65 Valiant 273 dual exhaust

Mopar Exhaust Systems

  1. LAP Valiant

    LAP Valiant Active Member

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    I'm considering true dual exhaust for my 273 powered 65 Plymouth Valiant. What will increase power? Crossover or H pipe? Plan to use stock manifolds also. Any advice or experience will be appreciated.
     
  2. toolmanmike

    toolmanmike FABO MODERATOR Staff Member FABO Gold Member

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    The experts claim the HP single exhaust is more efficient than a dual system. (Scavenging effect) I don't think I have ever seen a drag strip or chassis dyno comparison between the two.
     
  3. Ccas

    Ccas FABO Vendor FABO Vendor

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    The biggest performance exhaust upgrade you can make is mandrel bent pipes. Even with a 273, you can pickup 15+ hp over crimp bent pipes you'd get at a local shop. A crossover will help as well but not as much as the pipes. An x could give you another 8-10. The H help but only on the bottom end torque and at heavy throttle, it sounds a bit more traditional V8 sounding whereas the X starts to have a turbo type sound.

    If your motor has been worked with a bigger cam, these number will increase significantly.
     
  4. AJ/FormS

    AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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    Stock engine?
    Automatic?
    what rear gears?
    Where do you need more power? As in speed or rpm
    Wiki says you are at 650' elevation; does that sound right?
    If you're looking for off-the-line punch, there are better places to spend your money.
     
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    • LAP Valiant

      LAP Valiant Active Member

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      Thanks for the help - is like to get 30-50 more HP. It's a stick 273 2bl carb with torqueflite auto trans. Not sure about the gears but stock. What are the best bang for the buck power mods for this motor?
       
    • LAP Valiant

      LAP Valiant Active Member

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      I'm in San Antonio, Texas and that sounds about right for elevation.
       
    • LAP Valiant

      LAP Valiant Active Member

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      I"d...stock...
       
    • AJ/FormS

      AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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      OK first, you gotto know, that the 30 to 50hp is a pretty big step for a 273, and it's gonna come at some higher rpm like close to 5000. Below 2000 the power may be negligible, and from 2000 to 3000, the power increase is only gonna be 30 to 40 percent of the increase at peak.
      Now;
      5000 rpm with 3.23s and 77 inch tire roll-out (24.5" tires), will be ~38mph.
      3600 will be about 28mph.
      So, you will feel the power (with 3.23s) beginning at 28mph and peaking at 38 mph. then dropping off to the shift-rpm.
      Is that what you want?
      More likely your car has 2.76 gears, and the powerband with those will be 17% higher or from 33 to 44mph.

      If you want the powerband to start at a lower mph, then you will need a higher stall TC.
      Here is a power curve from a 5.2 magnum. Ignore the numbers. Just look at the shape of the curve.Notice the power at 1800 versus 2800.
      Your current TC might be stalling as high as 1800. On this graph that is 90hp. At 2800 on this graph, that is 160hp, or 78% more. This is a serious increase, and it's still the same stock engine. It will change completely the personality of your engine.
      The next performance increase would be rear gears. What those will do is bring the power down to a lower speed. 3.91s for instance, are 21% bigger than 3.23s and will bring 2800 down to 15 mph (instead of 28), and 5000 will be closer to 29(down from 38).
      Now is the time to pressure test your exhaust system. Ideally the pressure will not exceed 2psi. By 4psi, she is an ugly restriction, and you need to get after it. After the exhaust is taken care of, now is the time for bolt-ons. But at this point it's kindof an all or nothing deal; 4bbl, cam, and headers, as a set. The cam is the secret to unlocking the power. It does this by making the engine able to breath more air at higher rpm. But if the 2bbl is still on there, that's a problem. The headers will work everywhere in the powerband by amplifying what the cam is doing. But again, with the 2bbl on there, they can't do much for the topend. Headers can however, with the 2800TC, help the midrange. So those three work in concert.If I had to do them one-atta-time; it would be headers,4bbl, then cam.
      Happy HotRodding


      power-318.gif
       
      Last edited: Jul 30, 2020
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      • toolmanmike

        toolmanmike FABO MODERATOR Staff Member FABO Gold Member

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        Agree AJ. I rarely give out red X's but I all most did on that 30-50 horse statement. THe factory barely got there with a 4 barrel, 10.5/1 compression and a bigger cam, dual point, teemed with the large HP single exhaust.
         
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        • LAP Valiant

          LAP Valiant Active Member

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          That is valuable info. I think I'll save my $ for now. I'm just learning and you guys are a great resource. Thank you.
           
        • AJ/FormS

          AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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          You can still do the exhaust system pressure test to see if or at what rpm/speed, it is slowing your current combo down.
           
        • Cuda416

          Cuda416 Well-Known Member

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          Isn't it fair to say the biggest problem with the early A v8 exhaust is the exhaust manifolds, themselves? If that's true, he might start looking for magnum exhaust manifolds and do the required mods to make them work. Drivers side is the pain, but I think it's workable in this car. Tight, but workable. That factory drivers side manifold that pinches down around the steering column is pretty bad from what I've heard. Full disclosure, that's what we have in our 65, but I'm seriously looking at the mag manifolds. I already have the pass side jeep grand cherokee one with the 2-1/8 opening...
           
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          • LAP Valiant

            LAP Valiant Active Member

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            Does the pressure test need to be done at a shop?
             
          • LAP Valiant

            LAP Valiant Active Member

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            Driver side manifold does suck event for getting to spark plugs and my engine compartment is fairly clean except for rusty manifolds. Who should I look into purchasing manifolds from?
             
          • Cuda416

            Cuda416 Well-Known Member

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            The pass side is from the junk yard. Look for a 96-98 JGC with a v8. If you can get the drives side from a 68-70? 340, that's a good choice. If a mag, then I think the dakota v8 drivers side is good. You may have to cut the steering column "tube" and change the joint on it, but none of it's rocket science. There quite a few threads that talk about it at length.
             
          • toolmanmike

            toolmanmike FABO MODERATOR Staff Member FABO Gold Member

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            Magnums if you are on a budget. They flow better than the originals. The 71-74 are a little more expensive and flow about the same as the Magnums. The 67-70 manifolds are the most expensive and flow almost as well as a shorty header.
             
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            • LAP Valiant

              LAP Valiant Active Member

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              You guys are a huge help to this classic car rookie! Manifolds or headers will be the plan then maybe gears. Thank you all!
               
            • Cuda416

              Cuda416 Well-Known Member

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              I've been helped so many times here it's almost embarrassing. Always I thought I knew stuff.. I knew nothin' it seems. I'll help any way I can as a way to "Pay it forward"
               
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              • LAP Valiant

                LAP Valiant Active Member

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                Thank you Cuda416! We should have lunch or something and check the cars out since you're in my area.
                 
              • AJ/FormS

                AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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                no
                you just drill a hole in the pipe on the engine side of the muffler and braze a foot or so of brakeline into the hole. Then run some some vacuum line up into the cab, install a low pressure gauge in the end and finally
                take it for a power-run.
                Note at what rpm/speed the pressure peaks and the value.

                Less that 2psi is the target
                more than 4psi you can really feel the power loss*.
                But if it doesn't hit 4 until top of second gear, well, that's a judgement call cuz with 2.76s that's 74mph@4500, with 3.23s it would be 64 mph. This will change as you hop up the engine and it starts to process more air.

                * well you might not feel the power loss if you are used to it,lol. but if you go from 2 to 4 within hours, you sure will.
                 
                Last edited: Jul 30, 2020
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                • Cuda416

                  Cuda416 Well-Known Member

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                  Any pictures of this? Does the brake line go into the muffler at all like a pitot tube? or is it just to make a generic hose connection to the exhaust without melting anything?
                   
                • LAP Valiant

                  LAP Valiant Active Member

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                  Guess I'd need a working speedo to test it...
                   
                • AJ/FormS

                  AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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                  I put it in the pipe anywhere north of the muffler, but try to be within a foot or two of the muffler.
                  NO PITOT.
                  Just inserted a half an inch or so at 90* to the pipe.
                  Pressure is supposed to be equal in all parts of the system. In reality it tends to pile up at the muffler which is generally the restriction. Plus the gauge needle tends to be more stable at just in front of the muffler, cuz most of the hammering,it seems, is going on at the Y.
                  The first time I ever tested,was on a pick-up truck with a cat. I used 2ft of 1/4" brakeline, between the Cat and the muffler. Cuz I didn't know how hot that tube would get. That was overkill. Subsequent tests I used 3/16 line and down to 15 inches seemed adequate. The long line tho, lets you pinch it closed and fold it over; no brazing required. And you have lots of useful line left over. Plus if you just cap it, you can use it again later, after the restriction has been removed, to prove to the customer, that he paid to have the right part removed. This gets to be real important when you have to replace an expensive Cat.
                   
                  Last edited: Jul 30, 2020
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