[RileyR]

Do the tiny hole mod, if you need a catch can. You may then find you don't need a catch can. Or, need to empty it as often.


YMMV of course, especially at higher boost/power levels. But make sure you do the basics first.

I'll do a search on the tiny hole mod!

The basics being the tiny hole mod?

Ill test a cheap catch can and if no gunk in there after a while it will come back out

[plohl]

I have used a few catch can methods - the car was primarily a track car at the time, but I still do a bit of street driving.

The duel catch can method:
I found the intake side catch can was dry most of the time - only after a track day with lots of laps - or a race weekend would I see anything in it, and it wouldn't be much.

The one between the PVC and intake could fill up after a few tanks of fuel - or track day. I was using mishimoto's compact catch can for both - it's a very nice little unit. Little being the main issue, but I think it would be fine for a daily.

Now, my engine had done a lot of track time! Compression was ok, but it used a little oil.

[ManiacLachy]

I have wondered if you could plumb exhaust side into the same catch can as the intake side and still keep the active scavenging by retaining pcv and plumbing back to intake? This is possibly going too far against what mazda designed.

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My setup is a NitroDann catch can, the intake manifold is capped off, both the intake and exahust ports on the valve cover go into the same catchcan, and it goes out into the intake piping. Which pretty much covers off everything you mention, including not venting to atmo. So yeah, you can easily do that.

[Lokiel]

If it was needed they would have fitted it from factory ...

The SE does have one from factory, Mazda understands turbo BPs have enough blow by they put a bracket to hold the PCV in.

I'm still unsure if Mazda did a half-arsed job with the MSM/SE's oil catch can set-up (as they did with everything else turbocharging the car) or whether they did all that was needed to make it functional for what they delivered.

If you've modified your MSM/SE for "MOAR POWA", remove the top half of your intake manifold - you'll be shocked about how much blow-by oil crap coats it and has "baked in" (it's probably similar in modified N/A NA/NBs too).

I'm surprised that Mazda didn't try to vent it from the PCV valve to the catch can and THEN to the intake manifold instead since the catch-can is mounted on the cold-side of the engine.

When people started their "MOAR POWA" modz on the SE/MSM, many found that oil was flowing through their intercooler, as was evidenced by oil pools in the intercooler coldside pipe. This happened because oil was being sloshed up out of the catch-can back up, and recirculated along with the air into the air intake system. A simple one-way check valve from the oil catch can fixed this issue.

Maybe Mazda was aware of the potential for oil sloshing out of the catch can so didn't want it going directly into the intake manifold (and subsequently into the cylinders, producing black smoke in the exhaust) and that's why they didn't use a catch can for the cold-side?

After all my reading on catch-can setups, I've decided that air-flow through the rocker cover is what's important to evacuate the blow-by crap into a catch-can and why the dual catch-can solution is best - unfortunately it's hard to implement due to lack of space

WHY: When the PCV valve is open, typically at idle or cruising, air flows from the hot-side to the cold-side due to the vacuum created, so any blow-by crap is collected in the cold-side catch-can before going into the intake manifold. When the PCV valve is shut, the positive pressure generated forces the blow-by out into the hot-side's catch-can (this is why it's a BAD idea to simply put an air filter on the hot-side's barb if you know that you have blow-by issues, it WILL collect blow-by and become a fire hazard).

The 2nd best solution is where both rocker barbs vent to a single catch can that has a breather or vents into the intake system. This system relies completely on pressure build up and does not take advantage of the ability to clear blow-by at idle or cruising speeds so is fine for a track car but not a street or double-duty car. Users of this option should also enlarge their rocker barbs too to minimise the positive pressure needed to vent blow-by (ie. you will be able to vent blow-by at much lower RPM).

[Nevyn72]

The 2nd best solution is where both rocker barbs vent to a single catch can that has a breather or vents into the intake system. This system relies completely on pressure build up and does not take advantage of the ability to clear blow-by at idle or cruising speeds so is fine for a track car but not a street or double-duty car. Users of this option should also enlarge their rocker barbs too to minimise the positive pressure needed to vent blow-by (ie. you will be able to vent blow-by at much lower RPM).

I would disagree with this in the case of a turbo charged car.

This is basically what I do with my SE (like Lachy above), however I have the catch-can re-circulate back to the intake before the turbo inlet but after the filter and MAF. This means that, at all times, the catch-can outlet will only ever see some level of vacuum, (often significant during boost when crankcase pressures are likely to be highest).

In other words the catch-can is always having air sucked out of it and hence is always actively drawing air out of the crankcase!

[RileyR]

Thanks Plohl,
I liked the design of that Moshimoto one, and found a cheap ebay replica to test out. Claims to have a 50 micron filter and the baffle and size is the same. I figured a small one will be fine if checked regularly.