Selecting a PCV Valve for a Modified Motor

The PCV (Positive Crankcase Ventillation) valve is part of the emission control system.

The PCV is one of the earlier systems, the concept being that any "blowby" gases and crankcase fumes are drawn into the inlet manifold under the carburettor (by the idle vacuum) through the regulating PCV valve. Then into the combustion chamber to be burnt off as part of the fuel load. The Crankcase has a hose that goes to the filtered side of the aircleaner (or has a separate filter in the air cleaner) to allow the air drawn from the crankcase to be displaced with fresh air. The crankcase environment is "essentially" fire proofed due to the crankcase being in partial vacuum.

In a new/rebuilt motor there will be very little blow by to deal with. Generally unless your motor has been built with non streetable radical specifications the standard PCV system will be ok.

Where a PCV system has difficulty working properly is if you have poor idle vacuum due to a seriously lumpy cam profile.

As for the other bits of emission control working with your performance motor that's a whole different kettle of fish..

in short,the pcv valve stops pressure build up inside the engine,as well as other things. if the valve gets clogged you will develop oil leaks. i have run .640 solid lifter cams in a sbc and didn't have a problem with the valve. it was making just over 600 hp.

Many years ago I had Echlin make us some PCV valves to use in an asphalt circle track car. It had low manifold vacuum (10-12 in Hg) and we had a carburetor size rule so I thought that we could get a few more cfm's by putting in a PCV valve in to increase power and of course help clean up the environment HAHA!!. The tech guy can't argue with that can he? I believe that the spring on the manifold side was much lighter than stock. We never really got to play with them much because the way that our motors were set up we were drawing the crankcase air from under the intake manifold, we drilled and tapped the manifold to screw the valve into the manifold, and it sucked too much oil into the fuel air mixture. We never did try them in baffled valve covers, that would have eliminated the oil getting into the fuel-air mixture. As for a supercharged motor you should have normal manifold vacuum under the carburetor (20-22 in Hg) so a stock PCV valve should work. If you have a 3/4 hole in the valve cover and a 3/8 hose to the carburetor try a NAPA 2-9229 for engines under 300 cid and 2-9210 for engines over 300cid. If you have very low leak down numbers you can use the 2-9229 on all sizes. Be sure to use a closed crankcase system, take filtered air from the air filter in one valve cover and and out to manifold vacuum on the other side. This will keep you from sucking dirt into the engine.

Many thanks! That was exactly the arrangement I was going to use, with the valve epoxied into the bottom of the manifold and drawing the vapor from the valley. Very tricky of you. I'm just gonna try it in my T bucket to make people crazy. And you confirmed my suspicion I might need some baffling to keep out the oil. I do have 20" of vacuum at idle. It's a GM crate 350 with a small blower, no other mods. I will put a breather in each valve cover and tinker up some baffling. I figure I can check the plugs to see if oiling is a problem. Again, thanks for the great and fun info.