Crank case ventilation... I know..
 

I'm sorry for posting yet another topic about this! But please help me understand something I can't seem to find. Also more questions :)

The miata from factory has 2 small vents out the valve cover. One goes right after the air filter, the other goes directly into the intake manifold. The thing I can't seem to understand is why is it engineered this way? As far as I can figure out, the intake manifold will see higher vacuum and the intake manifold side will kind of pull air out when on throttle. Is this all there is to the way the stock balancing of the crank case pressure is designed?

With a turbo build, would it be strange to route both vents to one catch can? Ovbiously you can't vent that catch can to the intake manifold, because it will see boost. But you could still vent the catch can pre turbo or vent to atmosphere, right?

Now another thing I was wondering.. The ventilation on the stock valve cover is somewhat restricted by baffling inside, and the ports are quite small. Does anyone have any idea what horsepower numbers will be safe on the stock venting? Or rather, at what hp will we need to start increasing the venting to keep the turbo happy? :) I know there will be a lot of variables... and maybe this is impossible to answer but if you have any experiences, please share! Thanks!

Basic idea on stock setup is that the PVC valve, the one entering the intake man. is a valve the lets the case vent any positive pressure out of the case, and into the intake, to be burned as combustable "air". Because you are allowing air to escape the case, you need a source of fresh make up air to replace it, which is the purpose of the line near the air filter, on drivers side. Its plumbed in to the filtered air only because thats a clean, filtered source, easily accessible. A functioning catch can really needs to be between the PVC valve and the intake, to get the oil trapped in the vented case air out before it gunks up the intake. The intake air side, is "intake" so IMO does not benefit by being run thru a catch can. Adding a turbo does complicate it a bit, and I'll let someone better qualified than me to 'splaine that.

I may have been wrong. Not sure if the driver side vent is just an inlet or both an air inlet and vent out when stock. But yes a turbo does complicate the situation :p The driver side vent will definitely vent out if manifold is under positive pressure.

Ty for your reply :)

I just read your other thread where you talked about aiming for 300 horsepower. Both of the stock passages are too small for 300 horsepower and should be replaced with larger ports and increased opening sizes within the valve cover. This is all covered in the other thread.

I've seen the pictures and I was thinking about doing so. Thanks for mentioning because this makes me more certain.. lol so many things to take into consideration and I'm unsure of everything! What size lines would you recommend? Would you still run the 323GTX on a closed loop catch can (only pcv side) at the 300+ whp level?

At 300 horsepower you are going to need both passages venting to atmosphere through large passages and hoses. But that's also in the other thread. Are you sure you read that thread? Lol.

It's engineered that way to keep the oil clean.

The nipple on the exhaust side of the valve cover is intended to let fresh air enter the valve cover, not to draw blowby out. That's what the one on the intake side, with the PCV valve inline, is for.

By constantly circulating fresh air into the crankcase, the atmosphere inside it is cleaner, and the oil has less chance to intermix with blowby, moisture, etc.


The design of an adequately-specified PCV valve is that positive pressure in the manifold will not be able to blow back into the crankcase. Obviously, PCV valves can fail, which is why it's not a bad idea to have a catch can inline with the exhaust-side port if you're planning to run high levels of boost.


I'm going to disagree with Six on venting the intake-side nipple to atmosphere. That one needs to go to the intake manifold via a PCV valve, in order to ensure that the constant flow of clean air through the valve cover is maintained.

Someone needs to pare down the Definitive Catch Can thread. It's like trying to read Mopart's original Paint Job on a Budget thread at this point, it's waaay too long and you spend time reading crap that's contradicted a few pages later.

RFC: https://www.miataturbo.net/engine-pe...-thread-99549/

The PCV operates to allow some crankcase gasses to be ingested during times of low load and moderate to high vacuum through a metered orifice. As the throttle position is changed to wide open the available vacuum drops to near zero and rises to positive pressure on a forced induction engine, closing the valve. Meanwhile, the blow by entering the crankcase past the piston rings is dramatically increased and must exit somewhere. This will either be through your gaskets and seals or through the crankcase vent in the exhaust side of the valve cover. On an engine making three times as much power as stock this will be approximately three times as much blow-by. If you do not provide an adequate pathway then your engine will leak profusely out of any available seal. Additionally, the vapors exiting the exhaust side valve cover port will be traveling at such a high velocity it will carry copious amounts of oil with it, filling a catch can in a very short amount of time.

I disagree with the notion that the PCV valve is only designed to bring fresh air into the engine. It is designed to allow the consumption of vapors for environmental purposes. When it is not allowing the consumption directly into the intake, the exhaust side port is expelling vapors into the inlet tract pre- throttle body.

The crankcase will always be under positive pressure due to blow by under a load. I challenge Mr. Perez to monitor the exhaust side port of his stock automobile and present findings of vacuum on that port due to operation of the PCV valve under load.

I've tried it both ways. You can do what you wish. As for me and my house, we will run large diameter ports and hoses from both sides to a catch can.

Hi. Sorry to be stupid but which other thread is the passage port enlarging covered?

See above

Thank you.