Large valved 273

Small Block Mopar Engine

  1. toolmanmike

    toolmanmike FABO Staff Staff Member FABO Gold Member

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    Nice!
     
  2. toolmanmike

    toolmanmike FABO Staff Staff Member FABO Gold Member

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    If we could get a custom piston made to fill up the large 340/360 chambers it would help. Remember the 360 head gasket issue with the small 273 bore? Lot's of wasted chamber hanging over the edges. Or we could cast a new head with valves closer together. There is quite a space between the seats. Bigger valves? No problem. Now we are notching bores again. You could get bigger valves in the small bore if you tilt them, like a Hemi! How about a 4 valve head? That's what the small imports do. Heck, Just put some domes in the bore, juice up the heads, slide a proper cam in and hold on tight. The 273 can only do so much. Someone mentioned supercharging. Sure could. One of those GM 3800 blowers would work perfect.
     
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    • toolmanmike

      toolmanmike FABO Staff Staff Member FABO Gold Member

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      Dang, there used to be a turbo option on Desktop dyno.
       
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      • nm9stheham

        nm9stheham Well-Known Member

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        I've been working on this issue for the last year or so. With top ring locations being what they are typically, you're gonna be limited to the high 400's, or maybe right at .500" for max lift. It depends on the exact piston. This limit is for a stock deck, and the top ring location is the issue.

        If one were to do custom piston, and would move the top rings down about .100", then you could push up close to .600" . Probably no ideal to move the top ring down but that is what you gotta do for that matter. Egge might be able to do that; dunno if you could just ask for the ring groove to be machined lower. It would depend on the basic casting.

        As for the valve size and flow shrouding... sure, you would not get max possible flow from a 2.02" valve being by the cylinder wall notch. But I would expect you to still get more flow that way than with a 1.88" valve and no notches.

        Turbo or super-charging makes a lot of sense.
         
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        • oldkimmer

          oldkimmer FABO Gold Member FABO Gold Member

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          The 2.02 will cause reversion because of the air bouncing off of the deck. If u want a 273 to perform build it like the old D dart with a modern cam and 302 heads. Kim
           
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          • Wyrmrider

            Wyrmrider Well-Known Member

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            too big does not work for lots of reasons
            too big for the ports for one thing
            and no reason for big exhaust at all
            I'd start with the late 318 heads if stock heads
            but then I'd use 30 degree seats with smaller cams in the 500 inch lift range
            you can notch by hand- no need to notch the exhaust
            flow does not like to hit top of cylinder, a little break can make a lot of difference
            just reminiscing I think we took a 2.02 chevy valve and reduced the diameter to get a decent margin for a 30 degree seat and back cut them would have come out in the 1.94 area (you can't just re seat a 1.94 valve as they get too thin- 2.02 valves are cheap in any case- and lighter than MOPAR valves
            measure twice cut once
             
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            • RustyRatRod

              RustyRatRod 30 Degrees Outta Whack

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              Just use a fly cutter the appropriate size. One and done.
               
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              • PRH

                PRH Well-Known Member

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                I’m def not following the reasoning here.
                The piston is nowhere near TDC when the valve is at full lift.
                 
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                • dibbons

                  dibbons Well-Known Member

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                  Should check the valve to piston clearance through the entire range of crankshaft rotation (2 full spins) either checking with clay or those light springs. If using the springs, must check at other piston elevations besides TDC because the piston dwells near TDC for quite some time as it changes direction from going up to going down. I think I know what I am talking about anyway.
                   
                • nm9stheham

                  nm9stheham Well-Known Member

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                  Roger.... The limit is the depth of the notch you can put in the side of the bore and not have the top ring get into the notch. If the notch can only be so deep due to that, then the lift can only be so much. I've been working on the premise to not allow the top ring to overlap even partially over the notch at TDC... sure wouldn't do the ring seal much good to allow it to do so!

                  I've been also looking at other pistons with a much bigger bore, and offset boring the block towards the intake side to get completely away from having to notch the bore at all with a 2.02" valve. Looks doable, but boy is it close!!
                   
                • nm9stheham

                  nm9stheham Well-Known Member

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                  Now I'M not following the reasoning here ???? (But hey, nothing new there!) The 273's 3.625" bore is so small that even with a 1.88 or 1.94 valve, that's gonna happen. That bore is smaller than the head chamber and you're gonna get this effect to some extent or another. Even 675 heads for the 273 and 318, does not change that; the chamber overhangs the deck. I would have to check to see if a 302 chamber did the same but I am pretty darned certain that it does.

                  See here for a 675 chamber on a stock 273 bore; you can see the shadow line of the overhang of the chamber. (The white disc is a simulated 2.02" valve head.)

                  DSCN2656.JPG
                   
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                  • toolmanmike

                    toolmanmike FABO Staff Staff Member FABO Gold Member

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                    Oh just notch the bore. It'll be fine! :poke: LOL
                     
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                    • pishta

                      pishta I know I'm right....

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                      How about just using a cheap 318 block with its .285 bigger 3.91 bore right off the bat? Youll pick up .1425 at the edge. Your getting pistons for it anyway and wont have the valve shrouding (or as much). I know its now a 318 but C'mon....everything from the 273 still fits in and on it sans the pistons.
                       
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                      • dibbons

                        dibbons Well-Known Member

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                        Last edited: Oct 12, 2019
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                        • nm9stheham

                          nm9stheham Well-Known Member

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                          But inquiring minds want to know ! LOL
                           
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                          • toolmanmike

                            toolmanmike FABO Staff Staff Member FABO Gold Member

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                            I 'm curious as to what difference a 1.88 or 1.94 valve would do. I wonder how accurate the Desktop Dyno would be for bigger valves?
                             
                          • oldkimmer

                            oldkimmer FABO Gold Member FABO Gold Member

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                            R u sure about that Mike? Kim
                             
                          • Wyrmrider

                            Wyrmrider Well-Known Member

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                            yr wrote

                            "The biggest things about notching the bore are keeping the notch inside the gasket sealing ring"

                            you want to keep the notches away from the edge of the combustion chamber
                            you want to keep both the notches and combustion chamber smaller than the gasket
                            put the heads on the block, dykem and scribe deck around the bores AND scribe the gasket
                            and then scribe the top of the block using a hook with the heads onor using the size of the chamber taken from a templet you make
                            with core shift you want to check each chamber
                            it does not take much chamfer to make a difference- a square sharp edge shears and is a no no
                             
                          • 273

                            273 Well-Known Member

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                            If your gonna notch I’d go with magnum heads, closed chamber more than enough flow.
                             
                          • toolmanmike

                            toolmanmike FABO Staff Staff Member FABO Gold Member

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                            Apparently not. LOL I shouldn't believe what I read apparently. LOL
                             
                          • ddartdude

                            ddartdude Well-Known Member

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                            TMM - sorry about the X's.

                            D/Darts had stock 920 heads - 1.78 and 1.50 valves. They are closed chamber swirl port heads. Mine ran a best of 12.95 at 105 with launch and shifts up around 6900 RPM's. The Doug's headers had 1 3/4" primaries and 3" collectors back then. The guy that raced my first D/Dart used a 36" collector extension when he took it to the strip. I think he referred to it as a "stinger".

                            In the TSB D66-HP -1 dated April 18,1966, there is a section entitled `Car Operation For Best Performance '. This TSB can be found on the D/Dart website and the Hamtramck site.
                             
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                            • toolmanmike

                              toolmanmike FABO Staff Staff Member FABO Gold Member

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                              I was just reading through this old thread. So very interesting. 1966 Dodge D/Dart ~ 'Mystery, Myth and Misconception'
                               
                            • toolmanmike

                              toolmanmike FABO Staff Staff Member FABO Gold Member

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                              I deleted my 2 posts. I don't like to have information out there if I can't back it up. It just adds to the confusion.
                               
                            • PRH

                              PRH Well-Known Member

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                              If a larger than stock intake valve is installed in a stock 273 head....... what’s the lift point at which the valve contacts top of a 273 block?
                               
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                              • toolmanmike

                                toolmanmike FABO Staff Staff Member FABO Gold Member

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                                Good question.
                                 
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