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Discussion Starter #1
I'd like to change my stock 70 Z28 PCV valve CV746C (K), to one that matches my idle vacuum reading with my roller cam that produces 11-12" at idle. I read that the 69 Z/28 part no 6484699 would be a good choice. What's the idle vacuum reading for a 69 Z/28? Is there a PCV cross reference chart available for all brands with engine vacuum specs.
 

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The CV746C is the same as a '69 Z28 and LS6 if I'm not mistaken.
 

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Discussion Starter #4
I bought one of these hallow fixed orifice pcv valves and it fixed my oil consumption problem. I tested the vacuum at the oil dipstick tube and with the stock 70 Z28 pcv and got 6" of vacuum when it should be between 1-3". The reading with the new pcv is 2".



Testing: http://www.underhoodservice.com/tech...v-inspections/ Go to heading Testing a PVC Valve
 

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auto zone does not show this pcv . where did you get it , pt # and how much? thanks
Part no 6484525, this is a gm part number. Use this part number to interchange , if still no luck, go to Napa. If still no luck use the CV 746C number. This was an AC part number.

Probably under 10$.

Autozone could not find it because they probably need to know what color your car was.
 

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so that im educamacated on this - what is the purpose of changing the PCV valve and the vacumn reading? How or what determined that you needed to change this??
 

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so that im educamacated on this - what is the purpose of changing the PCV valve and the vacumn reading? How or what determined that you needed to change this??
Oil consumption. My rebuilt roller motor has a vacuum reading of 6" in the crankcase with the stock 70 Z28 PCV valve.
For a properly working PCV you should only have a reading between 1-3" at idle. GM used this hallowed out PCV on
many many cars and trucks that consumed oil. They no longer sell the PCV 12572717. Mine was $10 in Canada at
Carquest. Part # 76-2698.
 

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Oil consumption. My rebuilt roller motor has a vacuum reading of 6" in the crankcase with the stock 70 Z28 PCV valve.
For a properly working PCV you should only have a reading between 1-3" at idle.
Where do you take that reading? Dipstick tube? Or at the PCV?

Edit: Never mind. I saw above where you said at the dipstick.
 

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so that im educamacated on this - what is the purpose of changing the PCV valve and the vacumn reading? How or what determined that you needed to change this??

The way a variable valve PCV works is that engine vacuum actually pulls the pintle valve closed. It tries to maintain the same crankcase vacuum under all conditions. So, high engine vacuum (low load, less chance of blow-by) the valve is more closed to limit both the "vacuum leak" at the PCV and crankcase vacuum. Low engine vacuum, (high load, more chance of blow-by) the valve is more open to try to maintain that crankcase vacuum.

They are made with different springs in them depending on application. If you use a PCV made for a big solid lift cam engine that typically makes low idle vacuum in an engine that makes a higher amount of engine vacuum, then the valve will be a lot more open at low load and idle than it should be resulting in a large "vacuum leak" and too much negative crankcase vacuum. Likewise doing the opposite (high vac engine PCV in a low vac engine) will result in a valve that doesn't create enough crankcase vacuum at low load because the valve will be pulled closed.
 
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