catch cans vs breather cans
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From: Central Cal.
The MAF HAS to measure EVERY bit of air going into the engine to accurately get the A/F ratio right. If you suck in air from another source, it throws that totally off.
Letting in unmetered air is NOT beneficial, it is a detriment.
Letting in unmetered air is NOT beneficial, it is a detriment.
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From: Wilmington, DE
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From: Central Cal.
Nothing to be confused about. ALL air going into the engine, whether going straight in or going thru the PCV system, MUST be measured (or "metered") by the MAF so the PCM will know how much fuel the engine will need. This is why the air supply for the PCV system must tap into the intake tube AFTER the MAF.I
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Coincidentally, we were just talking about this on that other site....
https://www.corvetteforum.com/forums...post1597229356
https://www.corvetteforum.com/forums...post1597229356
TECH Senior Member
Joined: Jun 2016
Posts: 22,293
Likes: 3,617
From: Central Cal.
Coincidentally, we were just talking about this on that other site....
https://www.corvetteforum.com/forums...post1597229356
https://www.corvetteforum.com/forums...post1597229356
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Unless you've got boost... 
Then you at least need a check valve on the intake manifold... And if you want a catch can to trap oil mist coming out of the crankcase while you're in boost, then it gets more complicated.
Then you at least need a check valve on the intake manifold... And if you want a catch can to trap oil mist coming out of the crankcase while you're in boost, then it gets more complicated. 
My LS1 had lots of oil in the intake and heads. My mechanic ran the pvc hose off the intake into the catch can and out of the catch can to the passenger valve cover. He blocked off the back of the passenger valve cover. He then ran a hose from the throttle body back to the rear of the drivers side valve cover.
I'm a MAF tuned car and if I cap off my throttle body and intake manifold ports and run valve covers that have 10AN bungs I can run my breather can and vent to atmosphere to relieve the crankcase pressure. This works like a champ and I never have to worry about the blowby mixing with my air/fuel mixture and reducing the octane, however be prepared to see smoke coming from the breather can and the smell of fumes when the engine is running. I've yet to see much in the way of oil in the breather can even after making hard pulls on the car. The only thing I did notice is that when I run the car in the winter months the breather can will get filled up with water from condensation.
Now when I run the LS7 valve covers and connect the PCV lines up. Driver side valve cover port and valley cover port connected to a sealed catch can and then into the intake manifold, and the passenger valve cover port to the throttle body port all the smoke that I was seeing is now being introduced back into the intake manifold and being reintroduced into the combustion chambers and through the exhaust manifolds. Ingenious no doubt! Lets take all that blowby which has oil in it by the way and send it right back into to the combustion chambers to do what? Well you pull an engine apart that's been running PCV for over 300,000 miles and see for yourself. Simply put PCV is nothing more than an emissions system.
Now when I run the LS7 valve covers and connect the PCV lines up. Driver side valve cover port and valley cover port connected to a sealed catch can and then into the intake manifold, and the passenger valve cover port to the throttle body port all the smoke that I was seeing is now being introduced back into the intake manifold and being reintroduced into the combustion chambers and through the exhaust manifolds. Ingenious no doubt! Lets take all that blowby which has oil in it by the way and send it right back into to the combustion chambers to do what? Well you pull an engine apart that's been running PCV for over 300,000 miles and see for yourself. Simply put PCV is nothing more than an emissions system.
Thread Starter
Joined: Mar 2011
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From: Wilmington, DE
i understand the whole metered air thing. so why run one with a breather on an N/A maf car? unless you dont. i bought the mighty mouse can in an effort to cut down on oil in the intake.....should i not use it since i have a maf and am n/a? unless someone can confirm as mentioned earlier that the mighty mouse has the valve that won't allow unmetered air in....
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From: Central Cal.
It's not about any valve that determines what is unmetered air. It's about where you SOURCE the air going thru the engine before being sucked into the throttle body, AFTER the throttle plate. This source must be from the air intake BETWEEN the MAF and the throttle body. No valve can determine that. No valve has "discerning" properties.
Well, on my Trans Am the hose was run from the TB to the rear of the driver valve cover. I'm assuming the air travels up the block and to the rear of the valve cover and back to the TB. Since that's internal engine air, it was already read by the MAF.
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The mighty mouse guy says the vent on his can is exit-only. So it only opens when the intake manifold check valve is closed due to boost.
So it's useful if you have forced induction. Not so much if your engine is naturally aspirated.
So it's useful if you have forced induction. Not so much if your engine is naturally aspirated.