Engine and Transmission Tech

My engine is running fine, but I've been wondering if I should refine my Positive Crankcase Ventilation installation. Do you guys have any thoughts to share about that?

It occurs to me that I must be sucking a fair amount of air through the PCV valve, thus I'm probably sucking an equal amount of air into the vented cap on the opposite valve cover. Maybe I should upgrade my filtration there?

Is it very helpful to relocate the vent off of the valve cover?

I don't know a lot about PCV valves... do you have any advice for choosing between the many, many available? I think orifice size must have a big effect on idle speed. Anything else I should think about?



Aside: I got curious earlier this week, and removed the lid from my plenum for the first time since installing EFI about a year ago. (It's only sealed by "Right Stuff" monkey snot, so I don't want to make a habit of removing and reinstalling it.) I was very surprised how clean the inside of the plenum was. There was a light oily film over the interior walls, but nothing that wouldn't wipe off with paper towel leaving the aluminum bright and shiny. The floor of the plenum seemed cleanest, though I'd expected it might the dirtiest part. That's all after several thousand miles, including the roundtrip to last year's BritishV8 meet. I glued the lid back on and verified that I didn't have a leak by using the old unlit propane torch technique. No sign of idle speed change... but once again I wondered if the propane leak test is really effective. (I've never had that test result come back "positive". Have you?)

Curtis, the first thing you must decide is whether you want your PCV system to conform to American practice or British practice. There are pros and cons for each. British uses an orifice on the inlet to the crankcase. The orifice should be filtered but can use a small element.

British uses a relatively large hose from the intake to the crankcase and places the crankcase under engine vacuum which is good for keeping oil from leaking past seals. When blowby pressurizes the crankcase (WOT) this large tube feeds it directly into the intake.

American uses a PCV valve on the suction line to the carb. There is no separate orifice but the PCV is a shuttle valve, often with a light spring or working against gravity. At idle (high suction) it closes and meters air through an internal orifice or bypass so that the carb mixture can be calibrated. Under heavier throttle (less vacuum) the shuttle opens and allows full flow. At max throttle the blowby gasses can force the shuttle closed but the large diameter hose (5/8" usually) connecting the opposite valve cover to the inside of the air cleaner allows the blowby to be drawn into the intake. This does have the effect of enriching the mixture as the blowby has already been burned once and is metered twice. With EFI often a computer controlled PCV valve is used.

Personally I like the British system but it requires a recalibration if used with American carbs, and for those engines the American system is easiest.

Jim

Well of course I have an opinion!

A north american style pcv is a pretty clever little device.
Under high vacuum situations such as idle and cruise. The valve is sucked closed and flows a very small amount of air.
It is assumed that the engine is not highly loaded so blowby gases are at a minimum.
Under load when vacuum drops the valve opens up and is able to pass much more air. This allows evacuation of the crankase gasses into the intake where they are burned off.
But it doesn't always work out that way.
Situations such as high speed deceleration cause high vacuum levels. Which close the valve.
But the engine is producing high levels of crankcase gasses which can now only escape through the vent.
This often will overwhelm the venting system causing oil to pile up in the valve covers.
Carbed engines generally run just fine with a pcv. But a big cam can cause vacuum to fluctuate enough that the valve is constantly opening and closing at idle.
Trying to get a stable idle in that situation is near impossible.
EFI opens up a whole new can of worms.
If you have a speed density system you're immune to most problems.
But if you run an airflow metered system such as the 14CUX then any air that is used in the system must be accounted for.
The air flow sensor needs to measure all of the air used in the system. So that means that the valve cover vents must be tied into the intake between the airflow sensor and the throttle body and the pcv goes to the plenum for a vacuum signal.
The problem now is that the pcv is supplying random quantities of air to the plenum.
The fuel injection is pretty strict about things like that and generally responds with a rolling idle, or worse.
The superior choice for me. On a smaller displacement injected engine. Or anything with boost.
Is the metered orifice style of crankcase breathing system.
The air drawn into the system is consistent. So central command (the ECU) doesn't get it's knickers in a knot.
And 1/2 of the venting available doesn't just disappear at inopportune moments.
Tuning at idle becomes a little more sensible. And no more sudden unplanned lean spots at full throttle.
Both systems are simple, and both have been made to work well. It's just packaging and preference I guess.
As far as choosing and actual pcv (the "valves" is redundant) ( just my own little pet peeve) :)
I generally head for the catalog and look for an engine nearest in size and power to mine.
Flow rate and vacuum (spring) rates vary considerably. So some trial and error will probably be in order.
As far as the propane test.
"Just say no to propane"
Unless you lean your idle out heavily you won't notice a thing.
But if you have an ignition leak you can find that pretty quick.
I'm a big fan of plain old water in a mister.
Spray it on with abandon and if you have a vacuum leak. # 1 you will hear it sizzle and #2 you will drop a cylinder momentarily.
But wait, as an added bonus to the 101 things that I can do under my sink without going to jail.
Add some ammonia to the water and turn out the lights.
Spray your ignition and it will give off a distinct blue glow where ever there is an insulation breakdown.

Cheers
Fred