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For those who have a CF airbox: 5" or 6"?

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  • For those who have a CF airbox: 5" or 6"?

    I would like to know what is the size of the big opening at the box (where the flexible reinforced hose connects). Is it 5 inches, 6 inches. Reason why I ask is that I am making a similar one for my 2002 racecar which BTW also has the S14 7inch aluminum velocity stacks.


    [email protected]

    1969 2002 racecar
    1989 M3 racecar
    e39 Touring

  • #2
    Re: For those who have a CF airbox: 5" or 6"?

    Originally posted by LeeVuong
    I would like to know what is the size of the big opening at the box (where the flexible reinforced hose connects). Is it 5 inches, 6 inches. Reason why I ask is that I am making a similar one for my 2002 racecar which BTW also has the S14 7inch aluminum velocity stacks.
    Mine is 4-1/2"?

    T

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    • #3
      Yes its 4 1/2". I measured it the other day out of curiousity.



      calvin

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      • #4
        Originally posted by mfanatics
        Yes its 4 1/2". I measured it the other day out of curiousity.



        calvin
        Thanks guys. I will make mine at 5" diameter for less intake restriction.


        [email protected]

        1969 2002 racecar
        1989 M3 racecar
        e39 Touring

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        • #5
          Originally posted by LeeVuong
          Thanks guys. I will make mine at 5" diameter for less intake restriction.
          I'm pretty sure that enough R&D went into this airbox design so that it is set at that diameter for a reason.

          T

          Comment


          • #6
            The 405 HP Z06 breathes through an orifice smaller than 4 1/2", I really don't think there would be any restriction at all with a 4 1/2" OD. There are guys running up to 600 HP through 85mm MAF.

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            • #7
              I could be wrong but isn't the modern induction system have variable geometry?

              I thought the larger the intake opening the more you will loose low-end torque but better for the high-end? Porsche GT3 and M3 CSL have smaller intake opening in lower RPM's. The smaller diameter would speed up the flow rate. However, when the intake is applied along with the mapping system (such as the Alpha-N) the aforementioned concept might not be the case.

              If any of you can tell me if I am right or wrong I would appreciate.

              Mark

              Comment


              • #8
                Originally posted by //Mous3
                I could be wrong but isn't the modern induction system have variable geometry?

                I thought the larger the intake opening the more you will loose low-end torque but better for the high-end? Porsche GT3 and M3 CSL have smaller intake opening in lower RPM's. The smaller diameter would speed up the flow rate. However, when the intake is applied along with the mapping system (such as the Alpha-N) the aforementioned concept might not be the case.

                If any of you can tell me if I am right or wrong I would appreciate.

                Mark
                From my research, what you are saying is correct as it applies to the air horns that attach to the butterflies inside the airbox. I really think that with a 4 1/2" or larger opening on the airbox, the air horns act like they are in free air as opposed to having some sort of "tuned" air coming into them. But then again, I am not an engineer so I could be all wet on this.

                Comment


                • #9
                  It's a metric size.
                  I have looked high and low for that correct sized intake tubing here in the US to no avail.
                  My hose guy (who handles all kinds of ducting too) said its a metric sized ducting.
                  I tried to clamp the 5" down to it and it sucked.
                  4" no way.

                  So I am still using the old original piece I got from Markus ages ago.

                  If anyone locates some metric hose let me know, I'm in for some.
                  Cheers
                  jimmy p.
                  jimmy p.
                  87 E30 M3 Prodrive British Touring Car
                  88 E30 M3 Zinnoberot - Street
                  88 E30 M3 Lachsilber - Race (#98 SCCA SPU)
                  92 E30 M Technic Cabrio - S14 POWERED!
                  98 318Ti M44, Base - Morea Green
                  04 Ford F350 - V10

                  Comment


                  • #10
                    Remember that 4-1/2" opening was designed for the 9000 RPM DTM inhaling ability.:cool:

                    T

                    Comment


                    • #11
                      Even though the copy CF airboxes on the market are of very good quality, it doesn't mean that the DTM machines were using ones with the same specs. I kinda remember airboxes that were way bigger than those copies and had a bigger opening. Let's remember that the more air available immediately, the better the breathing. Which is why a bigger airbox is better. I would not be surprised if bigger airboxes had 5 inch openings or bigger. Just a thought.


                      [email protected]

                      1969 2002 racecar
                      1989 M3 racecar
                      e39 Touring

                      Comment


                      • #12
                        The speed of flow at available pressure differential

                        Originally posted by LeeVuong
                        XXXXXX
                        Let's remember that the more air available immediately, the better the breathing. Which is why a bigger airbox is better.
                        XXXXXX
                        Correct me if I am wrong;

                        The only pressure differential created in the intake side of a normally aspirated engine is by the movement of the pistons [creation of negative pressure]. There will be no flow without the pressure differential. Agree so far?

                        Imagine the intake plenum as a control volume area (at constant pressure) and the input and output have to sum up to zero. The amount of torque generated is roughly proportional to the amount of air going through the cylinder head.

                        Let say, your motor is running at constant 4000 rpm, so you’re consuming a constant volume of air. How can you speed up the flow? The only way I can think of is by lowering the cross sectional area, sort of like putting a nozzle in your garden hose.

                        So, my “guess” is if you slow the air flow down by opening the plenum, you will loose torque at the low rpm. What I recall is that there were so many versions of Gr. A intake plenums and if what you are saying is true, we would have seen much larger plenums.

                        And then again, I could be wrong.

                        Comment


                        • #13
                          if more is better might as well get longer intake runners and throw some fileters on them and make a hood scoupe righ too it. problem with using something that gives you as much air as a dtm box you need a built engine to make it work right

                          Shes sold now :(

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