The Stovebolt Site Forums The Shop Area Engine Shop PCV System revisited

Like many others, I have tried to run a PCV valve, only to go back to the road draft tube.

I tapped into the top of the draft tube installed grommet and PCV valve, ran a 3/8" hard line around to the the stock intake. Used short sections of 3/8 tube to tie it together. Had to sleeve down the rubber tubing to tie in with the 1/8 NPT fitting on the manifold. Plugged the lower end of draft tube and used a vented cap on the valve cover. It seemed like there just wasn't enough volume of air (vacuum) through the intake fitting.

Has anyone drilled and tapped out the 1/8"NPT intake fitting to 3/8 NPT? Does this sound like a viable way to get increased volume of vacuum to the pcv valve?

I'm starting out with a freshly rebuilt 235 and would like to upgrade and close the venting system. I sure would appreciate any input you might have. Thanks in advance.

might work better to route it into the air cleaner?

Bill

You'll need at least a 3/8" diameter hose flowing its full capacity to support a PCV valve properly. Drilling/tapping for a 3/8" NPT elbow or nipple is a very good idea. Look at the fittings a later-model engine uses for PCV plumbing to get an idea what it's going to take to flow enough air, and plumb your engine accordingly. The smallest, most restrictive fitting in the line will establish the maximum flow, so don't put any choke points in there.
Jerry

If you like to go with PCV use the 3/8" lines and flared fittings like the 261 PCV system, giving you a wider smallest restricition, like Hotrod Lincoln suggests.
Also you must use a condensation filter before the manifold, which the early systems didn't have.

I personally think that PCV sucks...in more than one way.
What follows is my personal experience converting to PCV on a daily late 235.

2-1/2 years ago I rebuilt the head on a daily driver engine, well over 10 years old.
I installed a closed PCV system with special condensation filter and ran it daily in the city and on freeways.
Found a higher compression head cheaply last November and swapped heads.
After inspecting the fresh rebuilt head that ran for 2-1/2 years, I decided to never use PCV again...

Although the spark plugs showed ideal temperature and clean and lean combustion, the chambers and valves had nasty crusty build-up, especially the two inner ones, where the PCV intake is located on the manifold.
The built-up around the valve seats would have gradually further blocked fuel from coming in, losing power and performance.
I haven't seen any of this built-up inside the head which ran for at least 10 years without PCV.
Remember, I got this built-up despite using a special filter, which most PCV systems don't even have. And this filter removed large amounts of nasty liquids and required frequent draining.
No chemicals and fine wire brushes would dissolve the built-up - I had to scrape and chip it off with a chisel...

That is the main reason why I went back to the road draft tube.
The other is that PCV actually shuts down when the engine produces most of its blow-by vapors:
With wider open throttle, manifold vaccum decreases sharply and blow-by gases increase crankcase pressure with no way to vent - stressing seals, blowing out oil at every opening it can find and restricting piston travel at higher rpms.
The road draft tube does the opposite and evacuates more crankcase vapors at increasing speeds.
At least it always provides a large opening vent even at lower speeds.

The 3/8" lines like on the 261 PCV stop flowing, when there is no or too little vacuum in the manifold.
Pressure builts up with the only vent being the long 3/8" restrictive line into the manifold and the intake filter on the valve cover.


Maybe fresh rings and only occasional city cruising will give less built-up in the combustion chambers.
Maybe somebody who has pulled a head on such an engine after about 6-8K miles of using PCV can enlighten us...

I could well be wrong but I thought a non-vented oil-filler cap (and a sealed valve cover) was used on a PCV system?

The air has to flow through the engine. From a filtered vent such as a oil fill cap, to the pvc and intake manifold. Our 21/2 ton military trucks had GMC sixes with sealed everything, for water proofing. The PVC valve was installed on the side cover and connected to the intake. The vent side went to the air cleaner and pulled air from the cab when fording water bodies.

Here's a picture of a stock 1958 261. Sorry I don't have a picture of the fitting off the other side where the road draft tube was placed.