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  1. #1
    Join Date
    Sep 2004
    Location
    Santa Barbara, Ca
    Posts
    983

    PCV valve question

    Ok, this might seem like a stupid question but it is something that I have never thought about before. I have always run a pcv valve on my engine since the day I first fired it up. It started out mostly stock, but over the years it has turned into a pretty raspy street motor. Here's my question. A pcv valve is supposed to open during low vacuum conditions to allow crankcase pressure/vapors to be released back into the intake. With my 242/248 duration comp roller cam, I have no more that 5 or 6 inches of vacuum at idle. Is a pcv valve a bad idea for this type of engine? Are they harmful to performance? If my logic is correct, my valve is at least partially open at idle, sucking in un-metered air. would this give me an overly lean idle condition? For your info my carb is a 750 Edelbrock w/ manual choke. I have riched up both the cruise & power modes 2 stages over stock, and have changed the step-up springs. Any feedback on this subject would be appreciated.
    Andy



  2. #2
    Join Date
    Aug 2003
    Location
    Orlando, FL
    Posts
    8,749
    You should run it if it is a street car, you need some way of vacuming the crank case. Set up properly, the PCV valve will pull from the breather on the left front, to the PCV on the right rear, giving a cross venting and some minor cooling effect.
    If you do not run a PCV, you will either need a vacum pump, or evac tubes to the headers, or else you will be pushing oil out of all the seals.
    Yours is working fine at your duration, we run them on cams much bigger then that on the street.

  3. #3
    Join Date
    Sep 2004
    Location
    Santa Barbara, Ca
    Posts
    983
    That is what I assumed about the oil seal thing. But what about the air/fuel ratio? Wouldn't any given engine run leaner with a PCV valve when it is in the open (venting) position? It is sucking in air below the throttle plate, almost like an air leak. I know that back when our cars were new, PCV valve equipped cars had carbs that were calibrated differently to account for the PCV. The PCV system was designed to work with engines that produced 17+ in. at idle. I am trying to figure out if my valve is open at my low-vaccum idle, causing a lean mixture & driveability issues. The car has always run fine, but I wanted to know if big cams and PCV's were a bad or good thing. I will go drive my car with the port plugged, if the rain ever stops! Oh, well.
    Andy

  4. #4
    Join Date
    Aug 2003
    Location
    Orlando, FL
    Posts
    8,749
    Just tune with it.