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Username Post: TWO PVC Valves        (Topic#233281)
"6th Year" Silver Supporting Member
Posts: 919
Loc: Sanford NC
Reg: 08-27-03
01-10-10 10:44 PM - Post#1840232    

A fellow car club member has a chevy 350 that has a pvc valve in each valve cover and no breather. His engine builder set it up that way. It seems to me that the two valves whould work against each other but than what do I know. Anybody ever hear or seen something like this. Thanks


"11th Year" Silver Supporting Member
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Age: 74
Loc: Venice, FL
Reg: 01-05-02
01-11-10 05:27 AM - Post#1840281    
    In response to carolinakid

Negative pressure in the motor w/o a breather somewhere.

82 Monte Carlo SC, 355 w/Vortec heads, roller rockers, 670 Street Avenger, TRUE DUALS.
2009 Sub.5.3L LTZ 30 Gallons of gas @ $2.49/gal. bla bla bla.

"12th Year" Silver Supporting Member
Posts: 1366
Age: 67
Loc: Illinois
Reg: 04-28-00
01-11-10 06:58 AM - Post#1840311    
    In response to carolinakid

Odd. Tell him to pull the dipstick while the engine is running.


"15th Year" Gold Supporting Member
Posts: 18770
Loc: Colorado Springs, CO
Reg: 08-08-04
01-11-10 01:09 PM - Post#1840481    
    In response to Don57

PCV valves are supposed to be set up so that you suck in air through one valve cover, so that it can go down into the crankcase on one side, then back up the other and out the PCV. This creates an air circulation pattern in the motor.

If two are used, a vent would have to be installed in the crankcase to supply an air circulation pattern in both banks of the motor. But, since the person putting two PCV valves in rarely understands the full function of the PCV system, the crankcase vent does not get installed.

If you get an engine builder who doesn't really think things through - like "if one PCV valve is good, then TWO must be EVEN BETTER". This type of builder assumes that ALL of the air inside the engine is a result of blowby, so suction is all you need.

In fact, you need to dilute the blowby gasses to keep chemicals in the gasses from precipitating out onto the cylinder block walls, which will be washed down into the oil and then recirculating through the engine. This is why you only want ONE PCV valve on a motor. It dilutes and removes combustion byproducts, then circulates it back into the intake tract - which then burns the mixture more completely (which makes it cleaner), before finally going out of the exhaust.

Forum Newbie
Posts: 79

Age: 33
Loc: Oakland IA
Reg: 11-19-08
01-11-10 02:19 PM - Post#1840514    
    In response to Vaughn

does the engine in question have an intake manifold oil fill for the vent? i dont know if that would even work properly, just happen to remember that earlier SB manifolds where that way

"6th Year" Silver Supporting Member
Posts: 919
Loc: Sanford NC
Reg: 08-27-03
01-11-10 03:12 PM - Post#1840554    
    In response to scottsdale84

No intake oil filler

John Savannah 
Posts: 114

Loc: Savannah, GA
Reg: 04-06-04
01-11-10 04:15 PM - Post#1840609    
    In response to Vaughn

So Vaughn, should the breather and PCV valve be set up to coincide with crank rotation? breather on the driver and PVC on the passenger? Does it matter?


Dedicated Enthusiast
Posts: 5628

Reg: 05-27-08
01-11-10 05:24 PM - Post#1840670    
    In response to John Savannah

ONe of the "hot" tricks for increased hp is to run a vacuum in the crankcase. This supposedly drops the pressure under the piston and thus the pressure differential pressure which will increase torquee a bit.

100 CHEVY 
Dedicated Member
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Loc: Helena,Missippi,U.S.A.
Reg: 12-09-04
01-11-10 07:08 PM - Post#1840754    
    In response to 52chevybob

That would help on the way down,but what about on the way up?

"You gonna leave it like that?

Senior Chevytalk Moderator
Posts: 24269
Loc: Austin, TX 78748
Reg: 05-09-00
01-11-10 07:19 PM - Post#1840761    
    In response to 100 CHEVY

Some GM crate motors have both valves with a PVC and oil cap!! I know. My ZZ4 has oil caps on both valve covers!!

'95 Jimmy SLT (Bought @131,814)
'83 Silverardo XST - ZZ4 powered
'96 GMC Jimmy LS Ret. @236651 miles

"15th Year" Gold Supporting Member
Posts: 18770
Loc: Colorado Springs, CO
Reg: 08-08-04
01-12-10 02:43 PM - Post#1841114    
    In response to gchemist

Crankcase rotation doesn't matter.

It's true that vacuum in the crankcase increases ring seal and therefore power, but using dual PCV valves is a REALLY POOR way of doing it.

You need an actual vacuum scavenge pump to really make it work. Yes, there were exhaust pipe header collector vacuum valves out there to do this in some racing applications, but it didn't really provide enough vacuum to make a consistent and definitive difference.


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