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Old 06-03-2002, 08:28 AM
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Post PVC/Vacuum and a Heavy Cam

I have a 1967 GTO with an extremly heavy cam and I don't have any specs on the cam. My problem is that the PCV valve is not allowing enough vacuum at low idle. I was told something about installing a can of some sort between the block and the intake. If anyone has any info on this kind of setup or any other suggestions it would be appreceiated.

Ok guys this is whats going on. At idle the PCV valve is not opening enough, you hear a clicking noise, and because of this one of the cylinders is not hitting. But if you diconnect and plug the line going from the PCV to the intake manifold all cylinders hit good. Now I was told this was happening because the PCV valve could not handle the CAM at low RPM and that a vacuum tank between the PCV valve and the intake would take care of this problem. I don't know so any help is good help.
Rick

[ June 03, 2002: Message edited by: ramoore ]

[ June 05, 2002: Message edited by: ramoore ]

[ June 05, 2002: Message edited by: ramoore ]</p>

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Old 06-03-2002, 11:59 AM
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sorry I have no info on this for you but I just wanted to say that the 67 GTO's are nice....and I do belive it is called a PCV Valve....good luck with this.....Mike
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Old 06-03-2002, 05:34 PM
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disconnect the pcv this will raise the vacume,but now you will be faced with no crankcase ventilation, so get an oil breather with a port like came on mopars and a diverter valve like came on AIR systems.Cut the pipe on the diverter valve at 45 degrees,this will be welded into your exhaust about halfway in with the opening facing rear so when the exhaust passes it creates vacume on the valve now connect with a hose the oil breather to the diverter valve and now you will have crankcase ventilation and more manifold vacume.this can be done on both valve covers if you have a crankcase inlet somewhere around the intake like olds or older chevys oil filler tubes
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Old 06-03-2002, 11:44 PM
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Hey Rick! Check out Summit or Jegs OR any HP Parts catalogs, and look for Vacume Canisters! This is what you need for your loss of Vacume, and that is what the canisters are s]designed for! Cindy
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Old 06-03-2002, 11:49 PM
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summit and jegs both sell vacume reservoir canisters that build up vacume allowing you to retain your power brakes. it installs between your vacune outlet in the intake and the brake booster. that is one solution you might look into.

the other is the one i am useing on my 66 impala and i can tell you it works. find a hydraulic brake booster from a mid 80īs caprice or some diesel van. and fit that instead of your vacume booster. then connect it so that the pressure hose from powersteering pump runs to the booster, then connect the high pressure return hose from the booster to the pressure side of the steering box, and then run the lowpressure return to the fluid reservoir. it is so effective ist scary

good luck with it.
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Old 06-04-2002, 05:59 AM
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I have a 67 Mustang with a Crane hydraulic cam and roller rockers and a 600 Holley. My pcv connection on the carb is capped. One valve cover has a regular filtered breather and the other cover has a breather with a 5/8" hose outlet which I used to run a 5/8" hose to a breather filter inside of my drop base aircleaner. I have 14hg reading on my vacuum gauge at idle and is enough to run the power brakes. The other suggestions are also good ideas.
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Old 06-04-2002, 10:57 AM
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YOU KNOW TENGO VACUUM CAUSE FOR YOUR SCARY CAM. NORMAL. GETTUM VACUUM RESERVOIR AS PER ABOVE. OTHER IDEAS SOUND COMPLICATED BUT PROLLY WORK TOO. VAC. RESIES ARE CHEAP. LATE
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Old 06-04-2002, 05:33 PM
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I'm confused????? just how exactly is a pcv valve causing low vacuum????? How will adding a vacuum resevoir increase his pcv flow???? These are two completely unrelated issues.
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Old 06-04-2002, 06:23 PM
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Chuck, The way I understand it , he has low vacumn on account of his cam. I don't know about the reservoir deal. Around here if our cam is so radical that it results in low vacumn, we use a vacumn pump. This is what we do when we do not have enough vacumn to operate the power brake booster. I would not think that it would take very much vacumn to operate a PVC valve as basically that just amounts to venting the crankcase and drawing the emissions to the base of the carb so they can be burned and expelled through the exhaust. I can't see where the cam would effect that. Got me confused too Chuck!
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Old 06-05-2002, 06:49 PM
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pcv valves are not wide open, they have a built in restriction. it may be that plugging the line increases vac. signal to dist. advance?,moving timing up?. or if throttle blades are open too much at idle, you could be opening off idle port too much.
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Old 06-05-2002, 07:09 PM
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change the cam, if you get a cam with less duration and wider center lines the vacume will go up. if your running pump gas long duration cams with tight centerlines the timing will be way out of wack and you will never get the performance you seek,where as a smaller cam may give you more useable horse power over a wider band of rpms, the rest you can do with gears suspention ect to get the desired effect
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Old 06-06-2002, 07:28 AM
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PVC is a poly vynal chloride pipe used for plumbing drains and vents. PCV is the valve used for crank case ventalation. PCV I believe stands for positive crankcase ventilation. It just bothers me a little when people mix them up cause I use the pipe everyday. I have never heard of a pcv valve effecting the way the cylinders hit. However it will cause gaskets and seals to blow out due to excessive crankcase pressure. It can also cause fouled spark plugs if there is no oil baffle where the pcv hooks into the valve cover, allowing it to suck oil into the carb and in turn fouling the plugs.As stated vacum cans offered by summit and jegs or any speed shop is the way to resolve your vacum problems with brakes and other vacum operated accessories.
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Old 06-06-2002, 09:42 AM
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In this day of technology it seems strange to run a cam that cannot be identified. It may be worse than the next $150 stick and you don't know why. It may not be the PCV valve at all. In any event I would suggest that an open PCV valve would lower vacuum pressure because it is siphoning off vacuum. If you plug it and the engine runs that does increase the vacuum, but leaves nothing left to deal with higher vacuum at off idle. Yes, you can run a vaccum pump for the brakes, but I wouldn't want that situation on a car I drove on the street when a properly sized cam would work better on the street and also deal with the problem. Of course, you know that all diesels use vacuum pumps.
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Old 06-10-2002, 08:37 PM
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I know that you guys have all delt with the 'vacuum canister issue', and it is designed to help with the booster and vacuum accessories. What is apparent to me is that he has a low vacuum signal that is killing 'one cylinder'. More than likely, his PCV valve is hooked to a single intake runner and leaning that cylinder out as compared to all the rest. 'Ramoore', make sure that the PCV hose/pipe is connected to the center plenum of the intake so that all cylinders will share in the task of venting the crankcase. Run the cam you have, if that trips your trigger, but the PCV shouldn't 'kill a cylinder' if it is shared by all.
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