oooops on Piston choice

Mopar Performance Issues

  1. tooslow

    tooslow Well-Known Member

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    I have a street Dart , 446, hughes solid cam 250 dur at 50 575 lift ,worked Stealth Al heads ,single plane intake ect....
    230 miles on rebuilt engine , Engines a dog , noticed my pistons are .120" below deck and my compression is pretty low . 130 psi average . I told machine shop I wanted as close to 0 deck as possible( 10.5 0 or 11.0 to 1) but I was busy working and I let the machine shop put it together. I usually assemble everything myself and check all clearances, plasti gage ect... and take my time because its my money and its fun. The owner of the machine shop passed away and shop was sold
    Otherwise i would take the engine back to them and work it out
    Situation
    I want to change my pistons to a KB 237,s , I calculate that I should be much closer to 0 deck . I haven't sat down with a sharp pencil to figure everything out exactly yet
    Question
    With 250 miles on a fresh engine can I simply just change the pistons . Street car and not a race car
    Do I have to take everything re balanced again ?
     
    Last edited: Apr 30, 2020
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    • brian6pac

      brian6pac Well-Known Member

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      Just the piston weight will have to match.
      The rings can be reused if you keep them in the same hole
       
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      • Locomotion

        Locomotion Well-Known Member

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        Unless you have a weight/balance card and compression height of the existing pistons, you won't have all the info you need until you take the pistons off the rods. Hypereutectic pistons require more ring gap than conventional cast or forged pistons and piston-to-wall clearance should be checked against recommendations. OEM deck clearances are usually more than factory spec.
         
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        • tooslow

          tooslow Well-Known Member

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        • tooslow

          tooslow Well-Known Member

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          Thx guys
          Find a new mopar familiar
          machine shop in Chicagoland and take her over there , get the combo right .
           
        • moper

          moper FABO Gold Member FABO Gold Member

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          It sounds like the cam's just too large for what you have. You based the choice on the want, not what they delivered. If it's a street car, I'd just leave the lower end and revise the cam choice. KBs are much lighter than the pistons you're likely to have if they sit that low in the block. So you're talking about new pistons, removing the old pistons they'll be pressed pin (again based on the "short" compression height of cheaper pistons) , balancing the assembly, pressing the new ones on, a quick touch up on the hone, and new rings. I'm not sure a couple points of compression are worth the work if the rest of the engine is good. Swap the cam to something with 10* less duration at .050. Maybe a solid lifter if you want a little more torque off idle. Then run it and enjoy.
           
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          • jbc426

            jbc426 Well-Known Member

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            FWIW, Smokey Yunik was said to have swapped pistons out that were over 100 grams heavier or lighter without touching the balance in some of his race engines back i the day with no noticeable vibration.

            I changed the pistons and cam in my 494" RB with the consultation and help of Dwayne Porter. He nailed it with my combination. Great man to work with and highly recommended even if its just to chat with him.
             
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            • tooslow

              tooslow Well-Known Member

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              How do I contact Dwayne Porter ?
              Thanks for the advice but I want 550 hp so I pulled the engine and figuring out a good piston combo
               
            • AJ/FormS

              AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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              What stall and gears.
              550 hp at 80 mph, and 125 at 1700 does not a good streeter make.
              400 ftlbs at 35/40mph is funtastic

              If your dogginess is below ~3000rpm, look to cylinder pressure, or stall. But after 3500 it shouldn't be a dog anymore. Your solution could be as easy as a higher stall, IDK

              As brian6pac said, and as I found out; hypers at 11/1 need more ring gap than specified in the catalog. I had to take my engine apart and open them up. I also run my (SBM) skirt clearance a lil looser than specified. IIRC mine are at .0035, but I run a minimum coolant temp of 205*F, and a standard transmission, so my revs are always up.
              But I agree; 130psi with alloy heads , is probably not the greatest street combo, lol.
               
              Last edited: May 17, 2020
            • K.O. SWINGER

              K.O. SWINGER Well-Known Member

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              as previously mentioned that is a lot of camshaft for the street, it could be made to work with the right Supporting Cast, converter, high rear end ratio Etc. But as far as Pistons go I would look at Icon forged they will be close to stock piston in weight and will give you the compression you're looking for
               
            • rumblefish360

              rumblefish360 So close, yet so far away FABO Gold Member

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              If the new pistons diameter is the same and if the same metal make, then you can swap them in minus a possible rebalancing. Pistons need to be weighed.
              (You’ll need a rebalancing...)
               
            • 512Stroker

              512Stroker We are all here because we are not all there.

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              It's a bitch when a engine build does not meet your expectations, been there hated that.
              And it is not your fault.
              I wont even attempt on a recommendation to fix it right.
              If the machine shop screwed up a basic fundamental like piston height I would not trust much of the other work they did.
              Good luck
               
            • tooslow

              tooslow Well-Known Member

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              When i say street, I mean i drive it on the highway, frontage roads around my house and cruise towns rarely . The Dart has a suspension , DR Diff upgrades Brakes , Flaming River steering box , frame connectors , 6 point cage . I take the car to Road America for track days and Autobahn by my house . It isnt meant to be a Stop light car. I will drive it to Ohio if we have the Mopar event this fall
              4 speed (tremac 5 speed would be nice , someday)
              3.23 gears
              M1 intake
              EFI( had trouble with stalling in corners with a Carb)
              Note: I could not get a carb with floats to work with hard corners, people do it all the time but never could find the answer until EFI?
              stealth heads -stock
              Oil Accumulator
              3000(2500) to 6000 is rpm range
              I cruise 70 around 3000RPM on the highway

              From what you are all your saying you are in a agreement ,if I do not want to change my cam. I need Compression, 550 hp was my intent. I specified 11 to 1 at the machine shop and it is what I though i was getting. water under the bridge, getting on with it now. about 120 to 125 psi right now
              Hughes 250 dur 575 lift @ 50 ( aprox #s) is the Cam I have and would like to stay with
              Thx for replies
               
              Last edited: May 17, 2020
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              • 512Stroker

                512Stroker We are all here because we are not all there.

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                Please keep posting your results.
                I love Road America I used to live 8 miles away from the track.
                They ain't kidding when they call it "Gods Country".
                 
              • oldkimmer

                oldkimmer FABO Gold Member FABO Gold Member

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                U should be able to find a forged piston close in weight to what u have. Most likely will be heavier. Then u can take weight out of the piston to match to your pistons. Win win that way. Kim
                 
                Last edited: May 18, 2020
              • K.O. SWINGER

                K.O. SWINGER Well-Known Member

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                Your car sounds kind of awesome would love to see some pictures.
                 
              • Marcohotrod

                Marcohotrod Well-Known Member

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                what did the stock pistons weigh? was it rebalanced for the .120" down pistons and what did they weigh? I agree with Smokey and Ed Hamburger - swapping pistons within 10% weight - and below 6500 rpm can be fine without rebalance. what numbers are on .120" pistons? stock 440 pistons weighed from 820 grams to 870
                 
              • tooslow

                tooslow Well-Known Member

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              • tooslow

                tooslow Well-Known Member

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                512stroker
                MVP is the club I do track days with
                Are interested in going to the next event at RA
                 
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                • QuickDart360

                  QuickDart360 Well-Known Member

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                  I changed from stock 360 pistons to KB107 flat top pistons without any balance like 16 years ago. Did unknowingly!! Engine seems to be strong and no vibrations after all these years. Did follow ring gap recommendation as per instructions.
                   
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