whats better J or X heads

Mopar General Discussions

  1. toolmanmike

    toolmanmike FABO MODERATOR Staff Member FABO Gold Member

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    Exactly. I wouldn't doubt they are all a little different. They are a different casting as per the part and casting numbers.
     
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    • 318willrun

      318willrun Utube channel 318willrun

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      If a X or J flows 250 @ .400, what would matter is I'll trade my Speedmaster heads for them even up :)
       
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      • Garrett Ellison

        Garrett Ellison Amateur driver on public roadway, do not imitate.. FABO Gold Member

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        Being I didn't build my engine, I have a similar situation, smaller chamber (I'm suspecting early semi closed chamber 273 on right side) but haven't checked casting number yet. 145 PSI cranking pressure right bank and 135 PSI cranking left bank, with the intake closing 107 degrees after BDC. Either that or the right head or deck had a pretty serious cut taken from it. I tell myself that the flow restriction of the semi enclosed chamber evens the power out the output of those higher compression (10.4:1) cylinders to that of the lower compression (9.9:1) cylinders.:rolleyes:
         
        Last edited: Dec 29, 2019
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        • MerlinsMopar

          MerlinsMopar FABO Gold Member FABO Gold Member

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          You can pop 2.05 or 2.08 valves in j heads also. Same casting. Pocket porting depends on the machinist. I have 2.08s in a set and they are awesome! Your gonna pay to have the work done either way. Depends on what you want. Good luck with your project.
           
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          • 66fs

            66fs FABO Gold Member FABO Gold Member

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            Back in the day, you could tell the difference in power with the X heads. Not much but you could tell. That is why they were prized. No other reason that I know of. Both are great heads, 1.88 or 2.02. X heads only came on 340's. J heads came on any 360s and 71 up 340s. There are/were a lot more J heads out there.
             
          • SSG_Karg

            SSG_Karg Heeeeeres Johnny!

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            Still?

            81ED94A5-35B7-43E6-815F-74DE8D4B0210.gif
             
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            • Garrett Ellison

              Garrett Ellison Amateur driver on public roadway, do not imitate.. FABO Gold Member

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              AE5FB10D-A3DE-4CCA-8EC8-76299E8F3371.jpeg 9D219429-586F-4917-AA11-6B28FFE48E3B.jpeg Just a quick pic on how not to do it when your installing larger valves, this is a set of ‘73 340 587 heads I’m helping a friend go through.2.02/1.60 installation and new guides. The intake not being blended isn’t as bad as the exhaust, being the charge mixture will tend to jump, but there are still turbulent losses. Leaving the exhaust like this would be disastrous, though. hardened seat installation left a huge step over to the bowl and is going to have to be blended back in. Valves weren’t back cut either, and the valve job is getting a closer look (re-done on a serdi type machine with bowl and chamber cuts) too. It was destined for pure stock oval track use but it looked like they started running out of money after they got so far into the heads. The bottom end is a work of art, though. 0D34A58B-6CEC-4518-99D5-41CC8A73AEAB.jpeg
               
              Last edited: Dec 30, 2019
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              • dazedand confused

                dazedand confused Well-Known Member

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                true
                 
              • RustyRatRod

                RustyRatRod Bla de blizhibliz de blatde blizi bla bla FABO Gold Member

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                Do you think those are cast in differences, or possibly so small that they are from the molds moving around or some such? In other words, do you think those differences are intentional?
                 
              • RustyRatRod

                RustyRatRod Bla de blizhibliz de blatde blizi bla bla FABO Gold Member

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                Yeah some still think the X is worth more.
                 
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                • 318willrun

                  318willrun Utube channel 318willrun

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                  X is worth more to the restore guy.... To the performance guy, it's a guarantee that they get the 2.02. As far as flow, we could flow two X heads and they would flow differently. Casting on cast heads is far from consistent and perfect.
                   
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                  • MOPAROFFICIAL

                    MOPAROFFICIAL FABO Gold Member FABO Gold Member

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                    I think they are cast that way.
                     
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                    • RustyRatRod

                      RustyRatRod Bla de blizhibliz de blatde blizi bla bla FABO Gold Member

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                      Ok, cool. Then you think there's enough difference it was intentional. I've often wondered. Thank you drive through.
                       
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                      • MOPAROFFICIAL

                        MOPAROFFICIAL FABO Gold Member FABO Gold Member

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                        After doing a smooth of the x head bowl it did 226 cfm
                        Now I have a set of 2.02 j heads, stock ...3 angle with the steep bowl cut , no smoothing...and one port does 228cfm
                        It's the in between numbers that are different between them and yes the rest of the ports could be less ,like 218cfm OR near the same in flow.
                         
                        Last edited: Dec 30, 2019
                      • 12swinger

                        12swinger Well-Known Member

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                        The X always seem to have a little smaller chamber, probably a smidge more compression.
                        The J head as a rule I believe had hardened seats in particular the later ones.
                        I always like starting with a 1.88 valve head as you get a fresh seat to work with.
                        I have a set I’m thinking of wrapping up for a 340 build, kind of want a 2.05-1.65 but probably easier 2.08-1.60
                        Anyways I’ve got them pretty much ported and ready except for 3-4 holes in the intake ports I will have a master probably braze.
                        Great head up to around 500hp
                         
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                        • dazedand confused

                          dazedand confused Well-Known Member

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                          Thanks for all the feed back everyone. I was wanting to build a 400 hp 360 . I have lots of old issues of mopar muscle and mopar action but one issue of mopar muscle has an article about a guy that likes to take mostly stock parts and a few after market parts and he made 378 hp with: thermoquad,comp cam , J heads that had a light porting /polishing etc. After market flat top pistons and headers. My project 65 dodge dart 2 door post car weighs in at around 3300 Lbs give or take. I figure for a daily driver and once in awhile to the drag strip on amateur night a mostly stock 360 with performance cam 4 barrel , flat top pistons 10:1 compression , 360 J heads with ports gasket matched valve job bowels cleaned up ,air gap dual plane intake shorty pretzel headers from hedmen headers, 904 with racing guts full manual valve body, 8-3/4 with posi and 3:91 gears should be more then plenty for that little Abody car ?

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                          • ir3333

                            ir3333 Well-Known Member

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                            keep your compression in the lo 9's with iron heads
                             
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                            • Jadaharabi

                              Jadaharabi FABO Gold Member FABO Gold Member

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                              Why? When 340, 383, 413, 426w, 440 and hemis came from the factory wirh 10 to 12.5 to 1?
                               
                            • 318willrun

                              318willrun Utube channel 318willrun

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                              was the gas better in the 60's and early 70's ?
                               
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                              • Garrett Ellison

                                Garrett Ellison Amateur driver on public roadway, do not imitate.. FABO Gold Member

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                                A lot of users have problems with J heads and compression ratios over 9-1/2:1 with the J heads. Small block Mopar open chamber heads are somewhat lazy during the burn cycle but J heads seem to be the worst offenders whenever someone is having pinging on forum. 10:1 and over is where most people start blending race gas into there premium pump fuel.
                                 
                              • 12swinger

                                12swinger Well-Known Member

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                                Gotta run premium.
                                Conservative timing.
                                A bunch of us have the code to get in the gate at the local little airport, 3.97 a gallon for 100 octane LL
                                 
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                                • MOPAROFFICIAL

                                  MOPAROFFICIAL FABO Gold Member FABO Gold Member

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                                  My x heads were 69cc when I got them, stock 2.02 still in them, I ran them that way for a little bit.
                                  My stock original 2.02 J heads , are 72cc fwiw.
                                   
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                                  • MOPAROFFICIAL

                                    MOPAROFFICIAL FABO Gold Member FABO Gold Member

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                                    Very true.
                                    Even if its 4 cfm @this or that lift, it's a diff.
                                    Some of the later heads peaked sooner/lower in the lift..which is more usable with the tamer cams of that time.
                                     
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                                    • MOPAROFFICIAL

                                      MOPAROFFICIAL FABO Gold Member FABO Gold Member

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                                      The headers are the shortest leash on that dog.
                                       
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                                      • toolmanmike

                                        toolmanmike FABO MODERATOR Staff Member FABO Gold Member

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                                        Big time better. Nothing like the puppy pee we have nowdays.
                                         
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