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By 46yblock - 14 Years Ago
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The conventional thinking is that PCVs keep the engines cleaner. But what about drawbacks of the addition of relatively contaminated and heated air into the A/F mixture? Looking at all the components of a used PCV system usually shows heavy carbon deposits. I had one late model 2V metal PCV tube almost completely closed from gunk. Quite a few other OEM aluminum spacers with PCV connection also have been found heavy with carbon like deposit, meaning whatever isnt deposited goes into the cylinders. The late model 2V manifolds have the gases routed to and from the PCV going only to the front 4 cylinders. It seems like the front 4 would be lean or the other four rich. Curious, at least to me. Is there a tried and true inline PCV that can go between the late model valley with rear cap, and the carb/spacer? The PCV I am currently using mounts into a bushing at the valley rear. It produced the same effect as a vacuum leak, with hoses going to each side of an aluminum spacer. Ended up having to decrease diameter of openings at the spacer to about .180 inches. I have heard that a PCV has a greater tendency to produce oil leaks from the engine generally, compared to a draft tube. True False?
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By Nathan Soukup - 14 Years Ago
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Since I installed one it stopped leaking all together.I am happy with the way it runs.I ended up using one for a 300 six.The one for 302 seemed like it had a vacuum leak.My engine used to leak from every place that it could except the rear main.
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By joey - 14 Years Ago
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Mike, I also have removed the draft tube, and have the valley pan that is vented at the rear....and have a PCV valve between it and a port on my Demon carb, connected by short lengths of heater hose. At least this setup spreads the gases fairly equally to all 8 cylinders. I too was a little concerned about the amount of air passed into the A/F mix via the PCV arrangement. I read (probably on here) that about 3 cfm go through it under normal conditions. Trying to limit this, I switched to a AC Delco PCV valve that's actually intended for a 231 ci GM. It runs fine. I took things apart recently to check it out and everything's OK, although it's only been a coupla hundred miles. I plan on checking it regularly, as it is somewhat of a restriction.
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By charliemccraney - 14 Years Ago
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On my previous build of the engine, I used a PCV valve from some GM 305 V8 application. It had a nipple on each end. One end was the same size as the hose nipple dealamajig that bolts to the valley pan and the other was the same size as the port on the carb. I don't remember the application. I found it by looking at the pictures of various valves on advanceautoparts.com or napaonline.com. While I do still have the valve... somewhere... I have no idea where I've put it to see if it has a number on it.
Road draft or pcv, they will all clog, given enough neglect.
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By 46yblock - 14 Years Ago
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The vacuum leak which was produced by my PCV really surprised me. The PCV is for some engine around 300 cu in. It has one outlet with an adapter on top that will take a 5/16 hose and a 3/8 hose. The hoses then run to the 1 in. alum. spacer which has been taped for nipple fittings. So, the PCV gas enters the spacer primary bores immediately below the throttle blades, at a right angle. Maybe there is a venturi effect at the entry of the PCV gas which caused more flow than usual. I filled the inside of the brass barbs with lead and then drilled it out to create more restriction. Seems to work ok. I dont actually remember what the final size is inside the barbs. The PCV Charlie had in his earlier engine is what I have in mind. Hadnt thought of using a PCV for a much smaller engine (duh). Perhaps that plus a draft tube would be a possibility, if I get one of the earlier blocks with a side mount together.
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By oldcarmark - 14 Years Ago
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This is an obvious question.Is the valve installed in the correct direction?At idle with high vacuum the valve should close(or almost) working against a light spring pressure inside the valve.This should do away with the "vacuum leak" at idle.As vacuum drops off idle the valve opens to draw more air from crankcase.On mine I started with a valve from a 302 which was too big and went to a smaller valve from a 4 cylinder Fairmont or Zephyr.Worked much better. The PCV system was created to help air quality.Instead of oil and gas fumes dumping raw out the draft tube they were reburned using the PCV valve.As far as oil leaks this system should reduce internal pressure as the "positive" in PCV states.The open draft tube system is a passive system as far as air in and out.If you are tapped into the spacer plate that is the correct hookup point if not availalble at the carb itself as done with my Holley 390.There is a tube to hook the line onto.Some people have used the vacuum fitting at the back of the intake to draw fumes which will give lean mixture in the rear cylinders.You could also tap the intake under the carb as long as its central to the motor.
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By DANIEL TINDER - 14 Years Ago
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When I first installed a PCV kit (CASCO blue FoMoCo valve, and rear-ported alum. carb spacer), I was running max. ignition advance (Loadomatic). The leaner charge caused an alarming ping on acceleration. Instead of retarding, I cured the ping initially by inserting a restrictor into the PCV hose to the valve in the valley pan. Tinkering with the distributor has since allowed removing the restrictor, and the car runs fine today.
Suppressing leaks, eliminating noxious fumes/oil mist that contaminate the engine are a PCV's main advantages. Of course though, any motor fed a charge diluted with blow-by & oil will not produce the same power as one burning pure gasoline of the right A/F mixure.
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By MoonShadow - 14 Years Ago
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I once saw a mechanic plug the PCV for an old Pinto 4 banger. As soon as he did the smoke started pouring out of the tail pipe. Rings were bad. That says to me that the PCV relieves a lot of crankcase pressure. From what I'm reading here which PCV makes a difference. Is there one that is specificaly recommended for the Y-Block? Seems they all have different flow rates. I used a screw in type from a later FE. Still get some puff from the oil filler at times. Chuck in NH
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By PWH42 - 14 Years Ago
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The engine in my driver is a stock 292(except for .080 overbore) and the factory PCV from a 64 truck engine was too big.I had to put a restrictor in the line to keep it from leaning the engine way out.
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By Noob - 14 Years Ago
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Interesting about the `64 truck PCV being too much cfm and it vac leak effect... I'm gonna plum a `64 truck set-up to the bottom of the air filter base... will afix a wad of steel wool (the stuff from a typical oil filler cap/breather) at the port openning inside the filter base and then occassionally check for contaminate build up... shouldnt be an issue on healthy motor, right? Cheers... Brian
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By oldcarmark - 14 Years Ago
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There is not anywhere near enough vacuum in the air filter base to operate a PCV valve.It must be tapped into direct engine vacuum.It may work partially at higher engine RPM but at idle there is no vacuum to close the valve.If you have no place to hook the hose into like a carb spacer you could drill and tap for a hose nipple in the intake manifold in the centre area below the carb. The type of cap below can be used to create a true closed system.Run a fresh air feed hose from the base of the air flilter.The 64 truck valve is threaded and screws into the intake manifold.
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By 569104 - 14 Years Ago
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Mike, I used this (see photo) speedway motor breather cap with pcv and a draft tube on my 309ci Y. Seems to be working fine. I use it in the valley cover oil fill tube and run hose to carb spacer. 
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By oldcarmark - 14 Years Ago
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If you are using a PCV system you need to take the draft tube off and plug the opening with a cup type plug.Having the draft tube AND PCV defeats the purpose of the closed engine crankcase ventilation system.Fresh air is supposed to be drawn in from the oil filler cap.If you leave the draft tube on fresh UNFILTERED air is drawn IN from that source instead of the cap so you dont really get a fully ventilated engine.
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By DANIEL TINDER - 14 Years Ago
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Aside from all that dirty/unfiltered air contaminating the oil, when the system goes positive-pressure at high RPMs, the crankcase opening will likely pump out more oil, as the agitated lube (compared to the valley/valvecover space) is probably creating a heavy mist? Also, what might be the deleterious effects (if any) of the extra volume (vacuum driven) of cold fresh air injected into the lower crankcase?
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By 569104 - 14 Years Ago
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Ok, Please help me understand. If I block off the draft tube and run a pcv from back of valley cover and have a standard oil filler cap then wouldn't I just be pulling fresh "unfiltered" air through the cap and under valley pan into pcv and not venting the crankcase at all? I have a "filter" in the draft tube that is equivelant to the wire mesh common in the oil filler caps. I also have vents in the valve covers. It seems to me it would be better to pull the fresh air through the crankcase and valve covers and out the oil filler tube into the engine. Seems like this would provide better "crankcase ventilation" to me. What I'm not getting? Thanks for the help. Mike I don't mean to hijack your thread.
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By 46yblock - 14 Years Ago
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Greg, it isnt a hi jack, but even if it is so what. I have done a few myself  .
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By Ted - 14 Years Ago
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569104 (2/15/2011)
Please help me understand. If I block off the draft tube and run a pcv from back of valley cover and have a standard oil filler cap then wouldn't I just be pulling fresh "unfiltered" air through the cap and under valley pan into pcv and not venting the crankcase at all? I have a "filter" in the draft tube that is equivelant to the wire mesh common in the oil filler caps. I also have vents in the valve covers. It seems to me it would be better to pull the fresh air through the crankcase and valve covers and out the oil filler tube into the engine. Seems like this would provide better "crankcase ventilation" to me. What I'm not getting? Thanks for the help. The cap itself is filtered with mesh which filters the incoming air. The later model PCV systems went the extra step and ran a hose from the cap to the air cleaner to insure pure filtered air but tends to be overkill for a simple PCV valve retrofit. The intent of bring fresh air in from the opposite side of the engine from where the PCV valve is located is to set up an air flow that will pick up moisture and other contaminants from the valley area of the engine. Having the fresh air inlet and the PCV valve close together minimizes the amount of contaminants that can be pulled from the engine. I believe that answers what you are asking?
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By 569104 - 14 Years Ago
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Thanks Ted, So what your saying is the way I have my engine set up is OK and would actually provide better evacuation of crank case "contaminants"?
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By oldcarmark - 14 Years Ago
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Do you park your truck in the garage?Do you notice the smell of oil and fumes when you open the door after its been kept closed overnite?When you close off the draft tube it seals the block and you wont get that smell anymore.One more reason to use a PCV system.
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By Noob - 14 Years Ago
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oldcarmark (2/11/2011)
There is not anywhere near enough vacuum in the air filter base to operate a PCV valve.It must be tapped into direct engine vacuum.It may work partially at higher engine RPM but at idle there is no vacuum to close the valve.If you have no place to hook the hose into like a carb spacer you could drill and tap for a hose nipple in the intake manifold in the centre area below the carb. The type of cap below can be used to create a true closed system.Run a fresh air feed hose from the base of the air flilter.The 64 truck valve is threaded and screws into the intake manifold.You make a valid point regarding vacuum signal. What I was thinking of is a PCV with a very light spring (or maybe even simply an open line with a wire mesh filter) such that all but idle velocity would help vacate the "gases" in the valley and recirculate them in the combustion cycle. This shouldnt create a "vacuum leak" senario as might a light spring PCV plumbed directly to the carb circuit or intake manifold. There should still be negative flow throught the oil filler cap, there should be less fumes into the air/garage, and with a PCV or mesh filter inline, it should minimize the heavy partical contaminants being re-introduced to the venturis. Am I really way off track in my thinking? Cheers... Brian
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By charliemccraney - 14 Years Ago
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You can rig a test to see if there is any vacuum in the air filter housing. Fit a nipple to the housing, attach a vacuum gauge, and run the engine. Because it's such a large volume, I doubt it will produce enough vacuum, if any at all to provide the desired effect. It shouldn't be any worse than a typical breather system.
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By DANIEL TINDER - 14 Years Ago
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[quote]569104 (2/15/2011)
Ok,
Please help me understand. If I block off the draft tube and run a pcv from back of valley cover and have a standard oil filler cap then wouldn't I just be pulling fresh "unfiltered" air through the cap and under valley pan into pcv and not venting the crankcase at all?
I have a "filter" in the draft tube that is equivelant to the wire mesh common in the oil filler caps. I also have vents in the valve covers. It seems to me it would be better to pull the fresh air through the crankcase and valve covers and out the oil filler tube into the engine. Seems like this would provide better "crankcase ventilation" to me. What I'm not getting? Thanks for the help. (quote)
The original oil cap had an oiled wire mesh filter (good for keeping out pebbles and large bugs), the repro caps have a foam filter (I clean & oil mine frequently for increased filtration), but the best would likely be an oiled cotton K&N type that seals tightly to the filler pipe. I DID try one of those but found that it was too tight a seal. Cotton filter would get saturated with oil pumped up by high RPM crankcase pressure, and drip on the engine. Better to let some oil mist escape around the looser fit, unless a system could be rigged to drain the oil back.
I initially also assumed that a lower fresh air intake would be more logical, but I guess when the cool air enters the fill pipe, it fall down to replace the hot blowby gasses being sucked out from the rear of the valley.
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