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Freiburger, DCR, and testing.....

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  • #46
    Re: Freiburger, DCR, and testing.....

    Originally posted by joe_rocket45
    That's my point, WHEN is the intake considered closed? at 0.000" lift at zero rpm, and it gets closer to .050" as the rpm rises resulting in a greater dynamic stroke and hence greater dynamic compression.
    It's going to vary with different engines, but really how much does a piston move in the time it takes for the valve to move that .050"? I'm guessing it's neglegable, but some one more familiar with engine geometries than me would have to do the calculation. I'm guessing it couldn't be more than a one cubic inch (On a SBC 350 4" bore that's ~.08" of the stroke or about 2% of the stroke)
    Escaped on a technicality.

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    • #47
      Re: Freiburger, DCR, and testing.....

      I've never been one to get into the whole DCR bull that goes around the internet because most of it IS just bull. DCR, like regular missionary position compression ratios don't mean anything except give you a starting point and a general reference. Looking up how much DCR you can run with pump gas is another useless piece of information.

      As a point of interest look at the engines that were run in the Engine Masters Challenge. These are required to have a max of 10.5:1 static compression and run on 93 octane pump gas. The SBC we built had 260 psi of cranking compression and a DCR of right about 8.4:1, both pretty high for a pump gas engine but it run's fine ( just went 6.5's in my friends Malibu yesterday). On the other hand, running a stock compression Dodge Magnum 360 with a realistic 8.8:1 static compression ratio and a DCR of 7.8:1 could rattle the heck out of the pistons like my pickup does every day. There are too many other variables out there that affect the final outcome of the engine as it relates to fuel octane requirements. Chamber shape, piston and head material, coatings, piston and chamber finish, RPM and load on the engine, ignition timing, spark plug design and heat range, intake manifold design (fuel and air distribution equality), carbon buildup, sharp edges, and whatever else I can't come up with off the top of my head. Sorry if I'm I'm ranting, it's just that there are SO many things to consider when building an engine that focusing on something like DCR seems a waste of time. The best thing I can say about it is that if you break down what is going on inside an engine and what really the DCR is made up of then you can get a really good picture of how the dynamics of an engine work and how important the intake valve closing point is. Just realize that, as has been pointed out, RPM plays a significant role in stuffing the cylinders with air and fuel.

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      • #48
        Re: Freiburger, DCR, and testing.....

        Originally posted by Freiburger
        Originally posted by BlackoutSteve
        So, there is either more cylinder volume being compressed in to the combustion chamber, or less. That is what changes compression ratios. Period.
        OK, I'll stop arguing after this one because it's clear we're not going to agree.

        My basic point is this: a ratio is a fixed number: its one cylinder volume divided by another cylinder volume. In the case of a 350 Chevy with 10:1 static compression, it is 43.73 cubic inches divided by 4.373 cubic inches. Reardless of when you decide to calculate the starting volume (ie, no matter when you decide that the intake valve is "closed"), it is still a fixed ratio of one displacement divided by another displacement. When you are doing a ratio of the ci displacement (volume) of one container to the ci displacement of another container, it does not matter at all what the density or pressure is of the material inside the container.

        You say "there is either more cylinder volume being compressed into the combustion chamber, or less. That's what changes compression ratios." I say the volume of the chamber before and after the compression event never changes. Volume is a fixed space. Per Mirriam-Webster, the definition of volume is "the amount of space occupied by a three-dimensional object as measured in cubic units (as quarts or liters) : cubic capacity." I don't think you can argue that the cubic capacity of the cylinder changes with rpm or mass flow.

        Things DO change with rpm: It's the density and pressure of the air/fuel mixture in the chamber that changes, but those variables are never measured in a volumetric ratio.

        I'm done.
        Sounds more like a static ratio you're explaining. Static including the rpm -whatever that might be..

        Me too, I'm done. I'm still going with Reher Morrison on this. :-*

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        • #49
          Re: Freiburger, DCR, and testing.....

          Originally posted by BlackoutSteve

          Who's talking pressure now. ;)

          So, things do change with rpm huh?
          Yeah of course things change with RPM, but compression ratio's don't. The VE is also different than dynamic compression. They are correlative, but not the same.

          Originally posted by Freiburger
          OK, I'll stop arguing after this one because it's clear we're not going to agree.
          I'm going to go with this too, I may add here or there, but I don't see an agreement coming up anytime soon, especially since the topic makes it's way around different forums with regularity.
          Escaped on a technicality.

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          • #50
            Re: Freiburger, DCR, and testing.....

            Originally posted by revolutionary
            The SBC we built had 260 psi of cranking compression and a DCR of right about 8.4:1, both pretty high for a pump gas engine but it run's fine ( just went 6.5's in my friends Malibu yesterday). On the other hand, running a stock compression Dodge Magnum 360 with a realistic 8.8:1 static compression ratio and a DCR of 7.8:1 could rattle the heck out of the pistons like my pickup does every day.
            By the fact you can pin a number on the Dynamic Compression shows that it doesn't change.

            (Sorry it was posted while I was typing my last post :-X )
            Escaped on a technicality.

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            • #51
              Re: Freiburger, DCR, and testing.....

              OK, but just remember that rpm isn't part of the formula. ;D

              That makes it a "static DCR" in my book. :D

              Cheers. Thanks for the debate guys.. ;) :-*

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              • #52
                Re: Freiburger, DCR, and testing.....

                Speaking of debates.Will the plane fly?Nevermind!

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                • #53
                  Re: Freiburger, DCR, and testing.....

                  No problem ;D
                  Escaped on a technicality.

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                  • #54
                    Re: Freiburger, DCR, and testing.....

                    Originally posted by Freiburger
                    But my point is the chart is WRONG. It shows "blown compression ratio equivalent" (bogus concept) numbers that would never run on pump gas...but they do.
                    Yeah, I have wondered about that myself...

                    You bring up another term I don't get..."effective" compression ratio. I think that's the same as "dynamic." I think you mean "the blower motor makes more power with less STATIC compression then the NA engine because there's more air/fuel in the clinder.
                    Think if it as the compression ratio INCLUDING the blower....the air enters the blower, it gets compressed twice (first by the blower, then by the piston) by the time it's ready to light off.

                    Does that make more sense?
                    My fabulous web page

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

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                    • #55
                      Re: Freiburger, DCR, and testing.....

                      My last post also.

                      I want to leave something to think about...

                      The idea of DCR is that the engine only starts to build compression once air can no longer leave the cylinder.

                      at 1 rpm this would be when the intake valve is on the seat.

                      The common DCR calculators use a number generally about 1/2 way between the advertised duration and 050".

                      This is because as the engine is being cranked it is NOT at 1 rpm, closer to 500. and at that rpm the air lost after 050 is negligible.

                      It would then stand to reason that as rpm is further increased the point that compression starts to build is closer yet the 050", and at extremes could even be before 050".

                      also consider that a cam may have an advertised duration of 280* but a 050" duration of 224*, that's 56*.

                      This has NOTHING to do with air density of the incoming air, only the point at which compression begins to build.

                      I only leave this as a thinking point, and will not reply to posts. just consider HOW the formula works and ask yourself if it is accurate at 1 rpm....

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                      • #56
                        Re: Freiburger, DCR, and testing.....

                        "Think if it as the compression ratio INCLUDING the blower....the air enters the blower, it gets compressed twice (first by the blower, then by the piston) by the time it's ready to light off."


                        Not to me. See my prior argument that a ratio calculation includes cylinder displacement alone, not mass, density, or pressure of the air entering the cylinder or in the cylinder. Your opinion may vary.

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                        • #57
                          Re: Freiburger, DCR, and testing.....

                          I don't see the difference, since the blower is a positive displacement device...just like the cylinder in the engine....

                          My fabulous web page

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

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                          • #58
                            Re: Freiburger, DCR, and testing.....

                            Well, this has been quite a thread! I use DCR only as a “figure-of-merit”. It allows a quick and crude comparison of two different dimensions. You might get a “feel” for potential detonation characteristics, but there are so many variables, to put significant focus on DCR is naïve.

                            Secondly Steve, Dave is right. Get over the VE thing (as-in, inertia, flow, density, acoustical waves, Doppler-shift, or whatever). DCR is only a comparison of two linear dimensions and nothing more. It is the calculated volumes at BDC vs. the calculated volume from intake closing ABDC. You know that. Everyone knows that. And as stated, compression ratio is mechanically built into the motor. It will never change. It is the ratio of total volume squeezed into the combustion chamber. The density, pressure (blown, unblown, altitudes at Pikes Peak or Death Valley) has nothing to do with it, period! What you are trying to do is establish an equivalent figure-of-merit for VE effects. That is commendable and I would love to have such a value but it just confuses the DCR which is not defined that way.

                            A precise DCR is impossible! Before I get flamed off this list please let me explain. Supposedly, DCR is calculated from the point when flow stops as the intake closes. Now here is why it is a Bogus (Dave’s words) and misleading number. Nobody on this planet knows when or where the flow stops! And therefore, nobody knows when or where effective compression starts! You will hear valve closings of .050”, .023”, or nothing at all, but nobody knows because it is different for every engine. It is as different as everything that effects VE!

                            As the valve closes, at some point across the seat interface, turbulence will develop. Turbulence impedes flow all the way up to when it becomes supersonic and then flow stops. Things that influence the onset of turbulence are the orifice shape (i.e., the valve-to-seat interface), delta P, density, and viscosity (they are different). The seat shape could be 1 angle, 3 angle, round, semi-round, different degrees (30, 35, 45 etc) recessed, ad infinitum. Even individual cylinders in the same engine will have differences. Now consider the inability to predict when flow stops, along with rod angles, cam profiles, and everything else that influences VE, and you can see there is no finite number that can be calculated. It is all a crude estimate, and should just be a lower-order tool, out of many, that a smart builder has in his quiver to arrive at an optimized combination.

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                            • #59
                              Re: Freiburger, DCR, and testing.....

                              Originally posted by Saltfever
                              Secondly Steve, Dave is right. Get over the VE thing (as-in, inertia, flow, density, acoustical waves, Doppler-shift, or whatever).
                              It's not something to "get over", as you put it.
                              In a previous example of 43.37in³ entering a cylinder to create a 10:1 c/r.. What happens when a certain amount of that 43.37in³ is reversed back into the intake. That is a real-world occurance, just like the opposite happening a high engine speed.

                              DCR formulas rely on a rigid bunch of numbers that suggest that a cylinder induces exactly the same amount of air throughout the engine's rpm range to arrive at a never changing compression ratio. Sorry, but that is pure balony. When the cylinder's volume changes due to how well it breathes, that alters the CR. Continuously.

                              Like I said, I'm with Reher-Morrison on this one. ;)

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                              • #60
                                Re: Freiburger, DCR, and testing.....

                                I have been thinking about the use of the term "dynamic" for a static calculation. It is easy to see understand "static" compression because it is fixed by geometry and doesn't change. However, that static length at BDC moves to a new location and becomes shorter based on the when the valve closes. Maybe the fact that the length changes (becomes shorter) and does move to a different location (ABDC) it becomes a dynamic dimension! Kinda crazy . . . how does that work for your guys? ;D

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