You have to be careful with negative pressure too. That's probably the most important duty of a PCV valve. Some crankcases (maybe all, IDK) can develop a vacuum at some RPMs and sucking an air-fuel mixture into the crankcase can result in it igniting, blowing gaskets and seals out. I've even personally seen an instance of a 'ballooned' oil pan and heard of pans blowing off. The PCV valve is supposed to close in that case, forcing the crankcase to draw though its 'fresh air' inlet and keeping out the mixture.
You actually want a partial vacuum on the crankcase. Attach a vacuum gauge to the dipstick tube and a functioning system should have a 1 - 2 Hg draw. The pintle/spring in the valve will regulate how much the pintle opens/closes at various vacuum strength signal to arrive at proper function. You would have to experiment with valve orifice sizes (or better yet an adjustable one). The better way to ventilate the crankcase is through the rocker cover(s) and secondly, the oil fill tube on a intake manifold/valley cover with a vented/filtered oil cap (or if a closed system, the cap is closed (no vent) and draws fresh outside air through the ACL. FORGET - The 2nd design of the valve is that it will close in case of a backfire/misfire preventing the charge from reaching the oil pan.
I recall reading about low volume vacuum pumps for vehicles, I wonder it something like that could be plumbed in?
I agree that you have to be careful. The PCV valve is supposed to flow in one direction only to keep any buildup of vacuum or pressure situations from happening but I guess things can fail to function as planned. Years ago I was involved in an OT online group dedicated to a particular turbocharged vehicle. Long story short, I was running 25 pounds of boost in my particular intake and never experienced or knew of anyone else on that forum having any backpressure (vacuum) issues. But we treated the PCV valves as a maintenance item and not something put in at the factory that you never give any thought to again. The drawing in my above post shows this situation in the bottom example. Our turbo crankcases weren't totally sealed. We had what was called a "makeup line", a small hose from the valve cover to the air cleaner that could push crankcase vapors or pull clean fresh air depending on circumstances as needed. A lot of aftermarket air cleaners come with the fitting for a "makeup line".
Thanks for your input guys. Here are my thoughts so far: 1. This is an Chevy 261 inline 6 (early 60s) - Since I have a road draft tube and vents at the valve cover, there is no way any pressure can be build up - totally open system for now. 2. Still want to get rid of the draft tube, but make sure I don't contaminate my oil and cause unnecessary wear. I can see a few ways to tackle this. A - Put a filtered vent on the draft tube, and suck gases from the valve cover. I like this idea, since I think the vapor/gases are hot, they would probably travel upwards. B - The other way around (filtered vent on the valve cover/suck at the tube/crank case) That leaves me still with the decision where to get my vacuum from. As I said, I would like to skip manifold vacuum, since my Fish carb wasn't made for it and it's almost impossible to compensate it for a variable vacuum leak of unknown size. Also PCV valves flow from 2-12 hg, depending on the model, and that specs are made a secret (at least I found noting) so it's kind of a lottery, unless you order a bunch and measure them. Since I live in Germany , there is no NAPA or something like this - I have to order them in the US and because customs/shipping cost is abysmal these days - so there is no "just get a bunch and give them a try". So that brings me back to my plan A. Would it be feasible to put a filtered vent on the draft tube, and suck the gases out via a baffled valve cover breather. I got a inlet at my air filter that would see "venturi vacuum" (you learn something new every day here)? Thanks, Frank
Venturi vacuum is used , for example, to control the vacuum advance unit on a Ford 8BA distributor (among others). It's a load control distributor and venturi vacuum is what Henry's engineers decided would best control timing advance.
Besides what Center mentioned, it came back into use in the late '60 and beyond as an emission control device. If you owned a vehicle that had three (or more) vacuum hoses connected to an intake manifold valve with one going to the vacuum advance cannister, one of those hoses was venturi vacuum. The valve was coolant-temperature-controlled and the design was that when the motor was running cooler, the valve used venturi vacuum to change the timing to reduce emissions. You also could run into ones that had dual vacuum cannisters at the distributor, one connected to manifold vacuum, the other to 'switched' venturi vacuum. Problem was the motor could start to overheat sometimes, then the valve would switch back to manifold vacuum only until it cooled down. I had an OT car with a 351C that had an intermittent knock that I couldn't seem to get rid of no matter what fuel grade I used or where I set the timing. Disconnecting that valve and using manifold vacuum only fixed it...
You would have to read HOLLEY LOAD-O-MATIC THEORY to fully understand it - Proper advance is controlled by a varying mixture of VENTURI and MANIFOLD VACUUM by the SPARK CONTROL VALVE on the carb.
The system described in POST #39 was known as IMCO, and used manifold and ported vacuum signals - IMCOVacuum advance/retard (also known as dual-advance) was an integral part of the IMCO (Improved Combustion) emission control system launched in 1968. IMCO works on a principle of applying manifold vacuum to both sides of the advance/retard unit depending upon vehicle operation at the time. Under acceleration, you advance the spark. On deceleration, you retard the spark to reduce hydrocarbon emissions (unburned fuel). The IMCO system is controlled by a thermal vacuum switch (TVS) located at the thermostat housing. As coolant temperature warms, the vacuum switch vectors vacuum appropriately to control emissions.
Just ordered a Fram FV410 PCV valve, which could/should be in the ballpark for my 261 and gonna give it a try. Also got me a catch can. This is the setup I got in mind: I put a filtered breather on the Draft tube snorkel (and remove the actual draft tube of course). The other breather goes on the valve cover and a hose from there to the catch can - than to the PCV valve and manifold vacuum. Still not super convinced, but I give it a try.
You are more than welcome. That info is mostly FYB but the system is basically the same. FORD used it up until 1967 on inline sixes.
That will probably work. On my yblock 239 I found a grommet that fit in the road-draft tube and held a pcv valve, then ran a 3/8 hose to manifold vacuum. This kept the crankcase airflow the same direction as stock. For a pcv, I just used one from an engine that was close in cubic inches, in my case I used a valve from a 3.8L v6 ford. That engine had a lot of blow by, the road draft tube would work ok when moving but waiting at a light the engine would smoke a lot out of the tube. The pcv helped with that until I found a 292 to replace the 239.
Sooo.... that Fram FV410 PCV valve, that someone recommended just arrived. Turns out it is a fixed rate unit without a plunger - just a can with a tiny hole, and the hole is about 1.5 mm (0.078 inch). Looks like all that "you absolutely need 3/8" ID" just went straight out of the window. If I had knew that, I could have saved 20 bucks and just put a washer with a tiny hole in the hose - well live and learn. I mean really, how is this stuff rated anyway? If you look up the applications for the FV410 on the Fram website, they are all over the place from little 4 cylinders to 6 liter V8 engines. And they all are happy with the same 1.5 mm hole in a little can?
Valves are not rated as OEM did not release this information. They are designed for a specific application and when you go aftermarket, you have no idea if the OEM specs were duplicated. Even an OEM valve will possibly not be correct for an engine that has many miles (increased blow-by). The valve you came across is a vacuum metering device to correct some mixture problems on an emissions SBC. You either buy an ADJ VLV or you keep trying individual valves (vacuum gauge readings) until you hit home. It is not as critical as a later emissions engine and you can be more lenient with an early style because you have fat fuel.
Venturi vacuum comes from a port in the carb near the venturi. It is created via the Bernulli principle. Air rushing past an open port will create a vacuum in the port. Holley uses venturi vacuum to open the rear throttle plates on their vacuum operated 4 bbl.
So I'm pretty much back on square one. Is this "correction valve" the only PCV valve on this engines, or do they have a second one, that does the heavy lifting? The more I read into it, the more I realize the whole PCV valve thing is a mess. "Get one for your application" yeah right - if you have a 261 pretty much all of them are obsolete by now. Just learned that a Standard Motor Products V-237 would be right, and I could get it at Rock Auto, but since I live in Germany - shipping that $2.50 valve is $25 . So I would like a cross reference, maybe I can find something similar over here, but no dice. Ahhhhrgh!
I bought just about every pcv valve GM uses for common '70's-'90's 4/6/8 cyl applications. Most are very similar internally, just minor differences in the spring itself. I was looking for a valve to suit my application, so i tried just about every stock GM valve that i could lay my hands on. Here's some specs from the valves I cut open... NAPA# 2-9229 (the body of the valve is stamped 2088) GM 4/6/8 cyl '85-'95 Spring free height is .775" 95 grams compressed .300" NAPA# 2-9333 (the body of the valve is stamped 2221) Ford 5.0 & 460 '87-2002 Spring free height is .800" 110 grams compressed .300" NAPA# 2-9210 (the body of the valve is stamped 2072, pictured in previous post) GM 262/350/454 '80's thru the '90's Spring free height is .775" 117 grams compressed .300" There are different pintel shapes and tapers. All the above have the same dia holes in the internal "washer" that the pintle slides into. 2221 and 2072 use the same pintle with 4 cutouts in the large "nailhead" end and a short taper. 2088 has a pintle that looks like a sawed off nail, no cutouts in the nailhead and a very gradual large dia taper where it slides into the "washer". I also have two other valves in hand, they were special order from my local store. I didn't get them until after i had made my valve, so i have not bothered to cut them apart. They are... NAPA# 2-9230 (stamped V187) '80-'87 Chevette NAPA# 2-9246 (stamped 2108) '74-'79 350/400/454 All the above valves share the same 3/4" body diameter where they plug into the rubber grommet, and have the same plastic elbow pushed onto the top. I could not find a valve that did what i wanted it to do in my street/strip SBC application. Turns out you can customize a pcv valve fairly easily. Here is one of the above pcv valves for a sbc that's been cut open... You could buy several different valves and mix/match parts if you wanted to. I cut mine apart on a lathe, only takes a few seconds to trim away the edge of the crimp to open it up. I'm showing a cut apart valve for illustration, but I ended up taking a stock non-cut valve and just added a small screw so that i could adjust the closed orifice size.... A cut apart valve could be re-assembled using a short length of 3/4" id shrink tube, so you could mix/match springs/pintles with no welding required... The screw is 1" long. I just drilled a hole slightly smaller than the thread diameter, and the screw made it's own threads in the plastic elbow. Try to make the threads straight in the plastic, so that the tip of the screw comes out centered on the pcv valve's pintle. I mounted my modified pcv valve in the top of my oil separator, away from the valvecover. I've been using the same modified valve since around 2008, works great for me. Grant
Now that's an interesting idea. And way cheaper than those adjustable valves. Thanks for posting this. Frank
The Carter factory carburetor school did a demonstration for students to explain how "venturi vacuum" would change based on the location of the port. In the drawing, the 4 glasses have a liquid (water) and of course the top of the glass is open to the atmosphere. One end of a tube is connected to a port at a specific point in the venturi, and the other end inserted into one of the glasses. The different values of vacuum (negative pressure) will cause the liquid to rise in the tube(s). The different heights of the liquid in the different tubes demonstrate the differences in vacuum due to the port placement. Note that the value(s) of the heights are proportional to the air velocity at given points in the venturi. Jon.
I need to try this on my pickup, the pcv closes too much at idle and I get fumes out of the breather. As this drawing shows, what the screw allows is the fine tuning of the position of the pintle valve during periods of high vacuum, i.e. idling. The vacuum pulls on the valve moving it into a position that closes off the flow of fumes. Using the screw as Weedburner shows, you can limit the travel of the pintle valve to keep it open and allow more flow. That's what I need to do. Note that this will not affect the flow during periods of low vacuum, i.e. under accelration.
At idle the breather, whether exposed on the opposite side rocker, plumbed into the air filter, or the oil filler tube, should be able to hold a piece of paper. The dip stick should have a seal also. Mine uses a piece of leather. This is the standard easy test when installing or checking a PCV valve.
I converted a 1964 Ford and a 1970 Toyota which both came with road draft systems aka "stink tubes" to PCV systems, from slightly later but similar engines. They both were plumbed to manifold vacuum and ran great with no carb modifications. The carbs on both of those engines also weren't made for PCV valves.
From my experience, there is not one single "emission doodad" that makes any engine run "great". They are band-aids around band-aids that often require compromises and workarounds at other aspects, and there is always a price you have to pay (mileage/power etc.). For example: "After years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, and all manner of crude systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it." So if your Toyota actually runs great, after introducing more air into the manifold, it probably run way to rich before.
Okay, I'll rephrase that. My engines ran fine with the stink tubes, but released noticeable blowby smoke at idle. After converting to PCV systems, they still ran fine, but didn't smoke at idle. Maybe they were rich before. Maybe I merely adjusted the idle mixture screws out a little afterwards. I don't remember, it was a long time ago. I did not say that the PCV upgrade made them run great.
The Jalopy Journal
