Splash lubricated surfaces will suffer/fail from a high vacuum situation without changes to direct oil to all of these areas. Cam lobes, piston skirts, etc.

You will know how healty your motor is by how fast it idles LOL.
why not just put a pcv valve on the Y to the IM?

You're a fucking genius.

The post-filter pre-Turbo line will only ever do anything once your filter becomes completely clogged with dirt. It's not a good plan.

Why not run a slash cut into the exhaust with a check valve downstream of your oxygen sensor? See for example this picture I took today of a Cummins engineered by an OEM and real engineers. This produces actual vacuum.

I wish I could make this work in my car. I tried the vibrant e-vac slash cut with no success

So what you guys are telling me is that the best approach is OEM, but with a catch can between the valve cover and the intake?

Presumably I'd continue venting the exhaust side of the valve cover to atmo with a little breather filter.

What's weird is the only time I ever got oil through the PCV system was when I had that little catastrophe with the oil jet that took out a piston skirt and all of my oil pressure.

I kind of wondered how much vacuum would be behind the filter on a turbo without the addition of a venturi before the hole. I guess you're confirming my suspicions.

I wish I had the fabrication skills to make that work. I'm also a little scared by the videos I've seen of miatas with slash cuts that look like they're fogging for mosquitos when they get on the boost. This is a street car and I personally hate driving behind cars that blow tons of oil smoke.

I try to think about the engine's needs in big picture terms sometimes to help predict the scale. To make roughly twice the horsepower you have to burn about twice as much gasoline. You have to ingest about twice as much air and the engine makes about twice as much heat that needs to be dissipated. You have to have the capacity for about twice as much exhaust flow and about twice as much blow-by.

Since the PCV valve is closed at full throttle the design allows for the stock engine to expend its blow-by gasses into the inlet tract to be consumed by the engine. It does this through the one little hot side hose. If the engine is making twice as much power and still only breathing through that one side then the velocity increases significantly if the pressure within the crankcase is to remain the same. Or the crankcase pressure rises and starts overpowering seals like the crank seals or the cam seals.

If you ditch the PCV valve and instead add a fitting and a hose going to a catch can, then you have doubled the capacity for venting the crankcase because now you are venting both sides under load. Doubling the capacity slows the flow enough to allow oil to fall out of vapor suspension more readily prior to leaving the engine.

If someone were to want to make roughly three times the stock horsepower then it would logically follow that larger openings and hoses might be necessary to maintain a reasonable flow rate and keep large quantities of oil from ending up in the catch can.

If you get an occasional whiff of those vapors and find them offensive then you can add a slash cut and a check valve to the exhaust tract. But again, if the orifices and hoses are too small and the catch can volume is not great enough to allow the oil to settle out of suspension in the vapor then the oil vapors may be carried into the exhaust and create smoke.

Oil has a significantly lower octane rating than gasoline. I maintain that it is a bad idea to vent those vapors into the engine. Additionally the oil will create carbon buildup on the valves, combustion chambers, and pistons, which then become hot spots and promote more detonation.

I do not believe everyone should do just as I have done and I'm not trying to convince the world that I am right, but there are some simple conceptual generalities that might be hard to see if you're not stepping back and staring at the big picture.

Do you know this for sure, or just guessing? It sounds like guessing, because my testing shows otherwise.

Using the exhaust - I have some questions.

If you run a cat I don't know if it will work, because positive pressure?

​​​​how well does it work with a free flow system? Not sure..

This wouldn't be the first time that diesel tech does NOT cross over, so telling me it's proven to work, isn't enough to sell me.

It will have vapor come out because the crankcase is under pressure but it would do that if it was just a hose hanging out into space. It will also work if you have a very high velocity and are creating a siphoning effect like blowing over the top of a drinking straw. One of the unintended consequences of this is that oil will drop out of suspension and coat the inside of your compressor housing and your compressor wheel and your intercooler lines and your intercooler. When you turn the engine off it will all pool at the lowest point. Ask Ryan G how that looks because he's seen it first-hand. If I was to run a catalytic converter I would certainly mount a slash cut behind the catalytic converter. Not the least of the reasons being the oil vapor will destroy a catalytic converter. But definitely because you don't want more positive pressure.

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I really enjoy how each little car segment of the world by manufacturers has their own discussion thread on several things like "what's the best oil", or in this case "catch cans".

I've been in the Subaru game for some years now and just wanted to say I have run both stock, air oil separator, as well as dual and tripple catch can setups. All these setups had two things in common, they cost money and didn't work. I liked the catch can as it recycled the oil but what I found is that if the separator was not heated during the winter then you got a chocolate frosty that the crankcase couldn't over come. It was also too restrictive while running 18lbs of boost, smoke show in all the wrong ways. I even vented the oil cap, and no I'm not kidding. I then went to two and three catch can setups on the Subaru. They were too restrictive and had way to many potentiallities for vacuum leaks. I tried venting the cans to atmosphere and it worked slightly better. I then said screw it and just ran breathers for heads and crank case and I haven't looked back, the car runs fantastic.

I know that my experience does not apply to miatas per say but the logic/theory I think does.

I understand that a pcv is there to do 3 things:
1:. Allow for venting of crank case pressure
2:. Allow for air to be drawn through crank case to draw out bad combustion fumes
3:. Emissions

so 1 I'm ok with, we have pressure that's bad it needs to go there. It has to be vented or bad things happen.

2 I can see the benefits from this and I guess it makes sense

3 eh dicey subject but half of you guys are running megasquirt and a turbo so your concern of Arctic seals is likely not high on list of concerns.

I understand that there are some beliebers who think that a vacuum source on the crank case is a good idea and I don't disagree.

However given my experience with every setup I think one thing rings through though. An engine wants to ingest air and gas, that's all. No combustion chamber gasses, carbon, oil, z Ray's, or anything else. The amount of build up on both the Subaru and Miata we're not something I was happy with. I'm glad that we don't have to deal with egrs either because that would make it even worse.

I think the best setup is the scavenging pipe setup as posted earlier. I'm currently running breathers on everything. I believe it helps to have vacuum to help seal rings and draw air through the system but I stop at recircing that air into the intake manifold.

I'm sure some will disagree but I would like to hear different ideas and explanations if you disagree.

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Why would the pre-turbo slash cut not make a similar amount of vacuum as the exhaust? On the one hand the intake is a smaller diameter, but the exhaust is hotter so I'm not sure which velocity is higher. Also even with the same velocity the Reynolds number is going to be different because of the temperature, and I'm not sure if higher or lower RE makes more vacuum from the venturi and I'm also way too lazy to do the math.

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You I still don't want that detonation promoting vapor entering into a turbocharged engine with only a few timing degrees of margin between triumph and trouble. My motor was too expensive to play fast and loose with my chances.

I'm just VTA and not routing it into my exhaust. But for those who are curious how well those cheap Moroso kits with the check valve work, on my big block Pontiac I could set the breather hose from the valve cover down into a jug of water and rev the engine one time and empty the entire gallon out of the exhaust pipe in a second. And I was running one on each side. So, yeah they work pretty well.

This thread is a very good read and cleared up some questions I had; look forward to applying to my FE 390.

Some curiosity about a simple two can setup, one either side of the VC, tree hugger style, for the Miata:

MATERIALS:
VC Scrubbies: coarse copper preferred
Catch Can : Mishimoto, non-vented
Hose sizes ~ 10AN? Fat from the VC to the can, then smaller to the non-VC Port? Or keep all the same size?
Hose material? Something insulating, & protected form heat
Hose routing: up to can if possible (reads like a skinny tall can rather than short fatty)
PCV: Any recommendations?
Check valve: Any preference? Someone mentioned a check valve off a GTX?

Thank you.

When the air charge ignites it produces around 10:1 expansion form whatever volume the cylinder had, lets say that 1/3 of it get's used in moving the piston and turbo and cools down as it does that work, you still have 6.6 times the volume of your intake charge going out the exhaust, if you double the intake diameter on the exhaust pipe you will have roughly twice the air velocity that the intake has.
A slash cut will befar more effective on the exhaust than the intake.

Nice info!

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