PCV for lower vacuum?

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nm9stheham

nm9stheham

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My son and I are looking for a PCV valve that restricts flow more at a lower-then-stock level of idle vacuum. We currently have the standard Mopar 60's era PCV valve like from a SBM, which is an all metal, 90 degree one. Looking at the PCV valve plunger when the engine is at idle, it is not closing nearly as far as it should to restrict flow at idle, and the level of air flow is audibly higher than I normally hear through a PCV at idle. We have 2 of these older PCV's, one of which is NOS, and they act the same.

The engine is idling at 14" right now, and that rises 15" with the PCV blocked, with notably smoothing of the idle. So we are not looking at a radical change, just one that closes off better at 14-15" of vacuum. Any suggestions on alternate part numbers will be appreciated.
 
How much is stock vacuum? 14-15lbs seems like a decent amount to me.
 
A bone stock 318... I would expect up around 18-19 in. We have a Crane Z268 cam so nothing radical.
 
I have a pop up sprinkler tube in-line with my PCV valve and use the flow adjuster to set how much flow I want at idle. Cost $2 and works great.
 
I have made a nylon orifice that inserts on the hose side, and can be sized by drill bit. I have sucessfully used this method on older pre PCV engines
 
Looks like we solved this problem; I found some references on a GM forum about an AC Delco CV-789C, which was a PCV valve specifically for larger cammed 4 bbl GM motors.

That crosses to a FRAM FV-191, which we ran out and bought....all of $4. Just popping it on the PCV hose and starting the car, it was seen that the amount of air begin drawn through the PCV valve at idle was dramatically less, and idle improved and was lower. After some monkeying to adapt it into a new holder, and resetting the idle mixture, the idle is generally smoother and much more consistent. We can lower the idle a bit and have it be stable. Vacuum in neutral is still around 14"-15" at 950-1000 RPM, but in-gear idle at 750-800 RPM, vacuum is up about 1 to 1.5 inches at around 12".

The other bonuses are that:
- The idle mixture screws are more responsive to small changes.
- The curb idle screw now has better authority.
- The engine did not brake properly before when the throttle was closed; it wanted to run on more than it should. That is all fixed and the drivetrain engine-brakes with closed throttle like you would expect.
- And an off-idle flat spot is almost all gone; we suspect the extra air from the standard Mopar PCV was leaning the transition/progressive circuit's mixture.

We'll keep tabs on this to make sure the FV-191 PCV valve is pulling adequately at mid- and WOT. Thanks very much for reporting the other ideas; I very much appreciate it.
 
nm9stheham said:
Looks like we solved this problem; I found some references on a GM forum about an AC Delco CV-789C, which was a PCV valve specifically for larger cammed 4 bbl GM motors.

That crosses to a FRAM FV-191, which we ran out and bought....all of $4. Just popping it on the PCV hose and starting the car, it was seen that the amount of air begin drawn through the PCV valve at idle was dramatically less, and idle improved and was lower. After some monkeying to adapt it into a new holder, and resetting the idle mixture, the idle is generally smoother and much more consistent. We can lower the idle a bit and have it be stable. Vacuum in neutral is still around 14"-15" at 950-1000 RPM, but in-gear idle at 750-800 RPM, vacuum is up about 1 to 1.5 inches at around 12".

The other bonuses are that:
- The idle mixture screws are more responsive to small changes.
- The curb idle screw now has better authority.
- The engine did not brake properly before when the throttle was closed; it wanted to run on more than it should. That is all fixed and the drivetrain engine-brakes with closed throttle like you would expect.
- And an off-idle flat spot is almost all gone; we suspect the extra air from the standard Mopar PCV was leaning the transition/progressive circuit's mixture.

We'll keep tabs on this to make sure the FV-191 PCV valve is pulling adequately at mid- and WOT. Thanks very much for reporting the other ideas; I very much appreciate it.
Click to expand...

Is it the same diameter as the Mopar performance valve cover opening? I believe it is 1/2".
 
Yes, this is a fatter PCV valve so more monkeying around would be required for the later PCV valve grommet. Our valve covers are the 273 HiPo types, so our solution won't work for the later valve cover grommet. It does not seem to be too big of a challenge to just open up the hole in a standard valve cover and put in a larger grommet.

FWIW, I ran across another potential PCV PN that MAY do the same thing but you'll need to experiment with it to see if it works as well; at the cost, it is not much money wasted. This PN is FV184 and was used on a lot of GM 4 BBL 305/350/400/454 engines. It looks to have a smaller body diameter.

As a side note, I personally decided to put an actual PCV valve in place rather than a restrictor. The reasoning is that any restrictor that cuts flow adequately at idle to avoid idle issues will flow less than a PCV valve at mid and wide throttle openings, when RPM's are up and the engine will typically be producing more crankcase vapors that need evacuation. This may depend on the condition of the engine and the restriction size; some engines with good ring seal may be evacuated just fine with a small restriction. But in general, a restrictor does behave like a regular PCV valve.
 
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