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Freiburger, DCR, and testing.....

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

    Re: Freiburger, DCR, and testing.....

    Can you see what a 51 cc head would be Eric
    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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    • Re: Freiburger, DCR, and testing.....

      Re: Freiburger, DCR, and testing.....

      Jeff, 14.0:1 static compression, 9.3:1 DCR, 229 psi cranking pressure. That'll make some torque ;D

      Dave, I need to think about this a bit -- in the mean time, what exactly do you think is the "breach of logic"? We're taking a known volume of gas at a known pressure and applying a compression ratio to it -- then using some corrections for temp, VE etc to estimate cranking compression. Actually, I am not calculating anything . . . in fact I don't even understand all the nuts and bolts of the cranking PSI formula (just the simpler DCR ratio calculation) so maybe my description of how you estimate cranking pressure is off.

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

        Re: Freiburger, DCR, and testing.....

        Since you're taking me to school, I still need to understand how "compression ratio," or even an output expressed as a ratio, is used to describe or measure the differences in pre-combustion cylinder pressure based on inlet events that happen prior to the intake valve closing. Really. I still see it as a breach of logic, so there's something here I'm obviously not seeing.
        There's no way I'm taking you to school - you're the prof!

        But it all seems logical to me. The ratios give you a picture of the statics and what happens before the intake closes is the dynamic part. Different combos/rpms would lead to different pressures at the point the intake closes, and therefore different pressures after compression.

        Tampa, FL

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

          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.

          I agree that it is over rated. The point you made in another post is more accurate. The pressure changes from fill efficiency of the combination but not the ratio itself. It is fixed as your static compression is.

          When you choose a cam with a later closing point to "allow" a better static combination,it can lead to pressure problems in relation to octane in some situations. The engine is effectively a shorter stroke engine with less displacement and big sloppy cam. When you reach the point that your fill efficiency matches up you have the same displacement as your DCR creates, with your assembled "real" static compression value realized.

          If the quench is tight, and it stays inneficient past the "lugging" point the fuel octane can be lower than you would expect. But, if your engine displaces 400 cubes at 12:1 and your DCR at 8:1 displacement makes it a 325 inch motor, at full efficiecy it is a 12:1 325 inch motor. So if it is anything it should be called "Effective Displacement". The 8:1 DCR is less important to me than how much displacement at what static compression ratio am I getting.

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

            Re: Freiburger, DCR, and testing.....

            Bill,

            The idea behind matching compression ratio to cam (via a DCR calculator or otherwise) is to try to maintain cylinder pressure (and consequently TQ) when using a big cam -- or conversely to reduce/maintain cylinder pressure with a small cam. The goal is to maximize cylinder pressure by using the optimum IVC timing and static compression ratio without getting exceeding the detonation threshold of a particular octane fuel. When you maximize cylinder pressure you maximize TQ so more static compression is a way to keep that "big sloppy cam" from acting big and sloppy.

            Where I personally feel you need to watch out when using a DCR calc is when you are looking at extreme ends of the "norm." Especially real high static compression ratios with real big cams, excellent cylinder heads and a high VE. You just cannot take a 13:1 engine and put a 320* advertised / 280* @ .050 duration cam in it and expect to race it on pump 93 octane because the DCR is 8.3:1.

            I agree that some people make too big a deal about DCR, especially when not knowing how to use the info. But I will stand by my earlier point that a DCR calculator is a helpful tool (especially when building a street strip / pump gas deal), but it is just ONE tool and many things need to be considered when planning an engine build. There is a limit to what it can be used for and like any tool, it's important to know the limits.

            Comment


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

              Re: Freiburger, DCR, and testing.....

              HAHA....and all I asked for was a test. 154 posts later......

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

                Re: Freiburger, DCR, and testing.....

                Hey Eric my NEW : Digital comp Gauge says 178 psi some guys have all the right answers LOL :'( ;)
                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!

                Comment


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

                  Re: Freiburger, DCR, and testing.....

                  Bill,

                  The idea behind matching compression ratio to cam (via a DCR calculator or otherwise) is to try to maintain cylinder pressure (and consequently TQ) when using a big cam -- or conversely to reduce/maintain cylinder pressure with a small cam. The goal is to maximize cylinder pressure by using the optimum IVC timing and static compression ratio without getting exceeding the detonation threshold of a particular octane fuel. When you maximize cylinder pressure you maximize TQ so more static compression is a way to keep that "big sloppy cam" from acting big and sloppy.

                  Where I personally feel you need to watch out when using a DCR calc is when you are looking at extreme ends of the "norm." Especially real high static compression ratios with real big cams, excellent cylinder heads and a high VE. You just cannot take a 13:1 engine and put a 320* advertised / 280* @ .050 duration cam in it and expect to race it on pump 93 octane because the DCR is 8.3:1.

                  I agree that some people make too big a deal about DCR, especially when not knowing how to use the info. But I will stand by my earlier point that a DCR calculator is a helpful tool (especially when building a street strip / pump gas deal), but it is just ONE tool and many things need to be considered when planning an engine build. There is a limit to what it can be used for and like any tool, it's important to know the limits.
                  It looks like we agree. To me its the pressure increase from fill efficiency that makes the smaller displacement via stroke of the compression stroke governed by the cam timing that makes or breaks it. Octane needs are governed by load. My example of 325 ci 12:1 (400 actual with 8:1 DCR) will forgive pump gas better in a lighter and lower geared cat than it would a 5000 lb truck.

                  Both ends of the extremes is another good point there is no substitute for knowing what you ave and building it accordingly. Good detailed and scienced combinatons obtain desired results. Always.

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