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  1. #1

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    Help: Cylinder head cut spec?

    Hello,

    I was talking with my machinest today (www.engine-tech.com) and asked how much he was milling off my 1200r head and he said 80 thou to give 170psi compression. This doesn't sound right to me for 91 octane pump gas @ 700ft elevation. Shouldn't the squish clearance be 40-50 thou and remove 35-40 thou from the face? I don't believe that I want more than 150psi? Is this top secret stuff?


  2. #2
    2000 GPRXP 4tec shooter98xpl's Avatar
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    Don't know about top secret, I just send my stuff down to Jim1300 in Florida. $85 plus shipping for cutting the head to the right specs for your ski.
    SHOOTER

  3. #3
    Hydrotherapy's Avatar
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    send it to jim like shooter said...

  4. #4
    Flying Scotsman's Avatar
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    Don't know about top secret, I just send my stuff down to Jim1300 in Florida. $85 plus shipping for cutting the head to the right specs for your ski.

    I like that price sounds like a winner.
    SCOTT.........

  5. #5
    steach's Avatar
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    Quote Originally Posted by thebert View Post
    80 thou to give 170psi compression. This doesn't sound right to me for 91 octane pump gas @ 700ft elevation.
    I'm no 1200 expert but that sounds WAY TOO agressive for a rec ride. I would think 145-150psi should work and keep a safe squish.

  6. #6
    With 170 psi I dont think I would use below 93 octane. Someone (I think it was phillip) said he didnt see any diference in performance going from 150 to 170. If that is true then I wouldnt go any higher than needed.

    I did as the others said, send it to Jim and not worry about it.

  7. #7
    Moderator OsideBill's Avatar
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    At 170 psi I was running a mix of 91 and 110 4 gallons to one. I had my GPR head done by Pro-Tec for 91 and it was 155 PSI on Snap on Gauge

  8. #8
    philip_gpr's Avatar
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    yes .080 is too much

    yes you want the squish to be .050 (certainly no less than .040)

    I use 94 octane, reduced timing and rich jetting with my 170 lbs - you have to keep everthing in balance

    700 ft will lower compression your starting compression - I forget the actual calculation

    what was your compression you started with

    I think milling .010 would raise compression approx. 10 lbs

    the 1200 gasket has 3 layers - each layer is approx. .010 in - this means you can raise compression and reduce squish further by using 2 or 1 layers of the gasket.

    for example on my ski
    3 layers 150 lbs .060 squish
    2 layers 160 lbs .050 squish
    1 layer 170 lbs .040 squish

    when I went to 170 I changed jetting (richer) and I lowered timing at WOT

    at 170 I would not use Advent curves 84 or 94

    if you only have 91 octane available then you won't be able to raise compression as high as others that have 93 or 94 available

    I am not an expert in this, maybe someone else can help out with the details
    Last edited by philip_gpr; 01-17-2007 at 01:00 AM.

  9. #9

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    thanks for the responses.

    The machinist already has the cylinder head and is just getting around to cutting it now. I dropped it off with him 2 months ago. He's approx. 250 miles from me so I can't drive around the corner to get it. I would have sent it to jim if I would have known 2 months ago that he machines cylinder heads. The highest compression reading I got was 120. I'm running the factory ignition and I did not measure the squish clearance prior. The machinist mentioned that the factory squish clearance should be 90 thou. I will be using all 3 layers of the head gasket. I will tell him to cut the face no more than 30 thou and set the squish to 50 thou. This should give me 140-150psi. I will have access to 91 octane gasoline. This sounds safe according to philip_gpr's explanation?

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