Positive Crankcase Ventilation Plumbing

[stirlsilver]

katanaman,
Yes I know I require a PCV valve but i'm going to get one when I put my EFI manifold on, I think I might need to use two PCV valves since at the moment I have a direct line connected post TB and it isn't enough to keep a vacuum when you get up it.

GreenV8S,
I'm drawing the vacuum from one tappet cover and I have placed the vacuum gauge on the other, and I have used hoses that are ample in size. I just would like to know if 4psi of vacuum is too much...

I'll do a proper setup with my EFI manifold with two PCV valves but I just need something for the moment to stop the engine smoking on heavy engine breaking and stop the 1L of oil being consumed every 1000K's

Stirling

[GreenV8S]

katanaman,
Yes I know I require a PCV valve but i'm going to get one when I put my EFI manifold on, I think I might need to use two PCV valves since at the moment I have a direct line connected post TB and it isn't enough to keep a vacuum when you get up it.

Nothing you do will give you any depression under full throttle, so the crank case will inevitably pressurise. I've never measured crank case pressure on mne but I would have thought that it should be less than a couple of PSI. As long as you have a clear 3/4" hose from the rocker cover to somewhere at atmospheric pressure the crank case pressure should be OK unless you have a massively worn engine. I would recommend the EFI layout though if you're going for a breather into the intake, because that seems to work fine. Alternatively, just stick the breather into a catch tank and you will know exactly how much fluid is being blown out, should be no measurable amount of oil but probably loads of water condensing in there.

[stirlsilver]

I overhauled the engine about 2 years ago now, so it's still fairly tight. Thing is I didn't touch the heads other then re-seating the valves so i'm thinking that i'm loosing a bit of oil down the guides when I engine break. In anycase, I was runing the engine with no vacuum and with some vacuum it certainly stops the oil consumption.

Thanks for the info tho!

[Paul B]

I overhauled the engine about 2 years ago now, so it's still fairly tight. Thing is I didn't touch the heads other then re-seating the valves so i'm thinking that i'm loosing a bit of oil down the guides when I engine break. In anycase, I was runing the engine with no vacuum and with some vacuum it certainly stops the oil consumption.

Thanks for the info tho!

I have used a 1/2 hose from one rocker cover to the large vacuum port on the front of my weber, but I have put a dome nut inside with a 1/16" hole drilled through it, as a restriction. I used this as it was all i had the right size and shape, and so I can pull it out with needle nose pliers if I need to.

The other rocker cover has the usual small K&N filter on. I feel no paticular 'suction' if put my thumb over the stub the K&N goes on, but then I get no oil vapour/mist at idle or in traffic, and my idle speed didn't rise at all when I fitted the system so it is not allowing air to scream thorugh. I may open up the restrictor hole just a touch more to get a little more scavenging, but it's just an experimental thing at the moment. I don't know if I need a catch can in the pipe either, but examination will tell if is sodden with oil next time I pull it off.

I'm not even sure what a PCV actually does to be honest.

[JSF55]

I think i fitted a bolt in tyre valve to my rocker cover, and then the 1/2" hose to the base of the edelbrock again with a small K@N on the other one, worked fine all last year and no fumes in the car, john

[stirlsilver]

I have used a 1/2 hose from one rocker cover to the large vacuum port on the front of my weber, but I have put a dome nut inside with a 1/16" hole drilled through it, as a restriction.

I fitted a standard valve on the line between the tappet cover and the base of the TB so I can vary the amount of vacuum that the engine gets, however i'm just leaving it open now because if I close it up to restrict the flow the vacuum drops off quickly when you start to accelerate.

The other rocker cover has the usual small K&N filter on. I feel no paticular 'suction' if put my thumb over the stub the K&N goes on, but then I get no oil vapour/mist at idle or in traffic, and my idle speed didn't rise at all when I fitted the system so it is not allowing air to scream thorugh.

hehe, well with 4psi of vacuum you can hear it when you pull the hose off and when you start to take off the oil filler cap it starts to make all sorts of whistling noises... too much vaccum for sure so I ended up opening the hole in which it draws air through more to prevent it reaching such a high vacuum. I ran the hose into the cab of the car and it will be sucking, but then when you accelerate you feel the suck slow, stop and then start to blow... Suppose it's to be expected with 12:1 CR. I'll fit two vacuum hoses to try and maintain a vacuum for as much of the rev range as I can.

I'm not even sure what a PCV actually does to be honest..

I beleive it stands for Pressure Controlled Valve and it will open and close to try and maintain a certain vacuum pressure in the crank case. When you accelerate and you have some blow by you need to draw more gas out to maintain the vaccum.

[GreenV8S]

When you open the throttle the manifold depression drops off quickly, and at the same time the blow-by increases, you would not expect to find any significant depression in the crank case under firm acceleration. If you allow too much flow through the breather then you will throw the mixture out when you are off load.

[IanT]

Paul B:

Here's how I understand it now..
PCV is Positive Crankcase Ventilation.
The PCV valve controls the amount of 'ventilation' allowed to be 'pulled' out of the engine via manifold vacuum.

At high manifold vacuum, the PCV valve doesn't allow much flow, so does NOT become a big vacuum leak as I feared in my original question. So it shouldn't hurt your vac and ruin your idle.

At low vacuum, (under load; more blowby) is when the PCV valve starts working to pull the vapours out the engine.

The PCV valve should also block off flow in the event of a backfire.

I found a good explanation here:
http://www.jalopyjournal.com/forum/show ... =pcv+valve

[jefferybond]

Hi

My Rover V8 just has R/C breathers vented to atmosphere - I want to set up proper PCV though.

I've got the bits to bring air in through the air filter into right hand R/C.
Got a PCV valve from Real Steel (Fram) to 'pull' out of the left hand R/C.

I'll need to 'T' the PCV valve into the vac hose that runs from the Holley carb PCV port to the brake servo.

My question: sucking on the PCV valve offers very little restriction, so it feels as if I'm about to install a massive vacuum leak in the system! Do I need some kind of additional restriction in there somewhere?

:?:

I didn't use a PCV valve on mine, but I made a small brass restrictor that fits inside the hose from the rocker cover to the carb PCV port. The restrictor is just a brass plug with a 2.5mm hole drilled though. An old carb jet might work if it fits snugly inside the hose.

Basically I have it arranged like this:

The flame trap on left rocker cover connects directly to the base of the air cleaner
The hose on the right cover cover goes to the carb PCV port, with a restrictor inside the hose (hole in restrictor is about 2.5mm diam).

The extra air coming into the carb via the PCV port affects the idle speed and mixture slightly, but this can easily be corrected with the idle screws. Obviously the restricted hose can only remove so much fumes, so when the engine is at high speed/heavy load, the fumes come out of the flame trap side and enter the air cleaner base, where they are gobbled up by the carb!

Very simple and works a treat!

Jeff

[mgbv8]

Wouldnt you lose servo power if you hook the pcv to the servo line. Under decelerating vacuum, the pcv would feed air into the vac port of the carb and starve the servo of vacuum wouldnt it?
When I had this setup on mine I left the servo alone and put the pcv into the other large vac port on the weber. The other vent went to the air filter housing. My understanding is, when you are under power, excess pressure comes out of the engine into the air filter, when decellerating the pcv opens to scavenge vapours into the underside of the butterflies in the carb.

I have seen the vac pumps for the crankcase on a couple of cars. They were driven from the back of the alternator to maintain a constant suck on the crankcase.

I now just have two hoses with large inline filters that go out through my inner wings. I'm just waiting for my catch pots to arrive.