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  • exhaust gas flow question

    Assuming that a 2.5" pipe flows around 500cfm, 3" pipe flows 750cfm, and a muffler flows 700cfm. Would the flow of an exhaust system with 2' of 2.5" pipe followed by 6' of 3" pipe and a muffler flow the amount of the largest restriction? Or would it be proportionately scaled based on lengths of each section? I.E. would the flow be around 500cfm or around 690cfm? Thanks in advance guys.

  • #2
    It all works as a system, and it's not a question of flowing X, it's a question of at what backpressure does the system flow X?

    I can flow 700 CFM out of a 1" stock Civic exhaust... it might take 50 psi of back pressure to do it and be sonic coming out, but it'll do it.

    How much power are you looking to push through your exhaust?
    '18 Chevrolet Volt - Electric fun hatch for DD duty!


    DefSport Koni Sleeve and Spring Perch Buy!!!
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    • #3
      400whp through a dual exhaust.

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      • #4
        Why are you trying to do a dual exhaust ?

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        • #5
          I already have a dual exhaust. What I am trying to figure out is how bad is the 2.5" section hurting power production. I went 2.5" only in a section for clearance issues. 3" is probably doable, but difficult.

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          • #6
            200 rwhp out of a 2.5" exhaust is no problem really. The muffler you use is going to dictate backpressure way more than the difference between a 2.5 and 3" exhaust tube at that power level.
            '18 Chevrolet Volt - Electric fun hatch for DD duty!


            DefSport Koni Sleeve and Spring Perch Buy!!!
            http://www.nissanroadracing.com/showthread.php?t=5902

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            • #7
              Was that area in the front or rear ?

              I'd really suggest doing something 3" off the headers to middle of car then into a single 3.5 or 4" which would work very well.

              Though if you're still running those bbody headers you really need to start from the problem...

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              • #8
                The clearance issue area is where the headers leave the engine bay. Biggest issue being by the steering rack.

                I just got a new to me Millermatic 130 that I plan to try out by dicing up the retarded Imapla headers. Primarily to gain enough clearance to run a brake booster again.

                I have been thinking about setting up a scale (or near scale) flow model up to simulate the 2.5" restriction. At work I have equipment to measure flow up to 1000LPM. The large diameter piping would exceed our 1000LPM standard.

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                • #9
                  Sell off the headers and buy a cheap weld up kit from summit or jegs.

                  Or Stainlessworks grab bag bends are awesome, that's what i'll be using this time around for mine which will be 1.875->2.125->3" tri-y, then dual 3" to about the rear of the trans where it will Y into a 3.5" single...

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                  • #10
                    Originally posted by Z28ricer View Post
                    Was that area in the front or rear ?

                    I'd really suggest doing something 3" off the headers to middle of car then into a single 3.5 or 4" which would work very well.

                    Though if you're still running those bbody headers you really need to start from the problem...
                    What are you running on your turd? A single out the back/side? I'm thinking of side exit pipes with an X-over in the middle to help balance pressure instead of just straight off the headers and out the sides.

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                    • #11
                      I think that the mufflers are less of a restriction than the 2.5" sections of pipe. IIRC the mufflers I have flowed around 650 to 700 CFM at 28inH20, but they don't muffle much of anything. I am not really worried about the 2.5" pipes flow at 400hp; i am more concerned that the 2.5" section may contributed to bottle-necking the motor down to 400hp.

                      To some extent the extra hp may be trivial, since I cant get what I have to the ground efficiently. I'm just concerned that I may be leaving 10+hp on the table, for $40 and a day's work.

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                      • #12
                        Then put some 3" oval in there all baller like.
                        '18 Chevrolet Volt - Electric fun hatch for DD duty!


                        DefSport Koni Sleeve and Spring Perch Buy!!!
                        http://www.nissanroadracing.com/showthread.php?t=5902

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                        • #13
                          I hadn't even considered oval piping.

                          You maybe onto something.

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                          • #14
                            Really doubt you'll see any gains with the increase in pipe dia, the headers are your bottleneck right now.

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                            • #15
                              Originally posted by Beejis60 View Post
                              What are you running on your turd? A single out the back/side? I'm thinking of side exit pipes with an X-over in the middle to help balance pressure instead of just straight off the headers and out the sides.
                              Rear half of an Apexi Noir

                              Running off the headers and out the side is stupid.

                              At minimum your 2nd plan.

                              I've fabbed setups with dual 2.5" over to single 3.5 just behind the transmission for others, one with a spintech 3.5" single, the other just no muffler, giant tips.

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