scootchu

I had read numerous posts about the necessity of a PCV, but I haven't read much about which one to use.
If I am running a 86 blocked 302 that used to be EFI do I run the PCV part # that matches that even with a carbed setup? Or do I use the PCV for say a 1970 302 PCV part number if they are in fact different?

Just one of those things about rebuilt engines that puzzles me.

oxfordbp

PCV is a valve. any PCV will work as long as it fits snug. Literally all it is is a one way valve!

67t5ponycoupe

The system is used to evacuate the crank case. How much air is pulled into the engine from the PCV is metered by the spacer plate under the carb. The valve part is only there so that a backfire will not send flame down into your crank case. The PCV valve that you use to more about what will work on you configuration, in other words, what ever fits will work.

Starfury

While a lot of aftermarket manufacturers pretend that's true, it's not. PCV valves are sprung/weighted slightly different for different applications. The PCV valve is supposed to stay mostly closed under high vacuum conditions and WOT, but open up under moderate throttle conditions.

I run a common PCV valve designed for 80's chevy V8's. It's got a convenient 90* fitting at the top, keeping my engine bay that much cleaner looking.

oxfordbp

Interesting, thank you for letting us know. Is this just to keep maximum vacuum?

kalli

the following is my opinion to all this (based on logic, not knowledge) ... if anyone knows better, please educdate me ...

the PCVs (positive crankcase ventilation) job should be to
- get rid of contaminants in crankcase
- make sure there is no positive pressure in the crankcase (caused by blow by)

so one way of hooking up a PCV (and I believe this is the way it seems to be done on more modern cars) is that the PCV (sitting in valve cover) is hooked up with the airfilter.
On the other side there is a breather.
For the PCV to operate you need to pull (a certain amount because of the spring inside) on the outside side of the PCV and it will suck out of the crank case.
You can't create a vacuum in the crank case when you have an operating breather on the other side this way.
The only thing you do this way is to feed some crankcase fumes back into the aircleaner and the engine will suck that right back in to burn.
Since those fumes come through the aircleaner (mixed with fresh air) they are metered by the carb (carb does not care if it's fumes or fresh air).

Now if the PCV is hooked up the way it is with nearly all mucle/pony cars from this era, to the manifold vacuum, then in my opinion somethign important changes.
The pickup is under the carb, so this fumes/air combination the engine is pulling is bypassing the carburetor, therefore it is not metered.

if a PCV would be what most people claim, a valve that makes sure that it flows only in one direction (and nothing else), then the PCV would be a massive vacuum leak.
Means at idle where the vacuum in manifold is very high it would pull craploads of air/fumes straight into the manifold. The carburetor would nearly be bypassed completely
effectivley there would be not a hope in hell the engine would idle as the carburetor would not even get a proper signal anymore

So there has to be somethign else at play and that is what starfury mentions. The spring load.
If the engine vacuum is very high (throttle plate is closed) something in there has to make sure that it's nearly closed and only occasionally allow a bit air through there
I still cannot see any sort of positive crank pressure happening as we have a breather. But at least the crap is not getting straight into atmosphere, it is burned once by engine (whatever that might help).

there might even be PCVs that work for cars that use a closed system (no breather on other valve cover). They would 'switch' whenever there was pressure in the crank case, so it's sucked in)

but I am quite sure that a PCV that was designed for a car that has 30 inches of vacuum at idle (lets say a big block that uses a stock cam or one that allows fro bottiom end torque/pulling), cannot work properly with my engine
that pulls less than 10" at idle (caused by valve ocverlap).

In any way I believe that when there is no or very little vacuum (WOT), the PCV allows more air to flow (caused by the spring). Engine can deal with it easier I guess and therwill probably be more blow by caused by higher revs

As for the idle situation, I can even hear my PCV working sometimes at idle (open/close/open/close .....). But I picked one from the shelf years ago.
I wouldn't even know if they are sold for different vacuum at idle or similar .. ?!

Starfury

Mostly right.

If you look at a typical PCV valve, you'll notice a small weight inside it, sometimes with an assist spring on one side. This functions as a check valve. It will "seal" (not completely, but for practical purposes we'll pretend it does) when at the bottom and top of the valve. When the engine is at high vacuum (idle and cruise), the weight gets sucked to the top of the valve and flow is blocked, because these are situations when the engine is producing very little cylinder pressure, and therefore has little blow-by. This is what keeps you from having a massive vacuum leak. For those of us with low idle vacuum, well, that's why we have idle adjustment screws When the engine is at part-throttle acceleration, the weight is left kind of neutrally floating in the valve, allowing lots of the crankcase gases to get pulled into the intake.

The problem is with WOT. When the engine is at WOT (very low vacuum), the weight drops to the bottom of the valve, blocking flow until pressure builds in the crankcase and pushes the weight up a bit to escape into the intake. The problem here is that there is very little or no vacuum to actually pull those gases into the intake. This is the downside to PCV valves, but only really applies to the performance market because most people in the real world don't hit WOT very often at all.

In modern vehicles with mass airflow sensors, the PCV valve (if the car has one at all) has to pull from in front of the MAF sensor, otherwise you're getting unmetered air into the intake, which messes up your a/f mix. There's still a little bit of a vacuum in the air intake, and the PCV valve is designed for this low vacuum situation. However, a lot of newer cars have very little blow-by and can get away with nothing more than a breather hose plumbed into the airbox.

kalli

thx for clarifying that Starfury, appreciated. makes sense to me

tohj

[QUOTE=Starfury;7875962]Mostly right.

When the engine is at high vacuum (idle and cruise), the weight gets sucked to the top of the valve and flow is blocked, because these are situations when the engine is producing very little cylinder pressure, and therefore has little blow-by. This is what keeps you from having a massive vacuum leak.



Sorry dont agree on that.......when idle it sucks most, it does not block the valve, try to take the PCV valve off when engine runs idle, it sucks full vacuum, i have tried with the vacuum gauge and it shows same vacuum as an other full vacuum source., and yes I have tried differents PVC valves, with others weight, and there is a very small difference.
The PCV valve is for sucking funes out off crankcase before it gets into your olie and make it black and dirty(It will get dark after some time, but slower when you have a PCV system that works)
I agree on WOT no sucking there, but as you said we do not all go WOT all the time

tohj

I think most Carbs have a built in PCV port, so I think the carb can handle it, if I block my PCV port on my carb it runs like crab so my carb is adjustet to have a PCV vacuum leak aktive.

From a Carb manual

Vacuum Lines
All Speed Demon carburetors have three
vacuum sources on the baseplate that can be
used for PVC, distributor vacuum, diagnostics,
or any other vacuum operated accessories.

So full vacuum through a PCV at idle is no problem for a carb, when it is adjusted for this.

TOHJ