piston compression hight V rod length

Discussion in 'PSI Superchargers Tech Questions' started by torino, Jun 15, 2008.

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  1. torino

    torino New Member

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    i have a set of new rods that are only 6.971 long for my hemi, running blown alcohol with billet dart 96 big chamber heads and 14/71 standard blower braket type funny car . i need to go to the next bore size 4.310 but need custom pistion to get the correct compression hight , this will make that around 1.65 . my question is is this to much compression hight , will this make the piston to top heavy , what is a good compression ratio number to run in this type of car 11, or 11.5 ? or more or less ? .
    would prefer to keep the rods as they are new , but if not what combination dose any one sergest .
    the block is standard deck with a 4.25 stroke .
    thanks
     
    #1
    torino, Jun 15, 2008
  2. Mike Canter

    Mike Canter Top Dragster
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    What OD are you going to run? How much boost do you think you will make?
     
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    Mike Canter, Jun 15, 2008
  3. torino

    torino New Member

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    it is running 41 over making around 30 pounds only .
     
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    torino, Jun 15, 2008
  4. Mike Canter

    Mike Canter Top Dragster
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    Then my recommendation would be to run the compression ratio around 11.5 to 11.7.
     
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    Mike Canter, Jun 15, 2008
  5. torino

    torino New Member

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    To get to around 11.5 compression i would need a piston with a compression hight of around 1.65 , do you think this going to be a problem ?
     
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    torino, Jun 15, 2008
  6. Mike Canter

    Mike Canter Top Dragster
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    I guess I am a little confused. The compression height is what gives you how much the piston is down in the cylinder and not so much the compression. Although is plays into the computation. The height of the dome is the real factor in the CR. Normally you want the piston to be around .025" down in the hole. So when you say your compression height is 1.65 how much does that put you in the hole? What is the block deck height? Also what is the thickness of the head gasket you are going to use?
     
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    Mike Canter, Jun 16, 2008
  7. SoDak

    SoDak Active Member

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    Hey Mike,
    What's the "magic" behind putting the piston down .025"?
     
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    SoDak, Jun 16, 2008
  8. Mike Canter

    Mike Canter Top Dragster
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    Rod stretch from too many runs or during a miss shift or losing a sprag or anything like that. I got that measurement from several well known professional engine builders and have never questioned it. I am always open to something better or discussion. The other needed measurement I was told to go along with that is to install at least one piston/rod and the crank, then put the head on with the gasket. Then turn the motor upside down and undo the rod cap and with the upper bearing installed use a dial indicator and with the piston at TDC push the piston/rod up and down so it contacts the head and back to the crank journal and make sure you have at least .070 clearance.
     
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    Mike Canter, Jun 16, 2008
  9. torino

    torino New Member

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    now im confused , i was working on a piston done of 81.4 cc as per the arias piston catalog with there 1.28 compression hight 4.310 bore , this is a standard of the shelf pistion .my heads have 175cc chambers .
    with a 4.25 stroke , rods at 6.971 , compression hight of 1.28 and head gasket at 0.080 and a deck of 10.720 .
    i also have some longer rods avalible now at 7.140 and can change the head gskets to what ever we need to go to .
    any help with what way to go is very much appreciated .
     
    #9
    torino, Jun 16, 2008
  10. Mike Canter

    Mike Canter Top Dragster
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    A 6.971 rod with a 4.25 stroke (2.125 for computations) and a 1.28 compression height is a total length of 10.376". If you subtract that from your deck height of 10.720 will put your piston down in the hole at 0.344" which is way too much.

    A 7.140" rod with a 4.25" stroke (2.125") and a 1.28" compression height gives you a total length of 10.545". Subract that from a 10.720 deck height puts the piston down in the hole by 0.175". Still too much.

    To be correct with a 6.971" rod you need a Ch of 1.599 plus or minus .005. So 1.594 to 1.604"

    To be correct with a 7.140 rod you need a Ch of 1.430 plus or minus .005. So 1.425 to 1.435"

    Also a .080" gasket is a little thick to start out with. The normal planning thickness I use is .052 or .062" depending on what it takes to put the CR where I want it.

    We use a Ch of 1.377"

    Before you can use the math to compute the CR you have to put the piston in the correct position down in the hole.

    I don't want to lead anyone astray here so somebody out there please check my figures.
     
    #10
    Mike Canter, Jun 16, 2008
  11. Bob Kraemer

    Bob Kraemer New Member

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    Just did a quick check Mike, looks like your numbers are correct.
     
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    Bob Kraemer, Jun 16, 2008
  12. torino

    torino New Member

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    sorry guys , big mistake on my part , was ment to have been 1.428 compression hight .
     
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    torino, Jun 16, 2008
  13. torino

    torino New Member

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    guys
    so now that we have the correct compression hight 1.428 and the 7.140 rod
    with the 4.25 stroke with the 81.4 cc dome , what will the compression ratio be .
    will we be close to your recomended 11.5 to 11.7 ?

    cheers
     
    #13
    torino, Jun 16, 2008
  14. Mike Canter

    Mike Canter Top Dragster
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    With a 7.140" rod that will put the piston at 0.027" down in the hole which is where you want it. Now go and re-compute the CR using the piston 0.027" down with a .052" and a .062" gasket and see if any of those domes listed get you close to a CR of 11.5 to 11.7:1. Remember the gasket should be ordered with a .020 greater diameter than the cylinder bore diameter.
     
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    Mike Canter, Jun 16, 2008
  15. Mike Canter

    Mike Canter Top Dragster
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    Thanks Bob
     
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    Mike Canter, Jun 16, 2008
  16. Bob Kraemer

    Bob Kraemer New Member

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    With the parts you have now the deck height is .196 down in the hole. Going to a 7.140 rod will give you a deck height of .027
     
    #16
    Bob Kraemer, Jun 16, 2008
  17. Bob Kraemer

    Bob Kraemer New Member

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    Your welcome Mike. Man I type to slow you already posted the same thing I typed.
     
    #17
    Bob Kraemer, Jun 16, 2008
  18. Mike Canter

    Mike Canter Top Dragster
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    Thats OK. I learned to type fast with two fingers. Or figures compute out the same
     
    #18
    Mike Canter, Jun 16, 2008
  19. torino

    torino New Member

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    guys
    i put the sums into the calculator , with a .052 head gasket i only get
    10.02 CR and with a .062 head gasket 9.83 CR
    dose this sound right to you guys ?
    is a dome of 81.4 cc seem right ? thats all i can think can change the CR .any ideas ?
    thanks for all the help
     
    #19
    torino, Jun 16, 2008
  20. Mike Canter

    Mike Canter Top Dragster
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    You need a piston with a bigger dome. Try JE Pistons or CP Pistons and see what they have.
     
    #20
    Mike Canter, Jun 17, 2008
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