PCV Valve / Rocker Cover Breather

[Aidan@EP-styling]

Well all,

Ok I know that the pcv valve opens only under vacuum bla bla bla...

But both the pcv valve and rocker cover breather expell gasses that have blown down past the piston rings. So do the 2 things do the same job? Why is there two breathers???

[Dub-Se7en]

the pcv valve will spend a lot of time closed as the the engine goes into positive pressure (ie when you hit boost) , this is where the secondary breather helps

[Sean_GT]

Would the breather on the engine not having a filter or anything cause any problems because mine is just the rubber tube with nothing on the end of it

[Dub-Se7en]

Would the breather on the engine not having a filter or anything cause any problems because mine is just the rubber tube with nothing on the end of it

not really just might draw fumes into the car and splash some oil out.

[Aidan@EP-styling]

the pcv valve will spend a lot of time closed as the the engine goes into positive pressure (ie when you hit boost) , this is where the secondary breather helps

but then I dont see the point of having a pcv valve in the first place if the rocker cover breather can do the job on its own???

[davehart]

I suppose its there as a back up in case one gets blocked up.

[weeJohn]

The pvc valve is there to do 2 jobs. Remove oil and water vapour in the hot air at the top of the head. This is why there is a second skin to speak inside the rorcker cover, to allow these vapours to accumulate there and as a backsplash as well. Its second job is to help relieve positive crankcase air pressure.

Remember that the second pipe on the rocker, the one we fit beathers to, is where the stock recirculating dump valve used to connect onto.

Fitting a breather on there prevents the build of of crankcase air pressure. If its just a pipe there it will do the same thing, but you also get negative pressure sometimes as well, which means that dirt could get sucked in there and into the oil and into the bearings as well. The filter also allows air to flow through it when the pvc valve has vacuum applied to it, preventing a negative pressure in the crankcase and allowing good vapour release.

It makes no difference to performance sucking these vapous in at idle. We fit catch cans to prevent the heavy oil vapours getting through, as when the air is sucked into the can it drops slightly in pressure and allows the heavy vapours to accumulate in there.

The pvc valve is a 1 way valve to prevent boost pressurising the crankcase as was said before. If it is connected to the intake manifold, even if its through a catch can, on idle it will still suck the bad vapours out. The vapours will not get sucked out if vacuum is not applied to the pvc valve.

Really we need both for a belt and braces job.

[MWP]

The valve is also used on NA engines...
It also has the purpose of stopping backfires into the inlet manifold from igniting the crank case gasses (=BIG bang).

The benefit to pulling the gasses out of the crankcase is it helps prolong the life of the engine oil.

It still amazes me how many people install catch cans wrongly.

the pcv valve will spend a lot of time closed as the the engine goes into positive pressure (ie when you hit boost) , this is where the secondary breather helps

Actually most road going turbo cars are only under boost a fraction of the time (say 5%-10%?).
Th rest of the time the inlet manifold is at vac, so the PVC system is in operation.

[weeJohn]

The valve is also used on NA engines...
It also has the purpose of stopping backfires into the inlet manifold from igniting the crank case gasses (=BIG bang).

The benefit to pulling the gasses out of the crankcase is it helps prolong the life of the engine oil.

It still amazes me how many people install catch cans wrongly.

Actually most road going turbo cars are only under boost a fraction of the time (say 5%-10%?).
Th rest of the time the inlet manifold is at vac, so the PVC system is in operation.

I agree completely, people just disregard the pvc valve.

Someone went to the bother of designing a catch can so as there would still be this sort of system in place, but the can would prevent the less desirable heavy oil and water vapours getting into the intake.

A catch can is an improvement to the system, not an alternative.

[Mike_ep91]

so can the pcv valve coming from the rocker be connected to a catch can and have the outlet on the inlet mani blocked off??

[MWP]

so can the pcv valve coming from the rocker be connected to a catch can and have the outlet on the inlet mani blocked off??

No. This is what 99% of people do and is wrong.

Do this:
PCV -> catch can inlet
catch can outlet -> inlet manifold

The catch can should be the sealed type (no air filter on it).

[Mike_ep91]

No. This is what 99% of people do and is wrong.

Do this:
PCV -> catch can inlet
catch can outlet -> inlet manifold

The catch can should be the sealed type (no air filter on it).

what about the other breather then??

[MWP]

Goes to the same place it does from the factory.

[weeJohn]

If you have removed the standard recirculating valve, which is where the other breather was attached to, you can fit a small filter onto the rocker cover there.

When it was connected there it was connected to the efi pipe, so it "breathed" through the main inlet filter.

[Gee]

So mine is wrong? (Picture below)

I was told this would help the engine 'breath' and I have seen other big HP Starlets setup like this?

[munday]

The problem with venting the catch can back into the inlet is that you still allow oil vapour back into the inlet causing octane dilution

[Gee]

Would this cause a loss in MPG?

[munday]

Would this cause a loss in MPG?

Possibly, id be more worried about detonation. TBH you probably wont be getting very much octane dilution, but i wouldnt want to risk it

[weeJohn]

The problem with venting the catch can back into the inlet is that you still allow oil vapour back into the inlet causing octane dilution

Thats the whole point of a catch can mate, when the vented gasses pass through the catch can the vapours are "dropped" in the can to prevent them going into the inlet.

Not 100% efficient, some vapours will still get through, but the majority wont.

Gee in my opinion your set up is wrong. There is nothing in your set up that will remove the vapours from the crankcase into the can. Have you ever had to empty it?

This is my setup, please dont think this is a way of showing how yours is wrong as its different from mine, this is the way I do it.



I use 2 pvc valves so as the valve nearest the inlet mani closes under boost, preventing the can getting pressurised. The valve in its normal place stops anything in the can being sucked back into the crankcase. The breather filter does as it named, its on the other pipe off the rocker, where the standard dump valve hose was connected.

I get an emulsified oil in my catch can, like a melted bar of fudge.

I empty it about every 6 months, the ammount in it varies.

[Gee]

OK, so my setup is wrong.

The outlets are welded in place, so can't do nothing about that now unless another rocker cover is used.

How would you 'suggest' my catch can is setup?

[munday]

Yes thats true, but i still wouldnt want the minimal amount of oil vapour going back into the inlet, as well as octane dilution you will have a thin layer of oil covering the inside of the intercooler, doing no good for its heat exchange properties.

If you dont want to vent to the atmosphere (which is illegal) does your catch can have anything inside? If not i would buy some metal kitchin scourers (heavy duty so they dont fall apart from vibration) and stuff them inside to give more surface are for the oil vapour to condense on

[Gee]

Yes thats true, but i still wouldnt want the minimal amount of oil vapour going back into the inlet, as well as octane dilution you will have a thin layer of oil covering the inside of the intercooler, doing no good for its heat exchange properties.

If you dont want to vent to the atmosphere (illegal) does your catch can have anything inside? If not i would buy some metal kitchin scourers (heavy duty so they dont fall apart from vibration) and stuff them inside to give more surface are for the oil vapour to condense on

Is this in relation to weejohn's setup? And I thought venting to the atmosphere was illegal..they way you have worded it, it seems you mean the other way round.

[munday]

Is this in relation to weejohn's setup? And I thought venting to the atmosphere was illegal..they way you have worded it, it seems you mean the other way round.

Not referring to any setup in particular

Edited my post to make it more clear, you are right i meant venting to atmo is illegal

[Gee]

OK screw this.

Going to get a catch can that does not vent to the atmosphere. I will put a PCV valve back in place as where it should be.

So, is WeeJohn's setup correct? Do I need a catch can with 2 inlets? Or what about this setup that everyone seems to use? Is this OK or not? The hose on the other side of the catch can goes nowhere, so is this still venting to the atmosphere?

[WallaceGlanza]

Your old setup is venting to atmosphere yes. I was going to have the same setup but put the second hose t-pieced into the pcv valve hose so it still had a vaccuum source as in the original way toyota had it setup.