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 PCV Flow Rates 
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Joined: Sun Jun 23, 2013 1:41 pm
Posts: 378
Location: Phila. Penn.
Seems to be a dearth of publicly available tech data on crankcase evacuation. This makes it more difficult finding suitable matches (or hacking) for low idle vacuum and high revving engines. So when I stumbled across flow data in a shop manual, I thought I'd share it here. If anyone comes across similar data, please share. I'll graph 'em if you post 'em. Stock PCV valves, exhaust evac, vac pumps, its all good to know.

From Chilton's General Manual 1970.
Fixed orifice sizes to substitute for PCV Valve Testing at idle:

Engine (cid) Hole dia. (in) fractional drill
90 to 140; 0.056 - 0.075 or 1/16
141 to 200; 0.074 - 0.088 or 5/64
201 to 275; 0.087 - 0.099 or 3/32
276 to 350; 0.098 - 0.112 or 7/64
351 and up; 0.111 - 0.125 or 1/8

From American Motors Corp. 1984-7 Jeep SJ Workshop Manual, MR 253
pcvflow_WEB.jpg [ 47.22 KiB | Viewed 765 times ]
Note that these are not high performance engines. Both had all the usual pollution stuff of the era(EGR, fuel vapor, etc controlled by temperature switched vacuum). The straight 6 also had some electronic controls on top. This included a solenoid controlled valve in the PCV line.
Both engines were expected to pull 14" of vacuum at idle.

Last edited by Mattax on Fri Dec 12, 2014 8:33 am, edited 1 time in total.

Thu Dec 11, 2014 9:47 pm
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Joined: Fri Feb 08, 2013 2:37 pm
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Fri Dec 12, 2014 2:42 am

Joined: Thu Apr 02, 2015 2:20 pm
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So what does it all mean Basil? Am I supposed to read it as 3 cfm killed 13 inches of vacuum on the 360? Or running the 360 under a load with 3 inches of vacuum drew 4.5 CFM through the PCV?

Tue Apr 07, 2015 4:25 pm

Joined: Sun Jun 23, 2013 1:41 pm
Posts: 378
Location: Phila. Penn.
It means that at idle, which should be 14" or a bit more, about 2 cfm are being supplied to the engine through the PCV. A little more for the v-8 and a little less for the straight 6. At wide open throttle, the 6 may get a little more, but not much, whereas the v-8 may get as much as 4 cfm through the PCV valve. That's my understanding - the test is to be done with a ball type air flow meter. Be interested to know how many AMC and Jeep dealerships had one.... :roll: :O

Tue Dec 01, 2015 11:00 pm
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