crankcase ventilation revisited

[Jon W.]

Looking back at several posts from May of this year there was some discussion on proper operation and configuration of crankcase ventilation/PCV systems especially when combining the earlier and later engine components. Here's my situation: 383" SBC, late 80's roller-cam block, iron Vortec heads with the black centerbolt valve covers to clear roller rockers, RPM alum. intake, Holley. One valve cover has provision for both the oil fill cap and the PCV valve. PCV valve is connected to vacuum at carb base. Other valve cover has NO openings. The fitting on the open-element air cleaner base is plugged off. I've been running this setup for 4 years and approx. 18K miles on my '70 coupe.

My questions/concerns:
1.)it appears that I do not have a fresh air inlet source for the PCV system (unless the later engines used some other source?)- where is the air inlet to the crankcase on the engines that used these valve covers?

2.)would a change in my current setup be recommended in order to draw air into crankcase? What might the consequences be if I continue as is with current setup?

Thanks in advance.

[Terry M.]

Re: crankcase ventilation revisited *TL*

Jon,
I believe the PCV air inlet for those valve covers originally is through the opening you are using for the PCV valve. On the engines that use those valve covers the PCV vacuum is drawn through the intake manifold. That is there is an opening in the intake manifold from the intake vacuum passage into the crankcase that contains the PCV valve.
I believe you should provide a source of filtered air through the other valve cover - perhaps use another with the PCV opening and the oil fill cap. Then you can add oil from either side of the engine.
Your current set up is relying on combustion blow-by, mostly past the rings, for PCV make-up air. I would think this might increase blow-by, increase oil consumption and reduce combustion efficiency in the long run.




Tire Rack tire tech

[Clem Z.]

Re: crankcase ventilation revisited

also if you do have a negative pressure(vacuum) in the crankcase you are pulling in unfiltered dirty air thru any orifice that it can get in which is not good.

[Jon W.]

Re: crankcase ventilation revisited

Terry and Clem,
Thanks for your replies. If I understand correctly, the later model small blocks with FI pulled vapors from the lifter valley through a PCV valve or similar arrangement directly into the base of the FI unit's air plenum or intake runner. (This vs. earlier engine's arrangement of pulling vapors from the valve cover to carb base, or even from oil fill tube to carb base).

Another thought comes to mind..The PCV system seems like a type of controlled or metered "vacuum leak". How does this affect carb jetting and idle mixture adjustment? Also, how does one determine the functionally correct size PCV valve to use in a particular application?

Thanks again. Jon

[Duke W.]

Re: crankcase ventilation revisited

Carburetors used with PCV equipped engines were specifically calibrated taking into consideration the flow of blowby gas. Also realize that blowby gas is mostly unburned fuel air mixture with some exhaust gas. Under many conditions it is combustible and a flame arrestor of some type is required to preclude an inlet system backfire from propagating back into the system and exploding the vapors in the crankcase. PCV valves have this function built in via the anti-backflow design of the vavle.

Correct valves are listed in parts catalogs for all engine applications and the original Delco valve number is usually listed in the owerers manual. Proper PCV systme configuration is also detained in the AIMs for each available engine configuration.

On Corvette engines with valveless PCV system designs,there there are screens that function as flame arrestors. The flame will be quenched by the screen and not travel through it.

If you are building an engine from parts you need to have some basic knowledge of PCV systems and carburetor fuel calibration to design a suitable system. Unfortunately, most back yard hotrodders have little understanding or appreciation for properly functioning PCV systems.

Duke

[Jon W.]

Thanks Duke for your help. *NM*