PVC Attachment

[pflaming]

Reading the thread on 'smoking'. Is a PVC system now required on these engines? If not, why do I see so many engines with them on?

[Powerhouse]

I put mine on to reuse unburnt fuel vapor and also because I had excess blow by from the draft tube that was coming in the car. Needs ring job, etc. soon i'm guessing. It's supposed to help keep things sludgeless in the valve system too.

[JBNeal]

PCV systems aren't required, they are an engine improvement, much like putting in a 180 thermostat, modern detergent oils or ethylene glycol coolant. Chrysler put out several service bulletins for their vacuumatic crankcase ventilators to replace the draft tube on the flatheads. Evidently, Chrysler engineers recognized a benefit to crankcase ventilation of blow-by gases, but I don't think Chrysler management deemed it important enough to implement this as a design change as the road draft tube system was maintained for years (not including Power Wagon applications).

There should be two vacuum ports on the intake manifold, one for windshield wipers, the other a test port for setting the carburetor. Early PCV systems appear to route crankcase blow-by from the draft tube port through a vacuum check valve (PCV valve) into the intake manifold via the test port. Another tube was routed from the oil filler tube directly to the air cleaner.

What's puzzling to me is that the draft tube ventilation appears to bypass the air/fuel ratio metering done by the carburetor. I would think this would affect the carburetor settings as this represents a controlled vacuum leak. I am considering routing both PCV tubes to the air cleaner so that the air cleaner can trap any particulates as well as the carburetor to correctly meter the air/fuel ratio for maximized performance & efficiency.

I have studied other forum members' PCV installations and noted their performance gains, i.e. cleaner engine oil per mile & reduced sludge in the valve lifter area. But the nagging question remains: does feeding the intake manifold the crankcase blow-by gases adversely affect engine performance? if so, is this negligible? I am inclined to think there is a negligible effect on engine performance, but I haven't done any calculations or performance testing to verify this...I'll put that on my to-do list.

[grey beard]

Mr. 49 Dodge,

You, sir, are sharper'n the average bear. Quite obviously you have given the PCV issue some serious thought. AND you are absolutely right.

After I put my on PCV system - from VPW, a military package, I'm sure - I found that at part throttle cruising at a steady speed, I had a buck or surge. When you have a 4:10 rear, this can be pretty noticeable. Thinking about this whilst I drove around last summer, I tried pulling the choke out just a tad. The surge cleared right up and she ran as smooth as could be.

Ahaaa, I thought to myseklf, the PCV is leaning out the main metering circuit just a little too much. So I pulled the top off my carb and checked the float level - right on spec, so I couldn't play with that. After more thought, I removed the big brass PCV valve I got from VPW and substituted a much smaller diameter valve from a similar displacement modern engine - just used vacuum hose for the connections where the original had been threaded together on the steel pipe coming from the block.

This smaller PCV valve fixed the problem to a 90 percent satisfaction level on my part. I now believe the rig VPW sold to me may have been from a big block trick military engine - waaaay too much vacuum flow for my diminutive 218 pickup.

In summary, your concerns are well-founded. The pcv valve gotta' match the displacement of the engine, and a perfect match may require drilling out the main metering jet just a scotch or two - mebby two or three dirll numbers larger.

But that'll wait till this summer when I get my rebuilt distributor installed.

Good Luck

[MBF]

I moved the pickup tube on mine to the air cleaner housing. I was keeping an eye on the plugs and noticed she was running too lean when hooked to the vacuum port off the intake-even after switching to a smaller id pcv valve. I know the vacuum isn't as strong on the air cleaner (above the throttle plate) but I don't have to worry about running lean and burning a valve or a piston. If one had a later carb that was built for a PCV system-this problem could possibly be avoided. Mike

[pflaming]

Chrysler engineers recognized a benefit to crankcase ventilation of blow-by gases

Evidently the 'blow-by' gases are not waste or negative materials, interesting. To those who have responded, thank you. Got another thing to do.

[JBNeal]

Did some research on The HAMB and found some interesting observations regarding PCV systems on hot rods. An explanation was made that I reckon has filled in some information gaps for me with regards to the PCV valve. As y'all know, vacuum varies with throttle; low throttle = high vacuum & vice versa. The PCV valve's function is two-fold: not only is it a check valve to prevent backfire into the crankcase, but as vacuum varies, so does the PCV valve opening, controlling the source of crankcase gas extraction, something I had not thought about. With the PCV valve installed between the intake manifold & the draft tube port, crankcase gases are removed at idle but not under loading because the vacuum drops & the PCV valve closes. Under loading, crankcase gases are drawn from the tube mounted from the base of the oil filler tube to the base of the air filter housing, since vacuum is higher at the air filter during loading.

I reckon if the road draft tube is replaced with a tube & PCV valve to the intake and no tube is located from the oil filler to the air filter, crankcase gases would be scavenged mainly at idle speeds. Under loading, crankcase gases would filter out through the filler cap breather as pressure builds in the crankcase. If this pressure is high enough, the crankcase gases will "f@rt" through the PCV valve, which might affect performance, albeit intermittently.

Filtration of crankcase gases would not be necessary since they do not normally contain particulates that could damage the engine; only outside air requires filtration. Modern V-blocks have some filtration of the crankcase gases for emission regulations, something that I had overlooked.

One thing I noticed in the vacuumatic crankcase ventilator instructions was the replacement of the idle orifice tube. I assume this replacement affects the air/fuel ratio at the carburetor at idle to compensate for the added air being drawn from the crankcase into the intake manifoldat idle. Without this replacement orifice, the carburetor needs to be adjusted at idle with the new PCV system installed to account for this added air source.

Now I fully understand why Chrysler specifies the road draft tube replaced with a tube & PCV valve to the intake manifold (no load at idle) AND the tube from the base of the oil filler tube to the air filter housing (full load at speed). These are two separate scavenging sources for two separate loading situations, with a PCV valve to regulate variations in between. Mystery solved, now to get to work on the real deal...