Anyone ever run crankcase at vacuum?

[Darthomas]

About 15 years ago, had a 68 truck 225 with forged crank built up to replace a trashed engine in a 65 Dart GT.
Used a factory two barrel intake, and a carb from a 273.
Idle was a bit too lean, no matter what, so I tried sealing the breather cap, pulling a vacuum on the crankcase with the PCV.
Idle was then perfect, so I used it that way for years without any issues. I liked the idea that a motor can't leak oil if there's a vacuum inside.
Car's been off the road for 8 years, apart in a body shop.
Anyone else ever do this?

 

[nm9stheham]

Here is a possible reason for this:
- The stockish PCV's in the Mopars (the metal bodied ones with the 90 degree hose nipple) don't close down to the nromal lower idle air flow at 15" in or so of vacuum. (A PCV should partially close down to allow less air flow at idle to avoid a lean condition.)
- The 2 BBL carb may have been large enough to lower the idle vacuum to 16" or 15" or less. This would have allowed the PCV to open to its normal cruise opening that allows more air in, and that leaned your carb.
- Closing off the breather shut down the excess air and made the idle better.

 

[Darthomas]

I was happy with it and got 20 on the highway, I realized the mix was richer after the flow was diminished.
Tried 2 or 3 different valves with no difference.
Then I read about vacuum potentially improving ring seal, and some race applications for doing this.
Just wondered if anyone else ever did this for any reason.

 

[nm9stheham]

Different stock valves or other models?

 

[Darthomas]

Just stock valves, the older type that had to installed into the metal cap and locked down with a nut.
It sure did pull a strong vacuum on the crankcase, even at idle.

 

[nm9stheham]

Well, those are the ones I am talking about. We tried an old one and a NOS one on 14-15" vacuum and neither one shut down properly to the lower volume idle air flow. If you knew your actual vacuum level at that time, then this could be known or rejected as a possible cause. But I am sure that is a while back!

We went to a lower vacuum PCV on my son's 340; it is from a GM v* 4 BBL application (FV191) and shuts down the flow properly at that vacuum level. Sure makes the idle better.

Your trick is kinda the same as a restrictor in the PCV system than some guys do on really low idle vacuum engines.