Large valved 273

Small Block Mopar Engine

  1. oldkimmer

    oldkimmer FABO Gold Member FABO Gold Member

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    With the bigger open chamber of the 340/360 head u loose compression. So now 10.5 is down to 9ish to 1. So in effect u have to try to fill up the gaping hole u created. For that to happen u would need a Hemi type piston with a bump as big as ur fist and deep valve reliefs machined into them. Which may or may not be feasible. Kim
     
    Last edited: Oct 14, 2019
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    • 4spdragtop

      4spdragtop Well-Known Member

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      But if you are talking ratio, how does the differences in volume make a difference? It's a ratio so the differences/changes would be a mute point? I understand it would affect compression PSI.
      I also understand its math/physics.
      A 340 doesnt sit out of the bore as much? Sorry, brainfart here, but stroke is the same 273 vs 340? 6.123?

      Yep I understand the compression change with heads(open vs closed chamber)
      Thanks guys, trying to absorb.......beer is easier to absorb lol
       
    • 273

      273 Well-Known Member

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      The difference in chamber volume isn't as huge compared to cylinder volume. So say one bore in a 273 is 585 cc at bottom dead center and at top dead center 65 cc that would be 9:1 now say a 340 at bottom dead center 780 cc and 65 cc at top dead center would be 12:1 now if you switch to a 78 cc chamber it still would be 10:1. A 273 would have to go to a 58.5 cc chamber. it just gets harder and harder to get race cr out of small displacement like 12.5 plus.
       
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      • 66fs

        66fs Well-Known Member

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        The difference in chambers is not as large as one would think. Minimum 920 chambers are 57.3 and "J" head chambers are 64.7. You have to mill .040 to get there. That means about 8cc larger chambers for each. So unmilled 920 heads are around 65 cc and unmilled "J" chambers are around 73cc. I think unmilled 675 open 318 heads are around 68cc. It is better to take the chamber volume out of the equation and think of a constant chamber and the effects of the bore area at BTC and TDC. That will show, like I tried to give a picture of the difference with the 273 and 340 pistons at the same ratio, the difference in ratio of a large and small bore. In a street engine it matters little since we are limited by the gas octane available. All 273, 318 and 340's have the same stroke and rod length. 340's are close to the top of the bore at 10.5 and have no dome.
         
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        • roccodart440

          roccodart440 Well-Known Member

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          WHa is the max overbore possible with a 273? How deep is the cylinder notch? See where I'm going here? If you over bore .060, you have a .030 notch before you do anything, etc.

          COmpression...

          I don't know what is available in SBM aftermarket heads chamber wise compared to stock. But it would seem there should be a decent amount of choices and also a domed piston, block decking, head cutting are also options to improve compression, and for a street engine it would seem that something 10.1 to 11.5:1 with aluminum would suffice.

          ALso, I wonder if you could recruit SBC or SBF parts for one of these builds?


          Keep in mind this IS ALL THEORHETICAL DISCUSSION.

          IF you wish to conclude a 360 is an easier path and more sensical, well you are right. This is for the mind teasers and bench racers.
           
        • Wyrmrider

          Wyrmrider Well-Known Member

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          great comments 66fs
          and others
          Remember Stroker McGurk
           
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          • roccodart440

            roccodart440 Well-Known Member

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            According to EGGE pistons you can go .060 overbore.

            That's a .030 notch.
             
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            • nm9stheham

              nm9stheham Well-Known Member

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              A 2.02 valve hits the stock 273 bore by something like .065" per direct checks. So if you want to clear the bore with no notch you gonna have to do things like:
              • Offset the bore to take more off the intake side. Since this on a non-thrust side of the bore, you can take more out.
              • Skim a bit off the outer edge of the valve. There is not a lot of margin to play with but there is a bit.
              • Or, put a small notch near the upper edge. Once you get the valve head past the bore edge, the valve head angles more into the center of the bore and the clearance improves a bit.
              And yes, I have been looking at all sorts of alternate pistons for this. There are some that are up around 3.700" diameter that will be useful so that the cylinder can be both overbored and offset bored. My 273 block will take it.... one bore will get a tad under the minimum wall thickness I wanted, but I'll live with it.
               
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              • 273

                273 Well-Known Member

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                What’s the power goal.
                Every 50 hp can make a big difference expense to the build.

                Plus rpm goes up quit a bit.
                These are just guesstimates.
                Probably on the minimum side
                250 hp 1500-5500 rpm
                300 hp 2000-6000 rpm
                350 hp 2500-6500 rpm
                400 hp 3000-7000 rpm etc..

                300 and under is the dividing line can get away with mostly stock parts just cam 4bbl and headers and some mill work to get a decent cr. Above that starts to get more involved especially in head and cr.
                 
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                • 273

                  273 Well-Known Member

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                  If I ever go in Time Attack and built and engine for it would be a 273. Cause I feel there’s a rule advantage.

                  I go with custom pistons to get the most cr as possible, stock magnum heads bore notched, air gap with a two barrel, headers and a custom solid cam.
                   
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                  • roccodart440

                    roccodart440 Well-Known Member

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                    This head might work as an upgrade. Still iron, but not a 2.02 valve. I'm sure it highly quality and has more potential than the stock pieces.

                    Speedmaster As-Cast Cylinder Heads PCE281-1773

                    1.920 valve, 62cc chamber. (stock chambers ran 57cc to 67cc on 273's)


                    Ultimately, I feel if the 273 is used, the best route would be a factory head port, maybe an oversized exhaust valve, roller cam and a centrifugal supercharger feeding it.

                    That overcomes the anemic head and the cubic inch deficiencies.

                    It would easily make 400-500hp. YOu can look up articles where they built 283 gm engine and put the squeeze on them using stock heads.
                     
                  • oldkimmer

                    oldkimmer FABO Gold Member FABO Gold Member

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                    The alum bare head is only a few dollars more. But ofcourse it has a 2.02 valve, which we all recommend against. Kim
                     
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