PCV Catch Can Confusion

[cattaway07]

I am having a difficult time finding solid information on how exactly a PCV system with a catch can should be setup on a naturally aspirated V8 engine. My current PCV flow path is as follows; intake (before throttle body) ---> crank case ---> PCV valve ---> catch can ---> atmosphere.

Some people are saying that having both valve covers routed to the catch can and then to atmosphere is optimal, but this seems to negate the purpose of a PCV valve. From what I understand, having the PCV system connected the way it is done on my vehicle creates a vacuum and draws oil vapors out of the crank case better than just the blow-by alone.

To make a long question short, should the PCV system send vapor out both valve covers or draw air in one cover and out the other?

[sly]

Catch can should be on the breather side since that is the side that pulls the vapors into the intake. The PCV valve side to the intake doesn't tend to collect near as much if any oil unless there's massive blowby that just keeps the PCV valve open. Since the PCV Valve side is on the bottom of the intake plenum (box) or intake elbow (aero/whale) and the breather is on the other side of the throttle valve in the intake tube to create a pressure differential to move the vapors to the intake side, if you run both to the can and then to something, it should probably be the intake plenum/elbow and plug the tube port to help pull the vapors out strongly in all cases except WOT. During WOT, vacuum is weakest and won't do much anyhow. I'm not totally versed on the dynamics of the system, but this is my take on it after thinking it through.

[gadget73]

thats not a PCV system, its a "draft tube" at best

The PCV should be connected to manifold vacuum. Filtered air is pulled through the fitting upstream of the throttle body into the engine and the fumes are pulled through the PCV valve into the manifold where they get burned

If you need a catch can, it goes between manifold vacuum and the PCV valve, but honestly unless you have something wrong with the engine or PCV system it really shouldn't be pulling enough oil through that line to really need one.

[sly]

I think the real reason for the catch can is to deal with racing. Lots of wide open running that has little vacuum or boost allowing oil vapors to come out the breather tube into the intake.

[gadget73]

its also useful to deal with engines that have the baffles removed for whatever reason. Not a big deal on a SEFI vehicle but on setups where it pulls through the valve cover, if those baffles get cut out for rocker arm clearance a lot of oil gets flung around and it can get sucked up in the PCV system. It will also happen if that metal screen under the PCV is removed. Found that one out myself when I forgot to buy another and then a few weeks later had to remove the upper intake for some reason. I was pouring oil out of it and then I remembered the screen. Put that back in, haven't had problems since.

The small baffle Explorer lowers also have that issue, Ford's fix was to replace the wee bitty baffle plate with the big plate that was standard on pre-Explorer 5.0 engines.