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My turbo is in, and I’m blowing boost past the stock PCV and into the crankcase. This has prompted me to read the 7 million threads on PCV and catchcans here., none of which are conclusive. A plan is developing.
One of my options was to ditch PCV valves entirely, and go with a catch can and check valve a la the Deezums catch can thread. I always assumed that PCV valves were just check valves, so why not. Did some research, and figured out that PCV’s work in more complex ways. Good link here:
This link explains much of the strange behavior I’m seeing with the five or so different PCV valves I’ve tried. For cars without boost, they only need to keep pressure out of the crankcase during a backfire- a brief, forceful event. As it pertains to turbo cars: it explains why many of them don’t seal properly when the boost comes on. Unless the positive pressure comes on very suddenly, the valve doesn’t seal. Roll on the throttle slowly, blow boost into the crankcase.
They also seal in the other direction at high vaccuum (idle) to keep it from acting like a vaccuum leak, and keep it from drawing oil into the manifold. A simple check valve wouldn’t work at all in this direction, it would suck right through.
What I’m taking from all of this is:
- PCV valve good, check valve bad- at least as far as idle is concerned.
- a catch can is a good idea between the PCV vent and intake manifold, but using actual PCV valve instead of a check valve will probably keep oil out of it anyway.
- the way people are testing PCV valves here (blow in it, suck in it, hook it up to an air compressor) is bullshit. The speed with which you hit it with pressure is a critical factor that’s being overlooked.
- most PCV valves suck for turbo because they won’t hold positive manifold pressure unless they get it in a quick hit. I’m holding out high hopes that the GTX valve I have on order is designed to close more quickly.
Ditch it. It is an emissions device. When you hit boost it closes and your crankcase pressure all goes out through the other vent. If you have more Blow by than stock because you're running more cylinder pressure than stock because you are running a turbo then you will need a greater flow path. If the stock flow path is good for 100 horsepower and you are running 200 horsepower then you will need twice the flow capacity. If you are running 300 horsepower then you will need three times the stock flow capacity.
Running a PCV valve is a fucked-up idea for any aftermarket turbo car. If it leaks then you will have a boost leak. If it holds then you are still providing significantly more crankcase pressure than the system is designed for (because turbo) and are going to continue to have problems. I'm baffled why people continue to have problems with this. Not you personally but everybody else that argues, "OMG, I must have a PCV because Mr. Mazda put it there because emissions."
And for the people who feel like when they're in a vacuum situations they must have crankcase draw through so they can have wonderful fresh air, there is a tremendous amount of blow by gas being introduced constantly to the crankcase and needing to be vented out and it completely overwhelms the pitiful amount of vacuumed gas that is going into the intake manifold when the PCV valve is operational. This is why the factory has whatever comes out the other side going to the intake tract in front of the throttle body. And for argument's sake even if it was a tremendously high flow into the intake manifold at idle, anytime you are in boost it's closed and then what? It stops working. And if the flow into the intake manifold is too great in vacuum it closes as was discussed by the OP.
So then- VTA through catch cans on both sides? Or perhaps back to the air intake pre-turbo. I hate the idea of feeding that **** back into my turbo and intercooler, catchcans notwithstanding.
VTA. The oily vapor that comes out of your engine crankcase will cause detonation if your engine ingests it. And it reduces the volume of oxygen you're getting into the engine by giving you something other than fresh air.
Tried the GTX PCV valve that everyone recommends here. In the stupid blow/suck test, it seemed like it might work better. It didn't. Spool data was basically the same as the stock valve (which I'll post in my build thread).
Now that I understand how stock PCV valves work, I also see more deficiencies with the GTX valve at idle. My idle was intermittently 300-400rpm high, because the valve didn't close completely at high vacuum and it was acting like a vacuum leak. Basically, like the simple check valve in the OP.
Sixshooter is right. PCV is stupid for an aftermarket turbo car. Tomorrow I cap the IM and try it with both vents VTA.
So from what I’m reading, the PCV valve (cold side outlet) is an emissions device, and the vacuum from the manifold does little to assist in crankcase ventilation.
Therefore its better to gut the PCV valve, run it to atmosphere?
I wonder if this might be part of the p0300 issue I have...hmm. I've seen it at idle, I've seen it going up a hill after rolling off throttle and I've seen it wide open in fifth gear. Haven't been able to catch a freeze frame at anything other then idle though.
I consider ditching the PCV system the lazy way of going about the problem. Also, I need it in order to be road legal. So what I did is run -10 AN lines with welded bungs, a sturdy metal check valve (that black thing sandwiched between the blue&red AN fittings next to the catch can) AND a PCV. I did the tiny hole mod in the valve cover, too. The correct order is as follows:
This way the only section that sees boost pressure is between the intake and the check valve. The latter is rated up to 500kpa, so that should be more than enough LOL
So from what IÂ’m reading, the PCV valve (cold side outlet) is an emissions device, and the vacuum from the manifold does little to assist in crankcase ventilation.
Therefore its better to gut the PCV valve, run it to atmosphere?
No need to gut a PCV valve. I'm not sure how you would do that anyway.
Remove the valve and put a "push in" barb fitting in the grommet or remove the grommet itself and thread the hole for a fitting. Or you can buy a threaded bung to have welded in if you want to get fancy like some of us. It doesn't hurt to increase the diameter if you're planning on running any kind of power. You don't want any restrictions.
I consider ditching the PCV system the lazy way of going about the problem. Also, I need it in order to be road legal. So what I did is run -10 AN lines with welded bungs, a sturdy metal check valve (that black thing sandwiched between the blue&red AN fittings next to the catch can) AND a PCV. I did the tiny hole mod in the valve cover, too. The correct order is as follows:
This way the only section that sees boost pressure is between the intake and the check valve. The latter is rated up to 500kpa, so that should be more than enough LOL
Mo' pictures in my build thread.
All that work on the cold side, yet you’re still VTA through a breather on the hot side? And the added line resistance of the check valve AND the PCV, AND a couple extra feet of tubing on the cold side. And the turbo pushing more blowby into the crankcase than stock? I’m skeptical whether that’s helping emissions much, or vacuuming much of anything out of your crankcase. Does your catchcan accumulate oil?
re: PCV, do you have intermittent issues with idle going high?
Since I started this as a reference thread, perhaps you could post a link or a part number to the check valve you chose, in case future searchers are interested. Nice clean engine bay BTW. Thanks.
Last edited by Schroedinger; 10-30-2017 at 09:56 AM.
VTA. The oily vapor that comes out of your engine crankcase will cause detonation if your engine ingests it. And it reduces the volume of oxygen you're getting into the engine by giving you something other than fresh air.
Forgive the noob question, but when you vent to atmosphere (assuming that is what VTA means) does it have to be to a catch can? Is there an easier solution?
Also, am I missing something or is allowing the intake manifold and crankcase to vent by removing the PCV going to lower your boost pressure and essentially create a boost leak?
Forgive the noob question, but when you vent to atmosphere (assuming that is what VTA means) does it have to be to a catch can? Is there an easier solution?
Also, am I missing something or is allowing the intake manifold and crankcase to vent by removing the PCV going to lower your boost pressure and essentially create a boost leak?
Trying to wrap my head around this.
From what I’m understanding, you can put a breather on both sides of the valve cover. Just cover up the intake manifold side barb.
Tried the GTX PCV valve that everyone recommends here. In the stupid blow/suck test, it seemed like it might work better. It didn't. Spool data was basically the same as the stock valve (which I'll post in my build thread).
Now that I understand how stock PCV valves work, I also see more deficiencies with the GTX valve at idle. My idle was intermittently 300-400rpm high, because the valve didn't close completely at high vacuum and it was acting like a vacuum leak. Basically, like the simple check valve in the OP.
Sixshooter is right. PCV is stupid for an aftermarket turbo car. Tomorrow I cap the IM and try it with both vents VTA.
that's still not going to fix your spool problem.
if you really suspect a leaky pcv, add a check valve in front of it.
The GTX pcv is no better than the oe miata one. stop reading posts from 1995...
If only for idle purposes, I remain convinced that PCV < VTA for a turbo setup.
LukeG, that can work (see most recent post in my build thread). If you have leaky rings, the breather filter may get saturated with oil and crud. I will be putting a catch can in there with a breather on the other end of that.
10-28-2017, 06:24 PM
On PCV’s, check valves and catch cans
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