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Thread: How much compression will I gain....?

  1. #1
    turbo addict
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    How much compression will I gain....?

    Here is what I have....bone stock head from a 1994 Dodge Shadow. It has cracks between every set of valves and a groove cut between two cylinders due to a blown head gasket. The groove will be repaired as will the cracks , new valve guides, and a valve job. The question I have is...when I have the head repaired I will have them take off .040...any ideas on what the old compression was and what the new one will be. The motor is an efi one, not turbo.
    [SIZE=3][COLOR=#000000]47 Time NHRA/IHRA drag race champ-----84 Plymouth Horizon/1 of 84 HO equipped from the factory/1910 lbs, 2.5 with carb, nearly 315,000 miles-----04 SRT4/S2 with S3 turbo/12.17 @ 119/DOTs/93 Octane/SOLD-----2003 PT Cruiser GT, won a True Street class at the 2017 National Muscle Car Association World Street Finals-----2010 Toyota Prius/my delivery truck, 77.9 mpg best and NOT A PLUG IN, nearly 230,000 miles and way over 5600 miles per month! [/COLOR][/SIZE]

  2. #2
    Visit www.boostbutton.com... Turbo Mopar Contributor ShelGame's Avatar
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    Re: How much compression will I gain....?

    By my calculator, 0.04 off a Swirl head should take away about 6cc. Assuming it started off @ 44cc, that would give you a new chamber of 38cc. Which should be good for ~1.3 points of CR, assuming all else is equal.
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  3. #3
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    Re: How much compression will I gain....?

    Quote Originally Posted by ShelGame View Post
    By my calculator, 0.04 off a Swirl head should take away about 6cc. Assuming it started off @ 44cc, that would give you a new chamber of 38cc. Which should be good for ~1.3 points of CR, assuming all else is equal.
    Thanks....how about .045?
    [SIZE=3][COLOR=#000000]47 Time NHRA/IHRA drag race champ-----84 Plymouth Horizon/1 of 84 HO equipped from the factory/1910 lbs, 2.5 with carb, nearly 315,000 miles-----04 SRT4/S2 with S3 turbo/12.17 @ 119/DOTs/93 Octane/SOLD-----2003 PT Cruiser GT, won a True Street class at the 2017 National Muscle Car Association World Street Finals-----2010 Toyota Prius/my delivery truck, 77.9 mpg best and NOT A PLUG IN, nearly 230,000 miles and way over 5600 miles per month! [/COLOR][/SIZE]

  4. #4
    boostaholic Turbo Mopar Contributor
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    Re: How much compression will I gain....?

    Quote Originally Posted by ATaylorRacing View Post
    Here is what I have....bone stock head from a 1994 Dodge Shadow. It has cracks between every set of valves and a groove cut between two cylinders due to a blown head gasket. The groove will be repaired as will the cracks , new valve guides, and a valve job. The question I have is...when I have the head repaired I will have them take off .040...any ideas on what the old compression was and what the new one will be. The motor is an efi one, not turbo.

    Are you replacing the G head on Matchbox? That makes a big difference. I believe the G head stock was either 54 or 56cc. I don't have my book in front of me to check. If you're dropping to 38cc's as Rob suggested, you're going to gain more than 1.3 points...

  5. #5
    turbo addict
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    Re: How much compression will I gain....?

    Guys, a bone stock uncut g-head is between 56 cc and 57cc. A stock uncut swirl is between 50-51cc. Not even close to 44 cc. The #'s you might see in MP books or elsewear are minimum cc's for a given class. Not real world #'s.
    Angelo, you need to cc your head and do simple math to figure out real compression ratios. Forget reading what books say and measure yourself. You will also need to know how far down (or up) the hole the piston is. Bore size, piston dish or dome, crank stroke, and head gasket thickness and diameter. Stan weiss website http://users.erols.com/srweiss/index.html#jcalc has a spreadsheet formula you just plug #'s into to get your results. Doesn't get any easier. Here is example of bone stock swirl 2.2 turbo motor.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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