318 pistons to bring compression up

Small Block Mopar Engine

  1. C2ndLTpigeon

    C2ndLTpigeon Mopar or no Car! FABO Gold Member

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    Wanted to see if someone can point me to the correct stock type pistons to bring a 318 to the earlier 9.2 compression. I currently have a 73 core with flat tops. I see some for sale that have 4 valve reliefs are those higher compression than flat tops?

    Thanks
     
  2. RustyRatRod

    RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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    Well, let me bust one myth right off the bat, and that is, no 318 ever had compression that high from the factory. It didn't happen. Lemmie splain.

    Chrysler had terrible machining tolerances. They left deck heights tall, combustion chambers big. That resulted in compression ratios a lot lower than where they were advertised. Some were down in the 7s. Probably not the ones rated at 9.2. They were likely closer to 8.5......or at least 8:1. Sorry to break it to you, but that's just how it was.

    Here's what I would do if I was looking to do it on a budget and was building a mild street 318. Go on the Summit site and search 318 pistons. Choose the one with the tallest compression height. There are a few different ones. The stock cast pistons will be fine. BUT, if the pistons you have are already the tallest compression height (and they might be), then no need to buy more.

    You need to "blueprint" your compression ratio. In other words, look at all the factory specs for your year engine. Deck height, combustion chamber size everything. Have the machine shop deck the block to the factory spec. Have them CC the chambers to the factory size by milling the heads. You'll also want to use the factory style think steel head gaskets, IF you can find any. They're hard to find now. If you cannot, you can mill an additional .020 off the heads to compensate.

    Essentially, that's what you're gonna have to do. Of course, you could get some different pistons like some from KB, but then you still won't be at "whatever" advertised compression ratio they are rated for, without all the blueprinting work.
     
    Last edited: Jan 24, 2021
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    • RustyRatRod

      RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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      OR the other option is to leave it like it is and add a better intake, carburetor, cam and headers. After all, for every ONE compression point, you only increase power by about 3%. It's not really worth all of the trouble through the machine work to do that. Low compression engines can be made to run through other means.
       
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      • dirty white boy

        dirty white boy 50 yr old Juvenal delinquent

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        you can try a .030 over chevy 307 piston but then the pins wrong size and need to hone block .005 or a pontiac 350 piston with same issues on pin and have to bore block .030 for them then trim up the skirts,...lots of headaches and added machining! rustys right on every word in his post! KB hypers and blueprint it all out!!
         
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        • RustyRatRod

          RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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          Funny you ask about this now. @SGBARRACUDA (Roy) just listed these for sale.

          [FOR SALE] - 318 .040 Seal Power 285 NCP pistons,rods, crank

          Although they aren't the tallest compression height, they are close and really high quality. Plus, I know Roy personally. He's a friend and a very honest guy. That's a great deal for what he has. Of course, you'd have to bore your block if it's standard, but you're wanting pistons so...... and remember, an over bore does add a little bit of compression. Not much, but a little. That plus all the blueprinting could get you into decent territory if you decide to go that route.
           
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          • dirty white boy

            dirty white boy 50 yr old Juvenal delinquent

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            thats a nice rotating assembly,..local pick up or id be discussing payment options instead or posting here!
             
          • C2ndLTpigeon

            C2ndLTpigeon Mopar or no Car! FABO Gold Member

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            This is what I was thinking if anything, I just commented on his post before I made this post seeing if he will sell just the pistons and rods.
            Just wanted to check out my options and see if there was a way to bring compression up with taller pistons. Seem like an small overbore, and some work on the heads will be the best I can do.
             
          • RustyRatRod

            RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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            That's realistic. With smart parts choices, you can see 1 HP per cubic inch. That's a lot of fun in a A body.
             
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            • dirty white boy

              dirty white boy 50 yr old Juvenal delinquent

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              you can cut the deck to a 0 deck height too,..building compression ant hard!
               
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              • RustyRatRod

                RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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                LOL That'd be a pretty big cut, but it sure would work.

                ..and make no mistake. A 1 HP per cube 318 in an A body will be a tire melter.
                 
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                • C2ndLTpigeon

                  C2ndLTpigeon Mopar or no Car! FABO Gold Member

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                  Might as well since I will I have the block there for an overbore, I live 2 miles from Indio motor machine and they do great work.
                   
                • dirty white boy

                  dirty white boy 50 yr old Juvenal delinquent

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                  the taller the piston the smaller the cut...
                   
                • RustyRatRod

                  RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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                  Be careful though. It might be a BIG cut. Think about intake alignment and all that. I would see WHERE those pistons sit at TDC and go from there.
                   
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                  • RustyRatRod

                    RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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                    Well duhhhh.....lol Those are 1.720.
                     
                  • RustyRatRod

                    RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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                    That's almost .102" in the hole at TDC. Trust me, you do NOT want to zero deck height that block. LOL
                     
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                    • C2ndLTpigeon

                      C2ndLTpigeon Mopar or no Car! FABO Gold Member

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                      Yea that's quite a bit! Would it be better to take it off the heads then.
                       
                    • dirty white boy

                      dirty white boy 50 yr old Juvenal delinquent

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                      1.74 tallest cast i know of for a 318,.. the KB hypers are 1.82 i believe!!
                       
                      Last edited: Jan 24, 2021
                    • Slantsix64

                      Slantsix64 Well-Known Member

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                      get 318 LA Block Flat top roller camshaft pistons 1986-1991 they are taller than that the low comp 318 pistons I had a 72 318 that had the pistons .100 in the hole dog of a motor.

                      The Roller 318 pistons will leave you at .040 in the hole with 64CC 302 Heads you will be at 9:1, i milled my 302 heads down to get me at 9:5:1 Cr using a thin head gasket.

                      RockAuto
                      SEALED POWER 526P Cast Piston Info
                      2 - 5/64 Groove, 1 - 3/16 Groove, 1.745 CD, Flat Head - 9.0:1 C.R., 0.9842" Pin Dia. (Only 6 Remaining) (Size: 0.030" Oversize)
                       
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                      • Slantsix64

                        Slantsix64 Well-Known Member

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                        Or you can just get a 318 Magnum higher compression, roller camshaft with nice duration from the factory. Bigger Valves/better heads ETC

                        I have a stock magnum 318 in my barracuda with a 4 speed, hauls some serious ass.
                         
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                        • moparmat2000

                          moparmat2000 FABO Gold Member FABO Gold Member

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                          Rusty, I am looking at a similar build on a 1977 318. I was looking at the KB167s. The stock pistons before dissassembly are about .100" in the hole at TDC. Arent the KBs a bit taller, closer to a zero deck height? The block will need to be surfaced "decked" anyways. The heads I have are rockauto reconditioned 302s. I have not CCd them yet to verify. Going to borrow a burette from a buddy of mine to get a number. I am ready to pull the trigger on a set of KBs but dont want to if they wont help get the compression up there.
                           
                        • YY1

                          YY1 Well-Known Member

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                          I don't think we can argue that the 9.2 motors (67-68 and 85-91) did have higher compression.

                          Didn't we have a member that measured an LA roller at 9.18:1?
                           
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                          • CarfinaticII

                            CarfinaticII Under Construction

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                            I'm in the middle of a 72 318 build in the small block section. The KB 167's compression height is 1.810. Don't know what your stock piston height is. I had the block zero decked to the pistons and with .30 overbore I'm looking just shy of 9.7 static compression. That's with SM aluminum heads (65 cc) Don't know if that helps, just thought I'd share.
                             
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                            • moparmat2000

                              moparmat2000 FABO Gold Member FABO Gold Member

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                              I was going to pull them out within the next 2 weeks. Thats when I will have time to get out the caliper and see.

                              600600p415EDNmainimg-Compression-Height-Measure.jpg
                               
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                              • CarfinaticII

                                CarfinaticII Under Construction

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                              • Slantsix64

                                Slantsix64 Well-Known Member

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                                Yup that was me. Measured three motors an 86 88 and 90
                                 
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