This is not a question about where or how you installed your PCV valve, but something a little different. I have wondered about this for years and read all the PCV threads, but have never seen this discussed. If you look for a PCV valve in the parts store there is a rack of dozens of them. If I have, say, a stock 350 SBC I can look it up in the book to find the proper valve for that engine. OK, now let's say I bore and stroke it to a 383, put different heads on it, different manifold and carb, different ignition, bigger cam, etc. Now I can't look it up in the book any more. It's not really the same engine. So what are the criteria for choosing a PCV valve? If it is vacuum then all those changes probably result in less vacuum. Different PCV valve? If it is not vacuum then what is it? Or are they really all created equal? Can any one shed some light on this for me?
I just buy something that is close to the same displacement as I can. If you really want to know if they are different, then see where they are made US? China? Mexico? I bet that makes a difference too.
there are different fittings, of course...but also different calibrations. Look at an old parts book, and you'll see
There are differences with the different parts, I believe the size of the orifice and the point at which the valve opens & closes varies by part #, but I've never been able to find the details of individual valves. That would be great. There is the adjustable valve from Wagner, but they are expensive.
in the 80;s, Ford even listed a different PCV for high-mileage engines 'cause worn piston rings equals less manifold vacuum. and to the guys installing PCV in old motors that did not originally have them. The PCV system has to have a source of fresh air, which will be a hose from a valve cover to the inside of the air cleaner housing .
Original ones were threaded and screwed into the rear of carbs with straight, 45 or 90fitting. They also could come apart for cleaning. For a 383 I would find a Mopar 383 from the 70s or a FOMOCO 390 from 62 -up... good luck.
I had this happen to me with a car..replaced mine with a tune up and the car ran like ****...I put the old one back on and it was fine...I always wondered about this too
Jim nailed it in Post 3. There was that time when you could pretty well find spec on the pcv valve and pick one that pretty well fit you needs. Now finding one for the closest engine size and performance level as your modified engine is about the best you can do for the most part.
The only difference I've seen (apart from the mounting, case, etc) is a minimal bore size change. I have always just grabbed one that "looked about right", cleaned it and fitted it and kept driving. Maybe I just been lucky.
The orifice size is calibrated to allow the proper amount of vacuum to draw the fumes out of the crankcase. Late model cars that are computer controlled are very sensitive to the amount of unmetered air flowing in. The MAF sensor lets the ECU/ECM/PCM know how much metered air is going in. If the computer has to add more fuel than it is expecting it ***umes there is a vacuum leak. An incorrect pcv valve can cause this problem. Same goes with carbureted cars... They are jetted for a certain amount of air flow so too large an orifice can cause a lean condition ... Or rich if too small. It isn't near as critical on a carbureted engine but something to remember when you add a pcv system to a car that came with a road draft tube.
I have a heavy cam in a 292 SBF..At idle vac is 5"; at cruise [2k+] vac is 13/14..Talk about confusing a PCV valve..I didn't realise they had multable areas of airflow till a few years ago and probably did more carb tuning than if I had got a PCV from a 289Hypo or B302..Been looking at the Wagner but don't want to fix what isn't broke now.
Well, Thanks guys. There is a lot of good info here. But I still don't know how to pick a PCV for a modified engine.
I am no expert but I would just about guess that the answer is 2 fold. Vacuum to pull it open and volume. The mods to the motor in question will still probably produce the vac required to pull it open if not at vacuum once the throttle is opened and it is cruising. The volume is probably the difference. In out applications we are not necessarily using a PCV valve for pollution control, instead we are trying to control crankcase pressure. Your PCV valve is going to want to handle enough volume to keep your crankcase evacuated. If you are really pushing the envelope with your engine you may want to run a vacuum pump in place of the PCV to keep crankcase pressure under control.
I've used the header evac systems on my circle track cars, but I don't want to weld on my ceramic coated headers on the hot rod. I am running a standard 350 sbc PCV and not really having any problems. Just more curious than anything. Like I said, I have always wondered about this, but never saw any discussion on it. My concern is the stock engine made 18-20" of vacuum. Now, only the block is the same, and it makes 8" of vacuum and about 300 more HP. It just seems to me that a different PCV is required.
I'd love to see a chart that lists the valves by number, and then gives the orifice size, the opening and closing points, the mounting style and orientation of the valve, and the OD size for those that plug into a grommet on the valve cover. Would make it much easier for a guy to find a valve that would work best for his particular engine. I'm thinking of something like the old spark plug charts that list the plugs by part number, thread size/pitch/depth, and heat range. Why is this information kept so secret? It's almost like manufacturers don't want you to know for some reason.
I just matched up my engine (in stock form) on what Summit had. I would say it works, but maybe marginally. I will probably add another breather to the other valve cover. I get some oil fumes once in a while so I think it needs to breathe a little better.
I recently tried to find this information on the internet and ran up against a brick wall: "information not available". Grrrrrr. There are huge differences in the parameters of PCV valves but to the best of my knowledge no chart exists that is not tied to specific year/make/model engine OEM applications. Grrrrr. Not only does the amount of vacuum the engine pulls under specific conditions important, but so is the amount of blow-by and internal pressure it produces. Grrrr. Hopefully somebody here will come up with some answers.
Once upon a time I bought every GM valve cover style pcv my NAPA had, then cut them apart to measure the orifices and spring rates... In the end, I found it far easier to just pick a common valve and make it adjustable by adding an adjustment screw to the end of the plastic elbow. Here's a pic of the screw I added, it allowed me to hold the pintle off it's seat to make the orifice size effectively bigger... It's easy to cut one of these valves apart, also easy to put it back together using a band of shrink sleeve. I chose to put my pcv in the top of a homemade air/oil separator tank... Grant
Seems to me, whatever pcv valve you choose, if it maintains crankcase pressure low enough to prevent oil from being pushed out the dipstick tube or valve cover gaskets, it is probably doing its job. Phil
Since a pcv is just a controlled vacuum leak , like any vacuum operated accessory. Wipers, fuel pump or brake booster. Any valve that you install will be tuned with your jetting and mixture screws. As long as you use the same valve once you have tuned your application everything should be good, right?
Because I have one on my 40, was using the prescribed pcv for the motor. Had oil film on the vc s continuously, plus an oil leak seeping from the rear of intake, engine was hard to start under some conditions, Wagner pcv solved those.
Maybe here's a good place to review the operation of the pcv valve: If you look at @Weedburner 's pictures above, it's easy to see what he did. The screw prevents the valve from closing, so it will draw more air at low idle. Very simple, and a great idea. I think I will be doing this soon. I don't see a need to open it up either, should be able to just drill a hole and insert an appropriate sheet metal screw of the right length. Experiment with settings to eliminate vapors flowing from the breather at idle. A whole lot better than $129 for an adjustable valve.
Bobss396 - Any chance you have a link to that Summit info you refer to? I just matched up my engine (in stock form) on what Summit had. I would say it works, but maybe marginally. I will probably add another breather to the other valve cover. I get some oil fumes once in a while so I think it needs to breathe a little better.
I like it! In your $129 face Wagner! This setup is very similar to the breather setup on our 410 Sprint car engines. They were set up with a crossover tube between the valve covers with 2 small filters in the crossover instead of the catch can with PCV. I see something like this in my engines future.
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