How do I know the correct PCV valve for an engine....is there a chart for different vacuum ...my engine pulls about 15 psi on a vacuum gauge. I can hear the pcv valve rattling at idle and if I pull the pcv valve out of the valve cover and block the end of the valve with my thumb, the idle smooths out and drops a hundred or so rpm...this suggests to me the valve may be causing a vacuum leak...I think it may be the wrong valve, not sure....the number of (Fram) pcv valves that look identical but have different part nos. suggests the difference is internal, so how do I know which one to use......
There is a ton of info out there. Found a few things on a quick search. AC Delco has a good chart I could not find High Performance PCV Valve Shootout – Flow Test Results – M/E Wagner Performance Products (mewagner.com)
My PCV hose is connected to base of carb...can I plug this outlet and run hose to base of air cleaner...this would eliminate the vacuum leak?
The air cleaner is a common OEM location for taking air into the crankcase, not out of the crankcase. Please post some pictures of your setup.
I have always used the one for the car I got the engine out of. For my 49 It has a 70 Cad engine so I used the PCV for a 70 Cad. But that doesn't help if you modify the engine.
Looking at your photo, make sure there is a splash deflector in the rocker cover or you have the grommet with one built in. Mr. Gasket sold them at one time. If a system is correct, the suction at the opposite rocker arm cover cap or breather should be 1” to 3” of vacuum. If it holds a piece of paper that’s a good check.
I cannot ID that carburetor, but if the port you are using receives manifold vacuum, it should be fine. If it does not, the hose needs to be moved to a port that does. And yes, as has been mentioned, the PCV valve is a controlled vacuum leak. If your idle performance is not what you expect it to be with the PCV in place, you need to re-tune. Since it is a small amount of extra air that is not going through the carburetor at idle. That being the case, you might need to richen your idle mixture slightly to compensate.
But should I be hearing the pcv valve rattling at idle...sounds like a marble bouncing up and down inside, almost like the valve is not correct for the vacuum supplied from base of carb...like it needs a stronger or weaker spring inside, this is why I asked about the correct pcv valve for my motor....72 350, 4 bolt main, mild roller cam. the carb is a Summit street and strip unit built by Holley
Buy a pcv for your engine , get a name brand one . Or get the adjustable one THATS mentioned. You should not hear it rattle at idle , when I worked at Jaguar the x-type with a v6 had pcv valve rattle issues and they had a service bulletin to replace it with one with a stronger spring . cheap pcv valves are just that , cheap !
I read the Wagner read. I may have read wrong or not far enough into it. I wasn't sold. Or my look at valves wasn't accurate, they seemed to overlap different engines, from 6cyl to performance v8's. Just a note: If you have replacement [like from a restoration supplier] PCV hose a few years old, check it. In the old days the hose would get hard as a rock; what I have is a few years old and one is slimy on the out side and very soft; the other is as soft but not slimy.
Id hate to see the price.....prob cheaper just to buy several regular ones and try them till you get the right one.
I may be wrong, but isnt a pcv valve something introduced as part of smog measures and equipment, and not strictly needed? I'm deleteing it on my 348, it will have the standard breather/oil filler, and as its a new engine there shouldnt be any real blow by on the rings etc. that the stock breather cant handle. The guy building it says he has never fitted one to a rebuilt engine, either street car or drag race. Just a thought..
Positive crankcase ventilation is kind of an option, old engines just had breather caps and road draft tubes to vent the crankcase. Smog regulations brought on the carb vacuum based closed PCV systems and racers found horsepower advantages by pulling more vacuum using exhaust system vacuum or belt driven pumps. Tuned properly, the engines still run fine with or without them. I think most hot rods incorporate them because they help keep the engine compartment cleaner by avoiding the spread of oil vapor and the dirt that sticks to it. (My wife loves everything about hot rods except the smell of blow-by, that’s another reason I use them)
I did not see it in the picture buy make sure that you have fresh air coming in on the other side of the engine, typically though a filter on the valve cover.
A PCV system will definitely make your garage smell better and it does help with minor oil leaks especially with a rope type rear main seal.
A working PCV system will also help remove moisture and condensation from the crankcase. This is often a problem for cars driven infrequently and mostly for just short trips. Water can collect in the crankcase and contaminate the oil. As the oil gets up to full temperature it vaporizes the moisture and the PCV system draws it out of the crankcase.
Crankcase ventilation, as opposed to just venting, is very helpful for longer engine life. http://chevy.oldcarmanualproject.com/shop/1947truck/47ctsm0003.htm In the 1940s Chevy offered PCV kits as optional for trucks in heavy duty service. In the new millennium / modern era some manufacturers managed to get PCV wrong, with expensive consequences. Google sludge recalls for Toyota, Chrysler, Audi, VW and Saab.
It really depends on how you run the motor. At WOT you have no or very little vacuum so the PCV does nothing and you will blow oil out at the weakest point. But for normal driving a PCV works fine.
The Jalopy Journal
