There are many different PN's for PCV valves, but wonder how much they really differ.
Significantly. Different maximum flow rates, different minimum flow rates, different pintle profiles and spring tensions to provide different flow rates at different vacuum levels, yes/no spring-loaded closed...
If it fits in the grommet hole and has a correct size hose nipple, it "should work"
Don't you believe it! Consider the enormous number of different PCV valves that all look alike (same size, same shape, same fittings) but there they all are in any maker's parts catalog, each with its own part number and application. Companies don't generally do that for fun and yuks, in fact they do the opposite to the maximum possible degree (as few parts as possible for as many applications as possible), and yet...look at all those PCV valves. If that's not enough to convince you, read
this. The wrong valve in that case looked exactly the same as the right valve. The wrong valve was spring-loaded closed and had different calibration.
Consider that even for the same engine, the blow-by and thus PCV flow varies considerably over its life
But the engine's mechanical condition is not what determines PCV flow. The PCV valve is the limiting factor there. And even if we were to remove the valve and put in a plain elbow instead, then the limiting point would be the ~1/4" diameter passage in the carburetor's throttle body and/or the intake manifold's carburetor mounting pad.
2. Even among classic Mopar's, the PCV valves differed. My 65 SB & BB used one w/ metal thread & nut which screwed into a metal cap which fit over a "stove-pipe" on the valve cover, and yes they were smart enough to put the nut on the outside.
You are describing the '64-'69 valve (Chrysler bought them from Stanadyne and they looked like
this) with its curved metal outpipe pressed into its metal body. There were two of them: one for the 170 engine, and one for 225 through 426. Identifiable by black vs. silver end washer. These valves also serviced '61-'63 applications originally equipped with a valve made by AC held together by a removable threaded-on end cap or a snap ring and plate at the bottom (inlet), that is
this one, and a separate brass (
this) or aluminum (
this) elbow threaded onto the top (outlet). These valves could be easily disassembled for cleaning, and sometimes tended to make noise as the flat bottom of the valve shuttle rattled on the inside surface of the end cap or end plate. AC carried on selling these as aftermarket service parts well after Chrysler went to the Stanadyne design for '64;
here's such a valve packaged by AC with a nonstandard grommet and instructions for how to install the valve with the thread/cup/elbow/spring/chimney '65-down arrangement or the '66-up grommet arrangement described below.
The '64 pintle-style valve reduced noise and had a
self-cleaning action by the jiggling motion of the pintle. All these valves were attached to a stamped steel cup about an inch and a half in diameter -- by means of the threaded-on elbow in '61-'63 and with a speed nut in '64-'69 -- with a 2-legged flat steel spring trapped between the valve and the underside of the top of the cup. The cup was pushed onto a chimney on the engine valve cover.
I recall my 64 slant is different PN which pops in a rubber grommet
That
started in '66. And the valve was the same as in '65, but now instead of being attached to the engine by the speed nut/cup/spring/chimney arrangement, the bottom of the valve simply snapped into a rubber grommet installed in the (revised) valve cover. This setup lasted clear on up through the end of the engines we care about on here. In '70 the all-metal valve was replaced by a valve that had a metal valve body and a yellowish-white plastic elbow (
this one), which lasted through '71. For '72 the valve with the all-plastic body (
this one) was introduced, still available today from Chrysler and numerous other suppliers in a variety of plastic colours that may or may not indicate different calibrations. All these valves (the '64-'69, '70-'71, and '72+) snap into the same grommet. The plastic valve can be literally "pressed into service" on a pre-'66 valve cover
this way.
But all these different-year, different-style valves I just described have very similar flow and calibration characteristics -- note Chrysler's use of one single valve for an engine displacement ratio of almost 2:1 (225 through 426) and only using a different valve on the smallest (170) engine because the bigger valve's calibration just couldn't stretch quite that wide without becoming unsuitable at the other, big-engine end. This is a further evidence point against the idea that any ol' PCV valve will work if it can be physically installed.
Whenever searching for a vacuum leak, suspect the PCV valve
PCV valves rarely cause vacuum leaks. In order for them to do so, they must either be physically damaged (cracked/holed) or the pintle must be jammed in the open position
but the orifice must not be clogged -- those are very rare circumstances. Fortunately the PCV valve and its hose are topside, easy to check and eliminate as the cause of a vacuum leak.
I think the way it is supposed to work is that the valve is forced closed when there is a high dP across the valve. The intent is that it is closed when idling (high vacuum), so it doesn't mess up the O/F ratio. There is a small designed-in leak when closed.
That's right, though I'd quibble a little with your "designed-in leak" terminology. Under high vacuum (idle) the valve closes
down to its minimum flow.
if you have a lot of blow-by at idle, seems the PCV valve would just close off even tighter (greater dP)
The PCV valve cannot close "even tighter" than its minimum airflow position, which is determined by vacuum.
suggestion that blowby might make it rattle at idle sounds unlikely
Correct, that's not what's doing it.
[quote}the crankcase might pressurize even more[/quote]
If the breather is faulty or absent, or the engine is so completely whipped that the breather cannot pass the volume of blowby, yes, you can get crankcase pressure which can exacerbate oil leaks. If the breather is faulty or absent and the engine and PCV system are in good condition, you can pull a vacuum in the crankcase that can suck gaskets and seals in...and exacerbate oil leaks.
Engines before PCV simply vented the crankcase to atmosphere
Yes, but that's not the whole story. They used what's called a "road draft tube", that is a metal pipe extending down from the valve cover (where the PCV valve would be in later production), with its lower end far enough below the car to be in its slipstream. Above about 30 mph, the motion of air across the bottom of the road draft tube induced suction in the tube, which evacuated the crankcase. Nowhere near as efficiently or reliably as PCV (hence "positive" crankcase ventilation), though, and there were problems: draft tubes could get clogged with ice, snow, or mud. They could rust out so the bottom was no longer far enough under the car. City delivery trucks and postal delivery vehicles could go for very long periods of time without going fast enough to induce a road draft. On the other hand, let the engine get some wear on it and the road draft tube made a dandy self-rustproofer for the underside of the car.
