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Freiburger, DCR, and testing.....

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  • Freiburger, DCR, and testing.....

    So every board I get on somones allways rambling about dynamic compression ratio and how its a huge deal for selecting a cam. Im not saying I agree with it and im not saying I disagree with it. It seems to allways be quite the talk, some guys swear by it and some guys blow it off as a bunch of "Tom Foolery". So I say test it and end the discussion. Start with a very general small block chevy. A 350, 383, 400 somthing like that. Set the compression at 10:1 make sure it has some off the shelf 200cc heads and a dual plane intake. Parts that are very general that would work well with all the different cams. Then run a cam that sets the DCR at 7:1, then run a cam that sets the DCR at 7.5:1, then 8:1 then 8.5:1. Test for overall avrege power/tq, octane sensitivity{hows it run on 87,89,92 etc.}, and peak power RPM's this would really give everyone that loves discussing this somthing to talk about. It sounds like an old school cam comparo but its more about testing the DCR concept....discuss

  • #2
    Re: Freiburger, DCR, and testing.....

    isn't the DCR dependent on RPM?

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    • #3
      Re: Freiburger, DCR, and testing.....

      I dont know the whole theory, but i dont think RPM has anything to do with it, its more of a mechenical function equasion and dosent take scavenging or air flow charactoristics into account. It has to do with your static compression and intake valve closing point. Theres calculators all over the place that will do the math for you. According to theory, ideal DCR is around 8.0:1 Eric68 here runs his DCR a little tight at 8.4:1 but he says it ran fine on pump gas. My actual compression is 11.1:1 and my DCR is around 7.7 or somthing like that and i run 89 octane everywhere I go and at the track. I think it would be a cool comparo.....

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      • #4
        Re: Freiburger, DCR, and testing.....

        Oh, then the optimum DCR would depend on what RPM you're running....?
        My fabulous web page

        "If it don't go, chrome it!" --Stroker McGurk

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        • #5
          Re: Freiburger, DCR, and testing.....

          Squirrill, the way i understand the whole concept is that 8.0:1 is optimal for pump gas and has nothing to do with RPM. I could be wrong though....

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          • #6
            Re: Freiburger, DCR, and testing.....

            The term "dynamic" means "moving"...which implies to me that it must have something to do with RPM. Maybe I just don't understand the whole thing....


            My fabulous web page

            "If it don't go, chrome it!" --Stroker McGurk

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            • #7
              Re: Freiburger, DCR, and testing.....

              That makes 2 of us,all I know about it is what ive told you, according to the whole concept I run way too much cam but my car seems to do ok for 89 octane I guess. I just wanted to see it tested to see how much it really matters. Im sure theres ALOT of people that would love to see it tested.....

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              • #8
                Re: Freiburger, DCR, and testing.....

                I guess another way to test it would be to run the same cam on the test mule motor and change the compression giving it the 7:7,7.5:1,8:1 and 8.5:1 DCR's.

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                • #9
                  Re: Freiburger, DCR, and testing.....

                  And what you'll see is the power increase as compression ratio increases, until it detonates

                  Sounds like a fun numbers game, that's all.
                  My fabulous web page

                  "If it don't go, chrome it!" --Stroker McGurk

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                  • #10
                    Re: Freiburger, DCR, and testing.....

                    Originally posted by squirrel
                    isn't the DCR dependent on RPM?
                    YES! Which is what all the formulas ignore.

                    There are also websites that talk the theory of DCR and claim that it's built-in and does not change.
                    "Dynamic".. Doesn't change? Now there's a contradiction.
                    The DCR does change with RPM and how the cylinder is filled at different air speeds.

                    Here's one..
                    "..the Dynamic Compression Ratio, is fixed when the engine is built and never changes during the operation of the engine."
                    Bullshit! :P
                    http://cochise.uia.net/pkelley2/DynamicCR.html

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                    • #11
                      Re: Freiburger, DCR, and testing.....

                      More keyboard racing, detonation occurs in motors at different times, because of more than just the compression and type of fuel used in it, no matter what the DCR is.

                      2007 SBN/A Drag Week Winner & First only SBN/A Car in the 9's Till 2012
                      First to run in the .90s .80s and .70's in SBN/A
                      2012 SSBN/A Drag Week Winner First in the 9.60's/ 9.67 @ 139 1.42 60'
                      2013 SSBN/A Drag Week, Lets quit sand bagging, and let it rip!

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                      • #12
                        Re: Freiburger, DCR, and testing.....

                        Steve and I had a good debate about this on HRF.
                        I see he is still wrong!
                        Cognizant Dissident

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                        • #13
                          Re: Freiburger, DCR, and testing.....

                          So wouldnt this be a good thing to test?

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                          • #14
                            Re: Freiburger, DCR, and testing.....

                            DCR is accepted as when the intake valve closes regardless of RPM, I tend to differ about this also. It must be defendant on RPM with intake runner length playing a huge role.
                            I did try to write a calculator that takes RPM, VE into account.

                            http://www.bid2race.com/DynamicCompr...Calculator.php

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                            • #15
                              Re: Freiburger, DCR, and testing.....

                              "Dynamic" is a total misnomer. And what people THINK OF as DCR is NOT rpm dependent. Unless you have variable valve timing, there is no variablity to a ratio based on engine rpm. What people CALL DCR is the volume of the cylinder from the point of intake-valve closing divided by the volume of the cylinder at TDC. This does NOT change with rpm. Cylinder PRESSURE changes based on rpm, but that has nothing to do with compression RATIO. (So, Blackout Steve, the statement
                              "..the Dynamic Compression Ratio, is fixed when the engine is built and never changes during the operation of the engine."
                              is not BS.)

                              IMO, the concept of DCR is true but overrated.

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