***273 Commando pistons and '302 heads???***

Small Block Mopar Engine

  1. 65Val

    65Val Average Length Member

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    Anyone using gennie 273 Commando pistons (10.5:1 comp.) with '302 heads? Do they work together? I know this has been asked before. I was wondering about piston dome/ combustion chamber shape issues. I have access to a set of factory Commando pistons and don't want to pay to have 'em swapped onto my rods if they won't work with the '302's.
     
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    • dart4forte

      dart4forte FABO Gold Member FABO Gold Member

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      I have a set of TRW L2222 forged pistons for sale that have been cut down for that purpose. What size intake valves?


      Taken off an article I have


      The 318-2bbl heads (4323302) used from 1985 on are a swirl port design with a closed (heart-shaped) combustion chamber design with a chamber volume of between 56 and 65 cc. The 4 heads I examined averaged 62cc in volume. The 302 head has 1.78 inch/1.50 inch valves and small ports averaging 54cc on the exhaust side and 118cc on the intake side. The intake ports have a more severe dogleg than earlier heads because the holes for the pushrods are larger - 11/16 inches. Cars equipped with the 302 head have a dished piston to keep the compression ratio from being too high. Some cars left the factory with nail head exhaust valves in 302 heads, others with semi-tulip exhaust valves, which add 0.6-0.75 cc to the chamber volume. There is an interesting excerpt in "Mopar Engines", page 72, describing how such a head was ported and made to flow as well or better than other small block cylinder heads. Apparently, this experimentation resulted in the master for today's Mopar Performance P4452758 cylinder head.
       
    • 65Val

      65Val Average Length Member

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      As far as I know, they are stock 1.78 intakes...same as the original wedge '920 heads that came off the engine. I could just stay with the 2 bbl ,4 valve relief pistons that are in there now,and use the thin MP gasket (.028 compressed), but I was hoping to try the Commando pistons for a bit more comp. I was concerned about the Commando dome hitting some part of the '302 combustion chamber....

      [​IMG] [​IMG]
       
    • dart4forte

      dart4forte FABO Gold Member FABO Gold Member

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      We took and cut some 1.88s down to 1.84's on my 273 with no problems. I'm usining the L2222 pistons. If you take the stock stats on the Hipo 273 such as the stock deck height, steel shim gasket, combustion chamber on the 920 head etc. you'll find that the actual CR on the stock 235hp was around 12 to 1.


      If you have access to a flow bench do a comparison between the 920 and the 302 in terms of port velocity and flow. You'll find that the 920 flows a bit better. One reason I think is the 920 was suited to flow wet air (carb) while the 302 was better suited for dry air (fuel injection). if you do flow test a comparison make sure you use a realistic 18" of vacume rather that the 28" of vacume. That way you'll get a better real world comparison.
       
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      • 65Val

        65Val Average Length Member

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        Not so much worried about flow and such as its only gonna be a cruiser (convertible, ya know). Just concerned about mechanical contact inside the chamber with different era parts being used together.
         
      • JBurch

        JBurch Well-Known Member

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        I did this swap almost 10 years ago for a friend; he wanted to do some up grades to his car, I helped.

        With a .030 cut on the heads, measured compression ratio came in at 10.48 to one, he stayed with 1.78, 1.5 valves, swapped in a Crane hydraulic cam, an H260, Spitfire headers, Holley Street Dominator intake. A nice wake up for his 65 Dart GT.
         
      • RedFish

        RedFish Well-Known Member

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        If you're not concerned with flow and all the other aspects of peformance builds, Why bother with the higher compression dome tops ? Todays fuel doesn't burn well under higher compression anyway. Stinky rich exhaust fumes wont be pleasent, especially with the top down
        I would use the 302 heads if for nothing more than their hardened exhaust valve seats.
        Research head to exhaust manifold conflits and test fit everything or you might crack your stock 273 manifolds.
         
      • cudavert

        cudavert DARTAHOLIC

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        I took a pair of 302's to the machine shop for a valve job and was told they didn't HAVE hardened exhaust seats. After doing a little reading online, it seems that Mopar only induction hardened the cast iron "seat" to a depth of around .015". Which could be taken away with a valve job seat grind possibly.
         
      • 65Val

        65Val Average Length Member

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        Probably right. I'm not going to chance the possible collision between piston and head. I think I'll just stick with the flattops that are in there now. Thanks for all the responses.
         
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