PCV/Catch can thread - just info

Michael Yount · Oct 28, 2004

Many of you know I've got catch cans on both my pcv valve vacuum line, and on the throttle body to valve cover line. They've been on the car for probably a year and a half/10k miles. During that time the oil has been changed 3 times. I've never noticed any reduction in oil level on the dipstick between changes.

Today for the first time I drained the catch cans. I got a total of about 6-8 oz. of oil out of it. This is on a car that's driven pretty easily, mild h/c/i/exhaust mods (about 275HP crank). The short block has about 65K miles on it.

I suppose I was surprised that it had trapped that much oil. The good news is that it seems to be pretty effective - I wouldn't have guessed that much oil would've accumulated.

90mustangGT · Oct 28, 2004

I would have expected more oil, but on my old motor.

Do you have any pictures or tips on how you made yours?

HISSIN50 · Oct 28, 2004

Jason, awhile back Michael was kind enough to shoot some pics of the setup to me. he did a very nice 'bargain' set up.

Michael, i would surmise the amount of oil you found speaks well for your particular level of blowby and general level of upkeep.
thanks for sharing your findings - i have been meaning to rig something up myself (soup can and fuel filter).

Michael Yount · Oct 29, 2004

Jason - send me an email at myount@senndelaney.com and ask for catch can pics/details, and I'll respond.

cardudeusa · Oct 29, 2004

Whats the reason for doing this other than to catch oil and see how much blowby?

Rick 91GT · Oct 29, 2004

Micheal, have you tried different locations to see the effects?

I was told by an old super gasser that the catch cam needs to be higher the valve cover or port you are tapping to prevent it from pulling oil. I was also told to mount it on the firewall.

I treid the front near the rad and it did pull more oil then when I had mine on the rear of the shock tower.

Where are your's located?

Michael Yount · Oct 29, 2004

On mine not so much to see how much blowby - the bottom end on mine is low mileage and pretty solid. It's to keep as much oil out of the intake as possible. Keeps things a bit tidier in there, and makes O2 sensors last longer.

Michael Yount · Oct 29, 2004

Haven't tried different locations Rick. Mine is located higher than the pvc valve on the firewall behind the passenger strut. The tube from the pvc comes straight up, makes a gently 90 degree bend towards the passenger side into a see through fuel filter - acts as a trap to knock out some of the oil, and makes a gentle downhill slope into the side of the catch can about 3/4 of the way up the side. Whatever oil the filter knocks out, it gravity feeds into the can. The vacuum connection to the can comes straight into the top of the can and runs directly to the manifold port.

Similar set up on the throttle body to valve cover nipple. I set the filter in the line from the valve cover nipple to the catch can so that when air is flowing from crankcase to throttle body (wot), I can knock out any oil before it hits the throttle body. But it's clear (no pun) looking at this filter, that there's very little oil moving through that part of the system.

Probably just means I need to spend more time with the throttle open more, and less time seeing how much gas mileage I can get.

Thomas302 · Oct 29, 2004

What is the point of the tube from the valve cover to the throttle body? I am using motorsport valve covers with no nipple and I just plugged up the nipple on ththrottle body, and its been that way for 2 years no problems. Should I put a tube there? Just wondering. Thanks

Edit: I do have a breathered cap on my valve covers.

TheUser · Oct 29, 2004

Thomas302 said:
What is the point of the tube from the valve cover to the throttle body? I am using motorsport valve covers with no nipple and I just plugged up the nipple on ththrottle body, and its been that way for 2 years no problems. Should I put a tube there? Just wondering. Thanks

I'm sure someone will come behind and give a better, more detailed reason, but I think the gist of it is at Wide Open Throttle, your crank case has a lot of pressure, which is then routed back up and through your valve cover to the throttle body and put back into the system, of course after the MAF, as to not read the same air twice. If you don't have that hose, you could possible run the risk of blowing seals because the pressure has to go somewhere. Some in your situation simply put a filter on top of the valve cover instead of the cap; some negative consquences are that you will most likely have an oil film all over your engine bay from it and you're letting out metered air.

Now I have a question....isn't that what the PCV is for?

matm347 · Oct 29, 2004

The PCV uses vacuum, since there is no vacuum at WOT, it no longer functions which is where the TB tube comes in. Even though the PCV will still allow gas to vent at WOT, it's main function is part throttle/idle.

I have my PCV capped off and vent the valve covers with a tube under the car...like they did back in the 50s n such.

Michael Yount · Oct 29, 2004

TheUser is close, but not quite there.

Under vacuum conditions, engine vacuum sucks on the crankcase through the pcv valve. It pulls any blowby into the intake manifold. Because you don't want to create a vacuum in the crankcase, there has to be a path to LET METERED AIR INTO the crankcase. That's what the little hose between the throttle body and the valve cover nipple are for. Under high manifold vacuum, metered air travels from the throttle body to the crankcase through the valve cover nipple. Some people who remove the hose and cap off those openings hear squealing sounds under vacuum as air gets pulled into the crankcase past the front or rear seals.

At w.o.t., there is crankcase pressure, and atmospheric pressure in the intake manifold - no vacuum. If the crankcase pressure is greater than the manifold pressure then crankcase venting will occur through the pcv valve/line AND through the valve cover to throttle body line. You can now see that the throttle body to valve cover line is bi-directional. Under some circumstances, air moves from the t/b to the crankcase; under other circumstances air moves from the crankcase through the throttle body. At part load when there is low manifold pressure and low blowby (crankcase pressure) the differential can't overcome the small check valve in the pcv, so venting of the crankcase occurs through the valve cover to throttle body line.

That's what that hose is for. There is absolutely no performance benefit to disconnecting/capping it off - and it may be detrimental. If you have a functioning pcv hooked to manifold vacuum, I'd hook it back up. For breathered valve covers, it's unnecessary.

Michael Yount · Oct 29, 2004

mat - "since there is no vacuum at WOT, it no longer functions"

Not quite right mat - the pcv has a lightweight non-springloaded check in it to prevent backflow. It takes very little pressure differential to open it. So, even at wide open throttle (no vacuum) if crankcase pressure exceeds atmospheric pressure by even a little bit (not uncommon), then the crankcase can vent through BOTH the pcv and the other little hose.

GT Trooper · Oct 30, 2004

Michael Yount said:
TheUser is close, but not quite there.

Under vacuum conditions, engine vacuum sucks on the crankcase through the pcv valve. It pulls any blowby into the intake manifold. Because you don't want to create a vacuum in the crankcase, there has to be a path to LET METERED AIR INTO the crankcase. That's what the little hose between the throttle body and the valve cover nipple are for. Under high manifold vacuum, metered air travels from the throttle body to the crankcase through the valve cover nipple. Some people who remove the hose and cap off those openings hear squealing sounds under vacuum as air gets pulled into the crankcase past the front or rear seals.

At w.o.t., there is crankcase pressure, and atmospheric pressure in the intake manifold - no vacuum. If the crankcase pressure is greater than the manifold pressure then crankcase venting will occur through the pcv valve/line AND through the valve cover to throttle body line. You can now see that the throttle body to valve cover line is bi-directional. Under some circumstances, air moves from the t/b to the crankcase; under other circumstances air moves from the crankcase through the throttle body. At part load when there is low manifold pressure and low blowby (crankcase pressure) the differential can't overcome the small check valve in the pcv, so venting of the crankcase occurs through the valve cover to throttle body line.

That's what that hose is for. There is absolutely no performance benefit to disconnecting/capping it off - and it may be detrimental. If you have a functioning pcv hooked to manifold vacuum, I'd hook it back up. For breathered valve covers, it's unnecessary.

Michael,

Would you recommend a breathered valve cover over the throttle body to valve cover hose for better ventilation? We discussed this the other day in reference to my rear main seal leak.

Michael Yount · Oct 30, 2004

On a naturally aspirated car, if you breather the valve cover with a working pcv set up on an efi car, then under vacuum conditions some unmetered air enters the intake via the breather/crankcase/pcv route. This seems to cause some cars/computers problems, and others don't notice any difference at all (see thomas 302 above).

For that reason I WOULD NOT recommend the breathered valve cover over the t/b connection on a naturally aspirated car - there's no performance benefit, and it could cause drivability issues - you won't know what it's gonna do to yours unless you try it. Besides that, it's tampering with the emissions system, which is illegal, although that doesn't seem to stop it from happening regularly.

BaXTeR3221 · Oct 30, 2004

Ok i'm following most of this, but what changes if you have forced induction, like a blower for instance?

Michael Yount · Oct 30, 2004

It gets complex in a hurry if you have boost. Your pcv valve has to be able to withstand the pressure without allowing backflow. The line between the t/b and valve cover has to dealt with as it will allow boost to the crankcase - and that'll blow some oil in a hurry.

Most folks simply say screw it, get a pcv valve off of a factory supercharged or turbocharged car, and put a breather on the valve cover to vent the crankcase when manifold pressure exceeds crankcase pressure. Some folks don't bother with the pcv and just vent the crankcase with breathers. Some route the vent lines to a remote mounted catch can and vent the can. Some vent the can to the suction side of the supercharger - but if you're blowing through the maf, you can accumulate oil on the maf hot wire. Lots to think about when boost arrives..... no pun.

BaXTeR3221 · Oct 30, 2004

yea this is all very confusing to me and its something i have to tackle in the spring, so i guess i should start reading up on it more.

HISSIN50 · Oct 30, 2004

intrcptrstng said:
yea this is all very confusing to me and its something i have to tackle in the spring, so i guess i should start reading up on it more.

the turbo stang forums should have more info (not all of us have forced induction).

good luck.

Michael Yount · Oct 30, 2004

To help with the confusion all you have to ask yourself is 'where is the pressure higher and where is it lower'? The flow will go from higher pressure to lower pressure every time. Any time you connect a hose or check valve as a part of all this, just keep that in mind, and you'll work your way through it.

PCV/Catch can thread - just info

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