30 replies to this topic

[LXCHEV]

Ok, obviously there are 2 reasons that I see why engines have a PCV valve connected from the rocker cover to the carb vacuum;

1. Pollution reasons - crankcase fumes are sucked back into the intake airstream and burnt/expelled inside the combustion chamber.

2. The vacuum helps relieve pressure from the crankcase

What I want to know is more info on point 2. Exactly how important is this? So many people just run breathers without a PCV, some use catch cans and breathers, also without a PCV. Does this have any effect on the engine? Obviously, if it's a heavy breather, it will be more likely to build up excess oil and possibly drip from the breather... but anything else???

[rodomo]

Old red sixes (179, 149 etc) used to have a steel pipe coming out of the rocker cover and pointing to the ground, a bit like a walking stick. The oil caps weren't sealed and thats how they breathed. I think the PCV's came in with 186? but could be wrong. If so that was about the same time as V8's in Holdens. The info I have is all Holden V8's were PCVentilated.

[FastEHHolden]

Even a fresh engine has blowby...an old text claims 40 litres per minute for a 2 litre 4 cylinder engine..so more for a worn 3 litre 6 cylinder.

The PCV shouldn't work at full throttle when blowby is high...as manifold vacuum is low and the spring should close the valve. Crank case pressure is then vented thru the pipe intop the air cleaner....so there is never a negative pressure in the sump.

at high manifold vacuum conditions crancase air is drawn thru the pcv valve into the manifold

[Litre8]

This system draws air into the engine through the oil filler cap or the air cleaner and circulates it through the engine. The air combines with vapors in the crankcase and exits the engine through a metering valve (pcv) mounted in the rocker arm cover. The air vapor mixture than re-enters the engine through the carb or intake manifold and passes the combustion chamber where it is burned.

In this case I think positive is in reference to the fact that unlike earlier crankcase ventilation systems (ie the old hook pipe) this system actively attempts to evacuate the crankcase of vapors.

[surfmaster]

The PCV is effectively removing vapours from the atmosphere and burning them by recycling the gas through the manifold and into the combustion chambers. As correctly stated above the older cars had a black tube that ran from the rocker cover down the engine to under the car where the burnt crap was exhausted into the atmosphere, (here insert global warming, drowning polar bears and a chronic beer shortage).

Older engines with a lot of air would fume straight into the atmosphere, PCV's limit some of that by recycling the fumes.

[ls2lxhatch]

When I was reading the oil bible it had a section on oil sludge and its causes. It appears that combustion gases and moisture in the crankcase are detrimental to oil.
http://www.carbibles...eoil_bible.html

One commonly held belief is that moisture causes or accelerates the formation of sludge. We do not disagree, and we wonder if SAAB's trouble with leaking head gaskets contributes to the moisture content in the crankcase. We also wonder if the relative increase in the length of the crankcase ventilation "circuit" is preventing the removal of combustion gases in the crankcase.

http://www.saabservi...ne_failure.html

[surfmaster]

When I was reading the oil bible it had a section on oil sludge and its causes. It appears that combustion gases and moisture in the crankcase are detrimental to oil.
http://www.carbibles...eoil_bible.html

One commonly held belief is that moisture causes or accelerates the formation of sludge. We do not disagree, and we wonder if SAAB's trouble with leaking head gaskets contributes to the moisture content in the crankcase. We also wonder if the relative increase in the length of the crankcase ventilation "circuit" is preventing the removal of combustion gases in the crankcase.

http://www.saabservi...ne_failure.html

That is why taxi engines last so long, they don''t have time to cool down and form water vapour and other corrosive mixes. Most of the damage to engines is done at startup and cold running, that is why short trips and stop start running on the way to the office is bad to the donk.

[LXCHEV]

Thanks for all the excellent input and comments so far. ls2lxhatch - yet another great article on that carbibles site, I hadn't read that page before, thanks for the link - I found that extremely interesting and educational.

So we have definitely found several things out:

* The extraction & recirculation of crankcase gases assists in reducing condensation/acid buildup/sludge buildup... and helps to keep the engine oil cleaner longer, and therefore increasing the life of the engine.

* Obviously it reduces pollution by not venting these gases straight into the atmosphere.

That first point above, I believe is only an issue if regular oil & filter changes are not carried out. Would you agree with this??

It seems a huge factor in determining the frequency of oil changes is the number of short trips/cold starts etc... as well as things like how long the car is left sitting around for, esp in colder weather etc....

Ok, next question.... so not worrying about pollution, nor contaminated oil...

Does not having a PCV connected have any other affects? I've heard people say without it, it places extra strain on gaskets, seals etc... is there any truth to this? (Let's assume we're talking a big cube V8, fitted with a fully breathable cap to each rocker cover). Could this place extra strain on the seals and stuff??

Considering the huge amounts of air that an engine pumps, would these allow enough air both in and out of the crankcase???

And lastly... for racing purposes - it seems most racers always prefer to run no PCV... to maximise the cleanest possible intake air into their engine. We did a test on the dyno recently with my Chev.... 1st pass with PCV connected.... monitored air:fuel ratio's etc... then removed the PCV from the rocker cover and plugged it..... we then did a 2nd pass, and we could definitely see a slightly neater line... HP reading didn't actually change, but it certainly seemed to clean up the intake air... which could mean running bigger jets and making more HP according to my dyno guy.

[FastEHHolden]

All after july 76...there is a good thread about it all...use the "My Assistant" link and search ADR 27 or 27A.

It depends on what style of rocker cover you have, as to whether your oil cap is vented or not.....there are 2 and three hole rocker covers...2 hole have vented...3 hole are closed.

[orangeLJ]

In reference to the idea of a non PCV motor (gray motors etc) wouldnt they, theoretically blacken (dirty) oil quicker as they didnt run an oil filter anyway?

so not running a pcv valve wouldnt have been the only cause of problems with oil in early motors.

[TerrA LX]

mine is weird. if i take out the pcv valve from rocker cover with engine running and block the pcv valve the engine runs like shit and stalls. anyone know why this could be?

DA is correct, this unmetered air flow is factored into the carby design and tune so even a rocker cover gasket or side plate gasket leak can make a carby hard to tune on a holden six of this vintage as the motor is suppose to be sealed.
And always ensure your oil cap has a good seal.

[LXCHEV]

Very interesting Caine! Thanks for the report! I've thought about fitting that same kit to mine - very common on drag cars... most people refer to them as "vacu-pans".... and yeah, the flow of exhaust gases down the extractors, creates massive suction, and pulls any fumes straight out of the rocker cover/s and shoots it down the exhaust system. Therefore your engine is not burning any of the old fumes like with a PCV.. but it's still relieving the crankcase pressure....

So do you have to weld in some kind of fitting into your collectors or what??? I'd be intersted to hear how easy they are to install... and if you notice any difference etc... keep us posted!!

[TerrA LX]

^^ pics please.