dfolse62

My 62 apparently had the pcv provision but I never had one and the port on the back of the AFB was plugged. I bought a new one (pcv) and a brass T and installed it. Before I connected the hose to the crankcase vent I felt it while the car ran. It was sucking a vacuum all the time. Is this normal? When I revved the engine up it sucked more, but the car runs. Should there always be a vacuum here? When does this valve close? Thanks

SWCDuke

It never closes. It's a metering valve, not an on-off valve. Orifice area depends on manifold vacuum, the more vacuum the less area. Otherwise high vacuum would suck the crankcase dry.

Critical pressure ratio also applies - the ratio of the ABSOLUTE downstream pressure to absolute upstream pressure. At any ratio at or below .528, flow through the office is sonic velocity and a further reduction in pressure ratio will not increase flow. It's determined solely by orifice area unless the orifice is a converging-diverging type as on a rocket motor.

(On a rocket motor with a converging-diverging nozzle the pressure ratio between ambient and the combustion chamber is well below critical. Flow is sonic at the nozzle throat and continues to accelerate to increasing supersonic velocity in the diverging section beyond the throat, so flow rate is a function of throat area)

Let's call atmospheric pressure 30" Hg., so the downstream absolute pressure for critical flow is .528(30) = 15.8".

Since 15.8" is absolute manifold pressure the corresponding vacuum that we can read on a vacuum gage is 30-15.8 = 14.2, so at any manifold vacuum above 14.2 the flow past the throttle blades (and PCV valve) is critical - which is sonic - and any further increase in manifold vacuum will not increase flow.

This is one reason why engines that pull over about 14" of vacuum idle fairly smooth and engines that pull less than 14" have a noticeable lope. If vacuum is less than 14" airflow varies with any variation in manifold vacuum. If above 14" air flow choked and will remain the same even if vacuum increases.

The critical pressure ratio (and I re-emphasize it is a RATIO, not a arithmetic difference) is a simple concept that is important to understanding fluid flow systems.

Duke

dfolse62

Wow!! I guess you are saying that it will suck (technical term) all the time without affecting engine idle or performance.

Does anyone know if the original fittings, which are 1/4 pipe had any type of orifice in them, or were they just common fittings?

dfolse62

Also, Do you mean that there is always a vacuum at that point but not a significant air flow?

SWCDuke

If you can feel "vacuum" there HAS to be flow, but if the metering orifice is small you may not be able to "feel" the flow, but you will sure feel the vacuum if you plug the hole with your finger.

Duke

SWCDuke

Remove the valve and draw an oral vacuum on it to see how the flow varies. The harder you "suck" the less the flow. Like I said, it's a metering valve.

Then blow on it an you will notice the air flow stops. This is an anti-backfire feature that will prevent a carb. backfire from propagating to the crankcase where is can cause an explosion because most blowby is air-fuel mixture that is combustible.

The PCV system maintains a continuous flow of fresh air into the system, which mixes with blowby and draws it into the inlet system for combustion. The purpose of the metering (PCV) valve is to maintain a correct range of flow for all manifold vacuum conditions from idle to WOT.

Duke

dfolse62

Thanks, It did not seem to change the engine, but I wanted to know.

SWCDuke

I believe all the hardware is available from one of the Corvette parts vendors - maybe Paragon. JohnZ mentioned it in the last week. Try a search.

Duke

Plasticman

Duke,

Do the various PCV valves that Chevy has used over the years (especially from 1962 to early 70's) vary in the amount of vacuum and flow, or are they essentially all the same, with differences for fitment purposes (angled, straight, push in, screw in, etc.)?

Thanks,
Plasticman

EVERGREEN

Exactly my question too John. I went to NAPA and told them which engine is in the 'Vette and the parts guy handed me the part and it works just fine (cheap too) but I see in the various catalogs a whole range with no explaination other than some ridiculous prices for PCV valves!

EVERGREEN

babbah

Duke,

I have this question, I have a 67 435 Roadster and has a radical mechanical cam that has 8" of vacuum at idle. Should I be running a different PCV valve part number other than the one I have in there now that I think has a "D" on the plunger inside the pcv valve? Im running the fram pcv valve that is called for for a 67 Corvette big block....

I noticed at the parts shop that all the PCV valves when you look into the bottom, that faces into the valve cover, that they all have different letters on the plunger. (the part that rattles when you shake the valve up and down)

The one that is called for when the engine with the stock 435hp cam used to produce around 12" when idling...

Fram calls for a PCV # FV181 (in running this one)

Purolator calls for a #FV136

Are they the same....

Any advice on this?

JohnZ

Internal orifice sizes and valve spring tensions vary based on engine characteristics to maintain correct flow rates; just use the one spec'd for your engine and it'll work fine.

Plasticman

Thanks John.

That is what I thought, but it begs the question for those of us using aftermarket carbs: What PCV Valve should we be using?

Especially since the aftermarket carb manf. do not specify any particular PCV. And what if the engine is running a "stronger" cam, along with other mods. Never have seen an article or info on what PCV to use, and what changes or affects we should expect to see.

I am sure that "different" PCV Valves will affect the idle mixture setting, as well as the cruise circuit mixture. But never have seen this issue addressed.

Perhaps the difference is so minute, that it does not matter, but I cannot believe that, since GM did not design all of those different PCV Valves for the heck of it!

Plasticman

SWCDuke

Different valve numbers may look the same, but they have different flow rates.

Unfortunately, I have never found any flow versus vacuum data for any of these valves.

With the help of a friend, I did once flow test some valves that were the same geometry as the original valve on my Cosworth Vega. I was looking for less flow due to oil ingestion, but after several redesigns I ended up with a very simple PCV system with no valve. I plugged the fresh air inlet at the bottom of the crankcase (Oil backflow under high cornering load would puke oil into the air cleaner.), and just ran a line from the outlet on the cam cover to the air cleaner nipple (which formerly supplied the fresh air to the crankcase inlet), so what I essentially ended up with is just a vent from the crankcase to the air cleaner housing, and I also built a liquid trap into the line - just a tee and a clear vinyl line with a plug that routes to the bottom of the car in a position that is easy to visually check and drain.

I can get away with this system in the CV because it has port injection and does not need an anti-backfire valve. Since the manifold is "dry" a backfire cannot propagate into the air cleaner and then to the crankcase where it can cause an explosion. My '91 MR2 has effectively the same design, which is were I finally got the inspiration. GM probably didn't realize that this was possible since they had little port injection experience at the time, so they installed a typical PCV system architecture for a carbureted car.

For a "custom application" or cam change, just stick (like John said) with the OE valve or use an OE valve from a similiar displacement engine. The PCV system can accomodate a fairly wide range of flow rates, so you only need be in the ballpark.

In some years SBs and BBs used the same valve, but I wouldn't carry this to extreme - like using a valve originally designed for a four-cylinder engine in a V8.

Duke

PS My simple crankcase vent design is probably not the best. A "flow through" system that draws in fresh air is better. Such a design is actually "ventilated" whereas my design is simply a "vent". With the advent of better oil additives and PCV since the sixties crankcase sludging has become a thing of the past, however, some recent Chrysler engines and maybe some Volkswagon engines have had a problem with oil sludging. I don't know the details of the crankcase ventilation on these engines, but a simple vent could contribute to sludging, especially if the car is used for short trips that causes a lot of crankcase condensation. A flow though design would help avoid this in short trip driving where the crankcase doesn't get warm enough to boil off condensation that would occur the first few minutes of driving since the flow through design would help purge blowby before it can condense.

For '92 Toyota redesigned the 5SFE crankcase ventalation system to a flow through type with a valve, which may have been an acknowledgement that the simple vent design was not the best for some driving conditions - like short trips.