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Old 03-23-2011, 05:29 PM
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Does PCV valve size matter?

I've noticed that there are several sizes of PCV valves . Does it matter what size you run on any particular engine?
The reason I'm asking is I have a 1960 caddy motor (390 ci) that originally had a downdraft tube,and I have converted it to a PCV system by putting a baffle under the valley cover and tapping a hole (its an aluminum cover) and I bought a kit from a fellow who makes them up (the baffle came with the kit) but the PCV valve itself seems kinda small .It has 1/8" pipe threads on one side and takes a 5/16" hose on the other. Does that sound right for a motor that big?

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Old 03-23-2011, 05:58 PM
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That does seem pretty small exspecially for that caddy motor. I believe i would go bigger myself.



Cole
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Old 03-23-2011, 06:37 PM
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The PCV valve is a spring-loaded valve with a specific orifice size designed to restrict the amount of air that's pulled from the crankcase into the intake manifold. This is necessary because air drawn through the valve from the crankcase has a leaning effect on the fuel mixture, the same as a vacuum leak. So airflow through the valve must be controlled within certain limits. At idle, air flow is reduced because little blow-by is produced. When the engine is cruising and vacuum is high, airflow through the PCV valve is at the maximum to purge the blow-by vapors from the crankcase. It's important to note that PCV valves are sized for specific engine applications. The wrong PCV valve for an application can flow too much or too little air causing drive-ability problems. So before you change the valve, I think you should contact the person from whom you got the set-up from, about how he came up with that particular valve & orifice size.
Rich
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Old 03-23-2011, 07:23 PM
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Thats why I was concerned I think he uses the same one for any application because it is small and not noticeable on retro engines. And I went with it because I didn't want to put holes in my valve covers,but it is down in the valley pan and I have 6 strombergs on top so even if the valve was larger it still wouldn't be noticeable
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Old 03-24-2011, 05:59 AM
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RICHARD STEWART 3RD - Is there some sort of "sizing" chart available for engine size/PCV selection? It sure would be nice to get the proper size valve for my Blown 291 Hemi, rather than just sticking SOMEthing in there...
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Old 03-24-2011, 06:08 AM
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I never really noticed that there were different sizes.

Good post, looking forward to reading up on this.

I just replaced it with the stocker, even after I put the 6-71 on her.
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Old 03-24-2011, 06:23 AM
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My engine is a 1960 which had the downdraft tube. in 1961 they used PCV on caddy 390's in California ,and in 1962 everywhere else. I been thinking maybe I should look into getting one that they used on them years. Not sure where they connected them---maybe they changed to valve covers with holes-----any one know? I'm still going to keep mine coming out of the valley pan ragardless
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Old 03-24-2011, 09:12 AM
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BUT,the question remains DOES SIZE REALLY MATTER---in relation to PCV valves ,that is
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Old 03-24-2011, 09:18 AM
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Hi

Measure the amount of vacuum in the crankcase. With the engine at normal operating temperature, block off the PCV breather tube or vent to the engine (usually the hose that runs from the air cleaner housing to the valve cover on the engine). Pull out the dipstick and connect a vacuum-pressure gage to the dipstick tube. A typical PCV system should be pulling about 1 to 3 inches of vacuum in the crankcase at idle...
Rich
PS If you see no vacuum, or find a buildup of pressure in the crankcase, the PCV system is plugged or is not pulling enough air through the crankcase. Also If the engine has a leaky oil pan, valve cover or intake manifold gasket leak, or leaky crankshaft seals, it will not be able to develop much vacuum in the crankcase because it is pulling in outside air
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Old 03-30-2011, 11:13 AM
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so, the main reason I went to the PCV from road draft was because I didn't care for the smoke coming out of the breather (where U put the oil in) also oil would come out of it and get on my firewall and windshield (hoodless car) I know this engine could no doubt use an overhaul,but that is out of the question now. Anyhow,took it out for a ride and oil still coming out of breather--just like I hadn't installed PCV. Been wondering what would happen if I leave hose from valley pan to intalke,but take PCV valve out ????

I really can't understand why it is breathing out the breather that is supposed to be the inlet ?

Last edited by yragat; 03-30-2011 at 11:37 AM.
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Old 03-30-2011, 03:53 PM
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Quote:
Originally Posted by yragat
so, the main reason I went to the PCV from road draft was because I didn't care for the smoke coming out of the breather (where U put the oil in) also oil would come out of it and get on my firewall and windshield (hoodless car) I know this engine could no doubt use an overhaul,but that is out of the question now. Anyhow,took it out for a ride and oil still coming out of breather--just like I hadn't installed PCV. Been wondering what would happen if I leave hose from valley pan to intalke,but take PCV valve out ????

I really can't understand why it is breathing out the breather that is supposed to be the inlet ?
Taking the PCV out and just using a hose would be a huge vacuum leak, and likely pull more oil into the intake and make the engine smoke because of it. It needs some type of restriction and baffling.

Possible causes of it blowing out the breather even with the PCV connection is that the engine is just worn out that much and has a lot of blow-by?? Engines of that era rarely made it past 75,000 miles without being dead-dog tired due to the oil back then and poor crankcase ventilation(no positive ventilation-PCV).

i know different PCV valves have different spring tensions and relate to the amount of engine vacuum, but I don't know of a rating chart for them. Being a Chevy guy I just used the PCV application for the '70 LT-1 350 370hp for my performance engines.
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Old 03-30-2011, 03:58 PM
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I just found out that they used a PCV valve in 62 caddy 390's----maybe I'll get one of them They look a lot bigger than the one I'm using(at least on computer pics) and try it. I do have a baffle under the valley pan.
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Old 04-09-2011, 06:56 PM
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I have to include a question into this topic:

My 383 stroker, 9.7 compression, today I just broke the PCV valve (removed and stepped on basically). The valve was new last summer so I replaced it today with what I thought was the same one. Now, I cant get my idle down unless I compress the hose going to the valve to restrict the amount of vacuum pulling on it.

So, Im assuming I replaced whatever one I had with a higher flow pcv valve. Question really is what does anyone recommend? I read that some 302, 305ci camaro, or 6 cyl valves seem to work.
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Old 06-11-2013, 10:42 AM
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PVC sizing

Bubba what size PVC did you use?

Quote:
Originally Posted by bubbahotep View Post
I have to include a question into this topic:

My 383 stroker, 9.7 compression, today I just broke the PCV valve (removed and stepped on basically). The valve was new last summer so I replaced it today with what I thought was the same one. Now, I cant get my idle down unless I compress the hose going to the valve to restrict the amount of vacuum pulling on it.

So, Im assuming I replaced whatever one I had with a higher flow pcv valve. Question really is what does anyone recommend? I read that some 302, 305ci camaro, or 6 cyl valves seem to work.
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Old 06-11-2013, 01:40 PM
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Quote:
Originally Posted by F-BIRD'88 View Post
There is no real "chart" or spec page for PCV valves.
But if your engine has a big cam in it and lower idle manifold vacuum you are best to try to use a PCV valve from a OEM high perf engine that had a big cam too. Like a 69 Z/28 302 70 LT-1 Z/28 360hp or a 427 425hp L72 or 430 HP L88 69 corvette.

A PCV valve from a 4 cylinder or six banger may work too. It should stay in the low flow position at idle, when idling in gear (auto trans)
You can manipulate the inner pindle valve by hand using a small screw driver to see the effect at idle.
There are different pindle shapes and inner spring tension that control the flow rate and flow curve of the PCV valve. If yours is hanging in the high flow postion at idle its essentually the same as a big vacuum leak and will upset the idle.
On a cammed motor you may have to experiment with a few different PCV valves to get it right.

The reason why there is no real specification information on the specs of a PCV valve is because it is a emission device and your government does not want you to "tamper" with that. So the manufacturers are not forthcoming with the operation specs of different PCV valves. They keep it all under their hats. To avoid EPA hassle.

If a company like Fram or Standard motor products or say Comp Cams or Edelbrock were to come out with a line of PCV valves for "High perf appliciations" Like when using a non stock cam that has low idle vacuum.
It is considered "tampering" and the EPA would be all over them.
I just checked Summit and I am only coming up with the same set up PCV valves for these models...but the problem is that they are designed for a zillion makes and models, just not anything high performance.

http://www.summitracing.com/parts/frm-fv178/
http://www.summitracing.com/parts/frm-fv181
http://www.summitracing.com/parts/smp-v158
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