Super charging my 67' 273 commando valiant

Forced Induction Mopar

  1. Namehtnad

    Namehtnad Well-Known Member

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    Heeeeeyy yoooou guuuuys!!!....

    Ive literally been losing sleep over what I was going to do with my 67 valiant with an original 273 commando....whether i was gonna drop a 360 and be happy, or build and force my lil 273....ive done did decided, (my grammar gone done her shoppin). Ima keep the lil'un and build it up to take the boost.

    The story starts as this...i bought the little gem for a song, it was a sad leaky lil guy, that was breathing like an asthmatic jogger, burning oil and fowling plugs, navigating the roads as an aircraft carrier would, and stopping with the power of 1 brake...coolant and oil dripping from every little spot in the engine, blown seals and diaphragms in the carb.

    I spent a Saturday and replaced all the oil gaskets, timing cover, hoses, even had a specialty gasket shop make a set of gylock gaskets for the oil filter mount...bought an edelbrock and slapped it on, replaced the gas tank....turned into a peppy lil guy....the little bastard still burns oil in number 1&3, just gotta replace them valve seals. It was at this juncture i was gonna sell the happy lil smoker....(whom only smokes at high rpm)...

    then I got into two motorcycle accidents. One netted me a nice little settlement...so came the question to the little lady...sell or supe? She said she wanted to keep it...so....I redid the entire brake system, replaced the worn gearbox and steering bushings....

    i came to find out i am going to have a very sizeable settlement from the first motorcycle accident which is going to require me to have neck surgery and permanent nerve damage down my left arm....

    I want this lil guy to be my little sleeper, i want to keep the original motor, and at this juncture....money is no object...im going to have the 273 rebuilt for boost, and have have the heads refurbished, new piston rings with larger gaps, (for expansion) and i want to keep the stock 10.5:1 compression ratio, and I want forged rods. Ive already sought out the demon 650cfm blow through, and have done due diligence on researching the kind of supercharger i want....paxton 1201850p. I have a nice little set of hookers im just gonna dump right under the driver door (to be installed when the engine is out) but this is as far as i know.

    I come to this forum with humility, asking for what else I need for my lil 4 door commando. As far as required engine work, knowledge on how much boost i can and should run...(im sure itll spend a few hours on the dyno to get it right, ) and other required supporting mods i havent considered, I'm at a loss. I have searched the all knowing Google and you tubes....and can't find anything for my particular dilemma. The other guy I saw here blowing a 273 went top mount. I cant fit that under my hood. I wanna keep original body work.

    Any and most advice, comments, and help is much appreciated. The continuation of this project should begin in February or march. Thank you for any time you take to give advice. I'd like to add im looking to get around a respectable 400 ponies

    ----this guy---
     
    Last edited: Dec 8, 2018
  2. Commando66

    Commando66 She's a beaut Clark!

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    Cool build. Pretty rare car to have a Commando 273 in a Valiant, let alone a 4 door! Post up some pics
     
  3. Namehtnad

    Namehtnad Well-Known Member

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    20180912_175445.jpg
    Old engine bay pic, the brake master cylinder is new, this was after i stuffed the edelbrock on. Cute lil 4bbl
    20180912_175546.jpg
    Long side of car. Original paint, no body damage.
     
    Last edited: Dec 8, 2018
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    • Namehtnad

      Namehtnad Well-Known Member

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      Yep, this little guy had it original from factory, checked the vin code and everything. Forgot to mention, the interior is very nice. The guy who had it before me had the entire interior re-done. No photo of that on this phone.
       
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      • wanta68cuda

        wanta68cuda Well-Known Member

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        It's going to make one heck of a sleeper!
         
      • thesiren74

        thesiren74 Well-Known Member

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        Cool little car, i always liked those valiants.
         
      • toolmanmike

        toolmanmike FABO Staff Staff Member FABO Gold Member

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        One of 101 built. Cool.
         
      • toolmanmike

        toolmanmike FABO Staff Staff Member FABO Gold Member

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        Even better yet, 50 4 speeds and 51 automatics. That's all Valiant 100 models, 2 and 4 door. No further breakdown by number of doors. LOL
         
      • Namehtnad

        Namehtnad Well-Known Member

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        Anybody have any advice yet?
         
      • toolmanmike

        toolmanmike FABO Staff Staff Member FABO Gold Member

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        Do your homework. Read and learn, rinse and repeat.
         
      • AJ/FormS

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

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        Man, I'm sorry to hear you're hurting physically, and wish you all the best.

        Well, notadvice but
        Have you researched 10.5Scr and supercharging?
        I have never done it, but I know you can only run so much total cylinder pressure with iron heads, and; depending on the cam, 10.5 with a supercharger cam, is gonna already be near the limit. It would be a huge disappointment if after spending your wad,you come to find out you can only run 2 or 3 psi before detonation breaks the pistons, or worse.
        I think turbos might be a little easier on her.
        __________________________________
        If I was in your situation, I would build my dream system;
        I would engineer a two turbo staged system, with very low Scr, and a little turbo up front on the passenger side, and a bigger remote mount at the back. Like you,I would target 400ish hp, because as a streeter,400hp is a lot, already having major traction issues. It's also, a lotta fun.
        The little turbo up front would start very early and make the engine torquey like a 360. And let it would let me run a small enough gear to hit 60mph at the top of first gear.Like 2.94s. It would also spool up the remote.
        The remote turbo would pump up the secondaries, after the tires hook.
        In my dream I mount fake sidepipes, that are actually the cold side from the remote, and no mufflers at all. In my dream the remote pulls air from inside the ventilated trunk. My dream starts with a 318 for a little more NA kick,(and easier parts selection) and it's easier to hit the 400/450 mark which, for a streeter,(ME), is all I could ever wish for, having a NA 360 already in that zone, and being very happy with it.
        My dream uses a big spreadbore carb on an old Offy DualPort, with the little turbo jamming the primary side, and the remote stuffing the secondary side. This system uses no intercooler, but the carb would get fresh cold air, and it may use water injection with open-chamber iron heads. But on a 318, I would install aluminums so I can run a lot of Cylinder pressure.
        But at Age 65, My dream,sadly, will have to remain a dream.
        Happy HotRodding.
         
        Last edited: Dec 10, 2018
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        • Namehtnad

          Namehtnad Well-Known Member

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          All chinese to me, my friend,

          However, you are never to old to make a dream a reality. Unless you're infirmed, then, yeah its too late
           
        • toolmanmike

          toolmanmike FABO Staff Staff Member FABO Gold Member

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          Your recipe for a supercharged 273 is off. Again, read and learn. Then read and learn some more.
           
        • Namehtnad

          Namehtnad Well-Known Member

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          It seems 10 psia is the standard for 10.5scr, with pump gas 91 and advance around 22°
          Unless I get off the dinosaur juice and stalk the corn fields. Much more boost can be applied before predetonation

          Maybe new heads are in order?
           
        • AJ/FormS

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

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          Or about to retire and try to survive until Judgement Day. No infirmity here, but you never know if satan will find a chink in my armor. Threescore and 10; fourscore if by reason of good health. That's 5 to 15 more for me, hardly worth starting another project.
          Whereabouts is Daygo?

          BTW nice moredoor
           
        • Namehtnad

          Namehtnad Well-Known Member

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          San Diego
           
        • AJ/FormS

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

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          I can't comment, never having built a supercharged system.
          But I do know that installing 2.02 heads on a 273 is normally not advised because of valve shrouding. With a Super, this may not be an issue. Your Commando Heads are sub-60cc items IIRC, so almost anything you bolt on there is gonna drop your compression,IMO, a good thing. Perhaps 62cc Magnums with 1.88intakes.

          I would surmise tho, that a 273 at 22*total timing is gonna be a dog until the pressure hits. You gotta have something left to drive on before the Super hits, which might be something like 85% of the time. This is why in my dream I start with a 318; that's 45 cubes, and a lotta low-rpm torque. And also why dual turbos; the small one is working right from idle, making the combo perform like a 360 most of the time.
          I think my dream could be adapted to a 273 easily enough.
           
          Last edited: Dec 10, 2018
        • AJ/FormS

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

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          10.5 Scr and say a 262 Supercharger cam is gonna put your NA (Normally Aspirated) pressure up to maybe 180psi. So I can't see this working on pump gas even normally aspirated. By the time you add 10psiG you will be up to 14/1 Scr and that's pretty good .............. for E85 or LPG.
          To run pumpgas, I think you will be better at 7.8 . But I still think you won't be able to run 10psiG. Maybe 8.
          Either way, off boost it will be doggy.
          IIRC it works like this; best guess.
          With iron heads about the max pressure you can run on pumpgas is 165.
          If that is true; 165 guage is ~190 absolute.
          10psiG is ..........................~40 absolute
          So subtracting them you get 150 absolute what you can design your engine for.
          150ab. is ~120 psi on your compression gauge, about.
          Designed for 7.8Scr, your 273 will make about 118 psi depending on the ICA, (your cam's Installed Intake Closing Angle) ( I chose 62* and 500ft above sealevel.
          That's my best guess working from memory.
          Now if your Intake temp is too high (no hoodscoop, no intercooler) you may get into detonation at 165psi cranking pressure, so I arbitrarily downsized your Supercharger to 80% to allow for that. And that is how I got to 8psiG.
          I don't think it's gonna make 400 like that..... but maybe...
          Best guess man.

          7psi, IIRC can double your power,so 8 would be 114% more and so to get 400, you will need 400/2.14=185 to start with, Normally Aspirated. That looks doable at 7.8 Static Compression Ratio and a low-overlap Supercharger cam. But, off-Super it will be less powerful than a slanty. I don't see headers being particularly helpful, other than being roomy enough to park 400HP worth of spent exhaust, for a couple of seconds atta time.
          Since a crank-driven super needs rpm to get to work, the bottom end will be doggy, until you install a higher than stock stall, and/or, more rear gear, or a combination.

          Keep researching cuz I might be really really wrong, cuz I'm going by an old man's memory.

          OOPs I see some math errors hang on. Ok fixed it
           
          Last edited: Dec 10, 2018
        • Namehtnad

          Namehtnad Well-Known Member

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        • Namehtnad

          Namehtnad Well-Known Member

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          Thank you for your info, ill take a good long hard look at this. I have yet to go to the local speed shop that does this on a regular and ask them as well, I'm sure they have too many hopefuls going in to ask about this stuff and never end up doing it. But....I think ill head down there today and ask. The more info the better.
           
        • AJ/FormS

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

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          That's a good plan
           
        • Namehtnad

          Namehtnad Well-Known Member

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          Hell, they only told me everything paxton did on the product description for their supercharger
           
        • subcom

          subcom Well-Known Member

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          That Paxton looks pretty straight-forward as far as install. I think 400HP from a 273 is probably very doable, though like others have mentioned, I'm not sure how much PSI it would require and what kind of fuel it would take to keep you out of detonation with those 10.5:1 pistons. If money really is "no object", I would consider pulling that 273 (after I paid off my mortgage and bills, lol) and saving it in the corner of the garage and replace it with a more boost friendly small block like a 360 or even a 318. Nothing rare you would cry over like your 273. Best of luck with the project. Will make a super sweet sleeper.
           
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          • toolmanmike

            toolmanmike FABO Staff Staff Member FABO Gold Member

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            That makes sense.
             
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