catch can for blow-through maf
 

i've been working on getting my pcv system fixed up lately, first fixing a problem with the pcv leaking under boost and now trying to eliminate a vacuum leak.

the problem is that there is nowhere to run the crankcase breather to (the line that would normally run to the intake, not the pcv line running to the intake manifold). with a blowthrough maf, if i place the line before the turbo in the intake tract it's going to be pulling vacuum, but it'll also be pulling in unmetered air at idle/low load. i can't place it after the maf because that would be after the turbo, and would see boost.

my current plan is to just cap the intake manifold and run both the pcv and crankcase breather to a vented catch can. that way there is unmetered air in the crankcase, but at least it has no path back into the intake manifold. of course this produces little to no crankcase vacuum which kind of takes the 'positive' out of 'positive crankcase ventilation', but it seems like the best i can do.

how is everyone else handling this with a blow-through maf?

EDIT... I don't understand what your saying, if the maf is in blow through mode it is between the turbo and the throttlebody and the intake port would be pre turbo so it WOULD NOT be sucking in unmetered air at all since the air has yet to pass the maf at that point. If your worried about fuel trims, get some logs before hooking it up and after to compair, but i really don't think it will be an issue at all.

Your plan would never pull a vacuum on the case (as you mentioned). As an alternative, install a check valve between the manifold and can then tie the two crank case ports to the other side of the can. At least then it is pulling vacuum when off boost and the check valve closes under boost so the crank case would be vented to atmosphere under boost.

The best solution is to run two cans in the factory configuretation but with the intake port moved pre turbo / SC. Its a good idea to install a check valve between the manifold and can since the PCV will leak some under boost and was not designed for boosted applications. This way you have a vacume on the case all the time (intake side under boost, manifold side under vacuum).

thanks for the help. i'm a bit confused though. if the crankcase vent goes back to the intake before the turbo, i agree that the air being pushed out will be metered, but the air being sucked in (which is the primary purpose of the crankcase vent, as the pcv has a check valve in it and will not allow air into the crankcase) will not be metered, and will eventually make it back into the intake manifold as unmetered charge. i noticed this from the idle/low fuel trims that we're going nuts. i've since capped the manifold to keep that unmetered air from getting back into the manifold and i have a far more stable idle (still have to log trims again, but i expect them to be improved).

so then is the idea that if i can get the crank vent tube to somewhere that is always under vacuum (in front of the turbo) it'll not pull in unmetered air? does this hold true at idle when there is very little vacuum in front of the turbo, or will a setup like this cause the same issues at idle/low-load as before?

thanks for all your help! this is driving me nuts trying to get it right lol.

Quick comment, as soon as you hit boost, air is not sucked in. Don't worry about your fuel trims too much on idle. The unmetered air going in while on vacuum is not much to worry about.

I see, your talking about air being sucked into the crank case... This is how the factory system is setup so that you can pull fresh air into crank case but it is post maf... I follow you now.

Sounds like a good reason not to do blow through to me... but i'm sure it's too late to change that for your setup.. I'll have to think about this one... If you just seal off the intake side crank case vent you won't ever be able to pull fresh air into the crank case to circulate through (hence the "vent" portion of pcv).

hmmm... off to the interwebs to search... biab

OK, so this is a common issue with blow-through. A few solutions to choose from if you have to run blow-through :

1. Cap the port at the manifold and T both crank vents together to your AOS / Catch can inlet and run a single outlet to the pre turbo intake tube, this should always pull a vacuum on the case, but will not provide a source of fresh air circulation.

2. Vent both ports to atmosphere... downside is will never pull a vacuum on the case

3. Run a dual can setup like factory (intake side port pre turbo). Down side is you will want to re scale the maf to account for the vacuum leak when manifold pressure is less than atmospheric.

I would probably try #1 first and see what it catches in the can. I might try #3 but I don't have to rely on a tuner to re-scale my maf for me since i use BRZedit.

derp, double post

I think I'm going to go with option 2 (similar to the d3pe can setup) venting both to atmosphere and capping the manifold. That will keep 100% of the hot nasty air out of the intake charge. I can add a real vacuum pump setup later if I see something that requires it (oil leak, etc).

Thanks for the help!

I run D3PE with no issues :thumbup:

i just put a mini filter on it lol :)

You really hate vented catch cans don't you, haha. I also still run the D3PE, and when I see what vents out of it, I am glad I do.

Yeah I want absolutely zero blow by going back into my intake tract. Only one way to accomplish that. I don't trust any can to remove all of it.

Sorry to bring back this old thread, but I've been lurking the forums trying to find help.

I have a issue with my car running low vacuum (.48-.52bar) and high STFT's at IDLE.

I have in the radium dual catch can set up with both of them vented to atmosphere, I noticed you said if they are vented to atmosphere I have to have my maf rescaled.

I've just done a complete build and have not driven the car as yet as I'm doing remote tuning with delicious

P.s I noticed @mad_sb hasn't been active since last year, hoping someone else can help me

just put a one way check valve in the line