Leak Down Numbers ??

Discussion in 'PSI Superchargers Tech Questions' started by Moore, May 26, 2010.

  1. Moore

    Moore New Member

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    What are acceptable numbers for a blown hemi, if you do a leak down? Has anyone pulled the heads and leaked just the chamber to see what the vales are doing? An how about each cylinder? Any info would be appreciated.

    Pmoore
     
    #1
    Moore, May 26, 2010
  2. Mike Canter

    Mike Canter Top Dragster
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    You had better have zero percent leakdown on the heads or you have problems.
     
    #2
    Mike Canter, May 26, 2010
  3. Comax Racing

    Comax Racing Member

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    Leakdown

    We built a leakdown plate for leaking the cylinder without the head on but I agree with Mike if your head is leaking you should almost be able to see it with the naked eye. As for numbers I'm going to let the experts answer that question. I'm kinda curious as well.


    Corey
     
    #3
    Comax Racing, May 27, 2010
  4. Moore

    Moore New Member

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    Been told you should check the chamber by seeing how much vacuum you can pull, not hold but pull. Any info on this process?

    Tks Pmoore
     
    #4
    Moore, May 27, 2010
  5. Mike Canter

    Mike Canter Top Dragster
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    Are you talking about doing a vacuum pull on the intake port and the exhaust port because they make a tool to do that.
     
    #5
    Mike Canter, May 27, 2010
  6. GregM784

    GregM784 Member

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    when the heads are off, fill the runner with methanol. Watch the valves. Wet = leak. :)

    Done it at the track numerous times.
     
    #6
    GregM784, May 27, 2010
  7. Moore

    Moore New Member

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    Mike, I'm talking about a vacuum pull on the combustion chamber. Install the plugs and pull on the chamber, get both valves at the same time. If you can't pull a vacuum, then put liquid in one port at a time and see where the leak is.

    Pmoore
     
    #7
    Moore, May 27, 2010
  8. Mike Canter

    Mike Canter Top Dragster
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    You can do that easier with a leakdown tester and a plastic tube to listen to each port. If you have the motor together and you do a leakdown and the percentage is high you will hear it hissing. You stick the tube in the intake runner and the exhaust port and if the valve is leaking believe me you will hear it. If neither is leaking then it could be the head gasket or the rings.

    You were asking what is acceptable for a leakdown percent. I would say the acceptable highest is 15% but would like them 5-10% but sometimes that is hard. If at a race I will accept higher without having to tear the motor down. You have to be real careful of false low readings if the rings did not lock against the cylinder wall. Sometimes you can see 40-45% then with a breaker bar on the blower pulley you can jerk the motor a little bit and the air will get behind the rings and it will drop down and be good. Before I will pull the head I will take out the pushrods and pump in the air and let the piston go down in the hole and see if I can get the rings to seal for the test.
     
    #8
    Mike Canter, May 27, 2010
  9. john348

    john348 Top Alcohol

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    Wizards make a alum plate for a hemi for using a leak down tester for the head or the block
    you will be amazed how much your head leaks with 100 psi air versus filling the port and watching for a "wet" valve
    we try to keep our heads under 1/2 percent 0% is hard to do
    nice to leak'm before you bolt them on
     
    #9
    john348, May 28, 2010
  10. SoDak

    SoDak Active Member

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    How do you deal with these big spring psi's and leaking valves. I've had it where the leakdown was acceptable, but when that valve is spun, in the lathe or grinder, its crooked. It appears the high spring psi "straightens" the valvehead.
     
    #10
    SoDak, May 28, 2010
  11. john348

    john348 Top Alcohol

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    IMO
    if it is crooked in a grinder you may want to grind it straight again
    then lap it back in
    I use 2.600 intakes and we never grind them, we hand lap them
    if they get bent replace or grind them then
     
    #11
    john348, May 28, 2010

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