Dynamic Compression and Aluminum Heads?

Big Block A body Tech

  1. JGC403

    JGC403 Well-Known Member

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    I know this has been discussed a lot on here, but I wasn't able to find an answer for my question.

    With aluminum heads they say you can generally raise the static compression ratio 1 point because the aluminum dissipates heat quicker.

    Now when you are inputting all of your variables into a dynamic compression ratio calculator what compression ratio would you put for aluminum heads.

    SO lets say your calculated compression ratio is 10:1, but you are using aluminum heads so it basically has 9:1 compression ratio.

    In the dynamic compression ratio calculator do you input your calculated 10:1 ratio or your "effective" 9:1 ratio for aluminum heads?
     
  2. rumblefish360

    rumblefish360 so close yet so far away

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    Try and not over think this. Squeeze his squeeze!
    You can get away with an extra point in ratio with aluminum. That doesn’t mean you should with a small cam or push the limits of octane.

    What is the combination your thinking of?
     
  3. MOPAROFFICIAL

    MOPAROFFICIAL Well-Known Member

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    10.1 is 10.1.

    The head material difference is in heat dissipation/transfer/weight.
    Dynamic # requires a intake closing event to calculate effectively giving the compression seen in the end of it.

    You could run around 11.1 static compression , but dynamic psi usually must be below 9.1 or you'll detonate on most pump gas. The cams intake closing degree 'early or late' will decide how 'high or low" that is.
     
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    • rumblefish360

      rumblefish360 so close yet so far away

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      Exactly! No 13-1 ratio’s and stock camshafts.
      :rofl:
       
    • JGC403

      JGC403 Well-Known Member

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      OK, wan't sure if the head material made a difference for the dynamic CR.
       
    • moper

      moper FABO Gold Member FABO Gold Member

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      IMO - material means nothing to dynamic ratio considerations. To be honest, static is more important, because all dynamic is theoretical anyway. Calculators for dynamic leave a shitload assumed, and it's best not to push to the max. Assuming this is a street or street/strip engine keep dynamic below 8.25:1.
       
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      • MopaR&D

        MopaR&D Nerd Member FABO Gold Member

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        Heat transfer to the head and cylinder walls has a measurable effect on power only at really low RPM, around 2500 max. Above that the cycles are happening too fast; before the hot burned gases have time to heat up the head they're on the way out the exhaust and a much colder unburned mixture is flowing into the cylinder.

        If you're building a truck engine for towing heavy loads I think it would make a difference but there are still many other bigger factors at play.
         
      • RustyRatRod

        RustyRatRod 30 Degrees Outta Whack FABO Gold Member

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        It doesn't work that way. At all. 10:1 is 10:1 is 10:1 regardless of what the heads are made of. The head material has nothing to do with compression ratio. Nothing. Nada. Zilch. Zero. It will have 10:1 even if the heads are made out of wood. When figuring for dynamic compression, use the static compression figure, plus any other variables (like the intake valve closing event) the calculator calls for.
         
      • RustyRatRod

        RustyRatRod 30 Degrees Outta Whack FABO Gold Member

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        Here is the calculator that I use. It figures static and dynamic at the same time. Some like, some don't. I like it. It's only as accurate as the input information.

        Compression Ratio Calculator
        Agreed a hundred and nine fifty million percent.
         
      • famous bob

        famous bob mopar misfit

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        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^THIS^^^^^^^^^
        combustion chamber size/ piston tops/ gasket thickness the same, , compression will be the same alum. or cast.----
         
      • rumblefish360

        rumblefish360 so close yet so far away

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        Squeeze is squeeze....
         
      • AndyF

        AndyF Well-Known Member

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        You should probably just ignore the whole concept of dynamic compression since it often causes more problems than it solves. Trying to run a high compression engine on lower octane fuel is a time bomb. A low dynamic compression ratio just means that at low speeds the effective compression ratio is lower because not very much air got trapped in they cylinder. This is a useless fact unless you are going to limit your engine speed to less than 2000 rpm. As soon as the engine comes up on the cam you have dynamic filling of the cylinder and full compression. So what exactly is the point of even thinking about the dynamic compression ratio? It is a useless concept. If you are going to run the engine then you need to pick a fuel that matches the static compression ratio. If you are just going to run the engine at part throttle then why do you care about performance parts such as aluminum heads?
         
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        • clinteg

          clinteg Well-Known Member

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          I’m on the other side of the static/dynamic debate. My personal opinion and from what I have learned from those who build engines, static doesn’t really mean much other than to calculate dynamic and dynamic means everything when trying to tune for pump fuel. When I had my cam ordered, it was custom ordered based on the dynamic compression ratio needed for the combo. After we ran the engine, it was dead nuts what they said it would do. Run on 91 octane with 11:1 old ass iron heads. Now if I were to put a different cam in and if it didn’t bring the DCR down enough, I don’t think there’s any way I’d be able to run pump gas safely, even with the same static CR as before. There are variables that aren’t accounted for in DCR, but i think it gives you a lot better and more information than static will ever tell you. That being said, I would NEVER build an engine to run on pump fuel based on the Static compression. I’d aim for the dynamic number. Just for the simple fact that my builder aimed for the dynamic number and it was perfect for my application. Just my 2 cents.
           
        • AndyF

          AndyF Well-Known Member

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          Sounds great except for the fact that it is impossible. The dynamic compression ratio is meaningless for a performance engine since it changes with RPM. Sure you can put a big cam in a high compression ratio engine and drive it around at slow speed but what is the point? As soon as the engine comes up on the cam you have a high compression ratio engine and you need race fuel. The only time dynamic compression ratio means anything is if you are building a very low rpm engine such as a tug boat. When you design a low rpm engine you have to be careful about cam timing since you don't have any inertia charging. For a performance engine the dynamic compression ratio is meaningless since you are aiming for inertia charging. If you get the engine design correct you end up with a little more than static compression ratio on a performance engine since you get a little bit of inertia ram effect at the peak. For a race engine you look at static compression ratio and then add a small fudge factor for inertia tuning. That is why all of the race fuel mfgs have charts which tell you which fuel to use based on static compression. They don't care about dynamic compression ratio. The dynamic compression ratio of a Pro Stock engine is probably less than 9:1 but who cares. It won't run on pump gas.
           
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          • Jesus Chrysler

            Jesus Chrysler Forgiving Sins Against Mopar Since 1983

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            That's how we did it back in my day!

            Hazzard 3:14. Jesse said to Bo & Luke: CR is what CR is! This is what you shall tell Rosco and Enos.
             
            Last edited: Mar 19, 2019
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            • rumblefish360

              rumblefish360 so close yet so far away

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              LMAO! That’s a good one!
               
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              • charles mill

                charles mill Wildman68 FABO Gold Member

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                great ? I am putting a 12.5 383 . done 30 over recon. rods bolts r recon rods new. am looking at mopar collector mag 4 advice. On heads also its a 6pk 68 cuda auto. mine as new. I play with a lot. 4sp? as of your ? 12.5 and their sale heads.? this will 4 a B BLOCK will b nasty.
                 
              • charles mill

                charles mill Wildman68 FABO Gold Member

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                AND AGIN Y CANT I PRINT INFO ABOUT HEADS I MAY BUY FROM MCG> ? ON THERE ADD 440 SOURCE. there heads sound and $ r good but cant print add 4 info to take 2 my shop 2 get ck out. why r they wrong . got $. DOES MCG AND 440 source make $ whit a nut as me. y cant I print spec"s on their product MY$...…………………………….. and solid. as a vet...……………..100% shot
                 
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