PCV experiment with low vacuum

A year or two back I initiated a thread regarding some oil consumption issues that I suspect were PCV related. I am running the Group 19 camshaft and it has low vacuum characteristics. The engine has about 12,000 miles and has been consistent since newly built with oil consumption.

I get about 10-11 inches at idle and about 14-15 inches at a steady cruise. PCV's are vacuum controlled and changing the cam can affect it's performance. In checking the chart in the 1970 TSM, it seems likely that the PCV valve is flowing around 20% more than it should at cruise and 40% or more at idle. I have been losing a quart about every 400 miles or so and the signs point to it being PCV related. I have the correct baffle under my R4B and a full metal pan gasket. Valve seals are good.

AMC PCV's are all the same and are designed to work with a stock cam, probably pulling around 20 inches of vacuum. After looking all over for a spring of similar dimensions to the oem one but with a ligher pressure and failing miserably, I gave up. I took a stock PCV apart and decided to modify the spring. Using a Dremel tool I shaved it down a bit and reassembled it. My very scientific method was to take a small bottle of Tabasco sauce and put it on the spring. In stock form the spring settled about 1/8". After I shaved it a bit it settled about 1/2" under my Tabasco bottle. Fortunately, my wife did not spice her dinner up or my comparison would have been thrown off!

I am going to try it out this spring, since the snow is finally mostly gone, and see what happens to my oil consumption. I have noticed in my initial run that the idle is a bit low. I wonder if that might not be because of lesser flow at idle bringing the idle down. A stock engine at idle is pulling roughly 1.5 cfm through the PCV but with my vacuum signal means I am likely pulling about 2.5 cfm. This may have brought the idle up a bit.

Any thoughts are appreciated and if anyone is interested I will post any noticeable effects of the changes in PCV. I suspect there are a few people out there who are experiencing the effects of running a stock PCV with a radical engine. Other makes have several options for variously rated PCV's, but as usual AMC'ers have to improvise or work with what we have!

Thanks,

Chris






Edited by ccowx - Apr/20/2018 at 10:42pm

I am sure it would for a non restoration application. I have seen this before and with some adaptation it would work like a hot darn. It is basically doing what I have done with much less trouble and more adjustability.

I am really thinking for those that want a stock appearance and/or don't want to blow over $100 on a PCV valve.

Chris

Post your analogies and results please.

I'm interested

I did a post back about 6 Pages ago. 69 390 using oil low vacuum. Interested in what your outcome is going to be I ended up taking my motor apart finding out it had no baffle or pan gasket. I did have a cam and I do have low vacuum. I think I found some vacuum leaks in the process haven't put a gauge on it yet. I did get it started again and it does smoke out the one side I'm assuming that it is trapped oil all through the exhaust system. So my next step is to get the tag on it and drive it a little bit keep track of the oil consumption. But in the back of my mind I'm thinking it's the same thing that you're talking about that vacuum intake sucking oil up. I read some people use canisters some people put steel wool down in there to slow the oil from splashing up just not sure at this point until I can put the car on the road and watching this post to see if somebody else has the same problem if mine stop smoking when I drive it I'll repost it

Just talking here, not saying I know, but a Lower Vacuum would REDUCE PCV air flow volume would it not?  Thought the PCV was a "weighted, restricted flow valve"?  Meaning there is a Restrictor built into the valve to determine the Air Flow Volume at a given vacuum drop AND a "Poppet Weight" that delays the onset of air flow until a given minimum vacuum level is reached?  Which also closes and stops PCV Air Flow before manifold vacuum drops too low...  So wouldn't than result in LESS PCV air flow with a larger camshaft and lower Manifold Vacuum Levels both at Idle and Cruising engine speeds?  Not saying I "know", just asking.