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  • Exhaust Burp..

    Hi guys,

    So after getting my 2.5" straight through exhaust (not cat) i have noticed an issue. When hitting about 4500RPM at full throttle my muffler will start "burping" not a backfire, but a "boof boof boof" sounds. Almost like the exhaust cannot get out fast enough.

    The muffler I got was a Magnaflow straight through design. So I guess my question is:

    Is this boost breaking up @ high RPM?

    or

    Is this the muffler just making noise?

    Has anyone else ever noticed strange exhaust sounds at full boost and high RPM?

    I have tried to gap my plugs smaller, as well as adjust my timing -> which did seem to help a bit.

    Thanks.
    84' 300ZX Turbo / '86 300ZX NA to Turbo (Retired)

  • #2
    So you dont feel any performance differences just a different sound from the muffler? If you are experiencing spark blow out you would feel it studder under boost.
    85 Z31 6.0 LSX turbo 766whp/792wtq
    04 GTO, LS6, big cam, porting, N20... underway for summertime daily driver.

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    • #3
      SATAN wrote: So you dont feel any performance differences just a different sound from the muffler? If you are experiencing spark blow out you would feel it studder under boost.
      ok. describe this studdering a little better because I'll get that occasionally however it'll typically be at about 3k rpm's and its when I hit full boost. Often times i'll muscle my way around a car without downshifting, so i'll be at like 2500 rpm's in 4th gear and just floor it. It'll give me the brief hesitation that is typical of a MAFmobile however at about 3-3.5k rpm's i'll get a VERY BRIEF hesitation that seems to last about 1 full turn of the engine.
      Funny stories!

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      • #4
        That doesnt sound like spark blow out. Its when you hit a high boost level and it feels like a very short repeted fuel cut. like you are bouncing off of a revlimiter with a fast (hystirisis?) it will cut hesitate maybe 4-5 times a second under full boost or close to full boost. Its solved by closing down the gap on the plugs or running more spark/juice to the plugs. blow out wont just happen at one certain rpm point though. Im not sure what you are describing. could be blow out... close your plugs to about .028"-.030" and see if that helps any.
        85 Z31 6.0 LSX turbo 766whp/792wtq
        04 GTO, LS6, big cam, porting, N20... underway for summertime daily driver.

        Comment


        • #5
          no, its not like a rev limiter in the sense that it completely cuts power and continues cutting power, and its not at any one specific RPM, its over a range of like about 2500-3500 and it'll only happen once for a brief instant. It kind of feels like wheel hop minus the hopping off the ground. I don't think i'm doing a very good job of describing it but it kind of feels like the same VERY temporary loss of power that you get when you give it full throttle from a light cruise only it'll happen during full throttle accelleration at low rpm and it'll happen just once and then continue making normal power.
          Funny stories!

          Comment


          • #6
            Holy jeeesh, you guys started up on this forum fast.. just to add..

            Some other car forum's I have read have also mentioned this symptom at high RPM and between gear shifts. They had said this is unburnt fuel igniting in the muffler, and it may be a drawback of a straight through muffler design.

            I also do NOT feel a power loss when this is happening. And have noticed that if I keep giving it gas, it will sometimes work it's wasy through and stop.

            As stated I did not notice this at all before when I had the OEM muffler. It happened as soon as I drove home with the new muffler and exhaust.

            The other thing I noticed, was when I set my timing to 15BTDC, this noise started happening as soon as I hit boost, at about 3000RPM. I put my timing back up to about 23BTDC, and only noticed an occasional sound at about 4200+ RPM in 2nd, 3rd, and maybe 4th gears. However this sound always seems to come from the muffler.

            The sound I had when running @ 15BTDC seemed to be coming from more of the downpipe area.

            Just to note my car is a NA to Turbo '86 . Im running at about 7.5 PSI max on 92 octane @ 20ish BTDC. Car idles fine, and has no other symptoms.
            84' 300ZX Turbo / '86 300ZX NA to Turbo (Retired)

            Comment


            • #7
              So...a shot in the dark....
              With-out any w/b information, My guess is that you are hearing the "ripeness" of the engine at different rpms with different AFR's.
              I say this because you mention changes in base timing altering where it occurs at. The fact is, these changes do effectively alter the end AFR of the engine at all given points.

              *I once new this guy that swore he could tune his car by ear.....then it blew up....lean is mean, but lean can bite you in the arse!

              I recomend that you disclose the amount of boost you're running and what other mods you have. I also recomend that you eventually get into a 3" exhaust...2.5" is just so small for a turbo car when upping the boost and wanting to let the "tiger out of the engine".

              To be continued.....
              feed me more info.
              Dan 8)
              I am here to help...

              Comment


              • #8
                Hmm interesting,

                Well all my mods are:

                -Team Green Filter (from you) mounted to MAF
                -9:1 N/A to Turbo
                -Rebuilt Turbo
                -2.5" Straight through exhaust to Magnaflow dual tip muffler (no cat)
                -Manual Boost Controller @ max 7.5 PSI ish (until i can feel safe @ 9 ish)
                -Current timing @ 22BTDC
                -Still have all emissions, - AIV and A/C


                That's pretty much all the serious performance stuff. Ripeness of the engine, interesting. Yah I know I should have gotten the 3" exhaust. It just seemed a bit to much to me at first. After having it done piece by piece I ended up paying about $400 CAD for all materials and labour. I guess it would have cost me about another $300 CAD to get a CM exhaust, which would have been the cost of my entire turbo swap just about.

                Yah, I dunno :roll: I just am so picky about everything. And was curious if anyone else heard this noise before. Maybe I just got weirdly unlucky with the setup.

                Oh well. I guess I will see if anything blows up.

                Cheers.
                84' 300ZX Turbo / '86 300ZX NA to Turbo (Retired)

                Comment


                • #9
                  well..here's a generic overview, the answer is simple...
                  One setting is better, safer than the other...
                  obviously retarding the timing a bit more is safer. The fact is, with-out certian information....no answer anyone here can offer is exactly correct, just best guesses.

                  If you really want to get serious about modding, get the tools.
                  AFR has alot to do with upgrading,tuning, diagnosing.
                  Get yourself a w/b or befriend someone that has one.
                  8)
                  I am here to help...

                  Comment


                  • #10
                    I appreciate the input.. one question.. what is a "w/b"? ---eeerrr I dont mean to sound like an idiot.

                    Thanks!
                    84' 300ZX Turbo / '86 300ZX NA to Turbo (Retired)

                    Comment


                    • #11
                      w/b=wideband (allows for accurate air to fuel ratio readings)

                      recomendation..
                      LM-1
                      Search Lm-1 or Innovate.
                      I am here to help...

                      Comment


                      • #12
                        verbatim wrote: I appreciate the input.. one question.. what is a "w/b"? ---eeerrr I dont mean to sound like an idiot.

                        Thanks!
                        wideband O2 sensor
                        85 Z31 6.0 LSX turbo 766whp/792wtq
                        04 GTO, LS6, big cam, porting, N20... underway for summertime daily driver.

                        Comment


                        • #13
                          i was going to say it's normal for a straight through exhaust until i read the loss of power issue my 90 240sx has a buddy club specII exhaust which turned out to be a 3" pipe with a resonator ops: and it makes that kind of noise but no loss of power or response
                          Stocker 87 na z31 and a slightly modded 90 240sx

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