PCV Valves, Vacuum Brakes & Forced Induction

[~JM~]

...Or a Tale of Two PCV Valves.

Drove the car yesterday after a top-end overhaul. My vacuum assist brakes performed poorly. Car has been down for several months. Ran a few vacuum tests today. With the new PCV valve installed (pictured left) I have 10" of vacuum at brake booster. With the old PCV (pictured right) I have 16" of vacuum.

I applied 20-30# of pressure to the opposite end. Here are my findings. The valve on the left prevents the boost pressure from entering the block/crankcase. The valve on the right reduces flow into crankcase, but does not prevent pressurizing the engine block.

I need a PCV valve that has the proper sized orifice to provide enough vacuum to operate the brakes & still blocks boost pressure from entering the engine block.

Any recommendation s?

Thanks.

[Steve Wood]

I have never liked the check valve or had good luck with it.

I always have taken brake vacuum from the block or the plenum so I have always had good  vacuum to the brakes no matter what was used in the pcv valve location.

New pcv's are not calibrated specifically for our cars.  I may have seen some of the originals on Highway Stars but I would not bet much on it.

A decent pcv may leak a little under boost but the volume should not be enough to pressurize the crankcase to any great extent and cause blow by in my experience.

[~JM~]

...A decent pcv may leak a little under boost but the volume should not be enough to pressurize the crankcase to any great extent and cause blow by in my experience.

The silver one above had substantial blow-by.

I dialed the regulator down on my compressor to provide 20-30psi to test them.

I have some .010" stainless shim stock. I might try to cut out a disk to fit into the check valve & drill a 1/8" orifice in it to see how that responds.

[Steve Wood]

I would try another valve