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  1. #1
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    Ideal turbo manifold design.

    I want to make a manifold for my 335i it has 1.25 inch primaries and i want to merge each banks 3 cylinders into 1 pipe and have the bank 1 and 2 pipes lead up to a t4 divided flange. What is the optimal pipe size to have those 3 pipes collect into? I was thinking 2 inch since 3 1.25 pipes flow around the same as 1 2 inch pipe but i feel like it may be over kill considering the pulses are happening all at once. Any and all advice would be appreciated!
    Turbo lag is the on ramp to the highway which is power.

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    On an older platform, I had a twin scroll full-race manifold that had 2" collectors. It was perfect IMHO. I'd suggest you look up the full race website. I can't post links for some reason but it's easy to find.

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    im just worried because if i go too big it will cause slower spool and the fact that the pulses come one at a time really have me thinking i should just keep a constant pipe size
    Turbo lag is the on ramp to the highway which is power.

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    if you stick with a .63 a/r it will spool quicker.

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    Click here to enlarge Originally Posted by WDBi Click here to enlarge
    im just worried because if i go too big it will cause slower spool and the fact that the pulses come one at a time really have me thinking i should just keep a constant pipe size
    Intuitively, this makes sense to me. On a given bank, there will be one pulse every 240 of crank rotation. You will never have two pulses at once going into the collector. It makes sense to use a constant pipe diameter all the way to the divided flange to keep exhaust gas velocity high.

    Either way smooth transistions from the primaries to the secondary should be a priority.
    Eppur si muove.

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    Click here to enlarge Originally Posted by ajm8127 Click here to enlarge
    Intuitively, this makes sense to me. On a given bank, there will be one pulse every 240 of crank rotation. You will never have two pulses at once going into the collector. It makes sense to use a constant pipe diameter all the way to the divided flange to keep exhaust gas velocity high.

    Either way smooth transistions from the primaries to the secondary should be a priority.

    This is what I'm thinking I just think maybe a small maybe .5 or .25 increase in pipe may be better that's my instinct but when I think about it in reality just the same pipe makes sense
    Turbo lag is the on ramp to the highway which is power.

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    Click here to enlarge Originally Posted by WDBi Click here to enlarge
    This is what I'm thinking I just think maybe a small maybe .5 or .25 increase in pipe may be better that's my instinct but when I think about it in reality just the same pipe makes sense
    velocity is a good theory to apply here....


    exhaust gas velocity will dictate which size primaries and collectors to use.

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    From my experience it's really hard to say because of the amount of variables. There is turbo size (turbine and compressor wheel diameter, weight, angles on the turbine wheel, length to turbo... )however there is a company that specializes in these things and they have software to calculate the exact diameter of the pipes and collector setup, it's German based. Grtz
    Last edited by nicklowrey; 08-10-2013 at 01:57 AM. Reason: spelling

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    @DBFIU

    could you chime in? would it be best to merge 3 1.25 inch pipes into 1 2 and run those 2 2inch pipes to the divided flange or should i merge them to 1 1.25inch pipe to account for pulses?
    Turbo lag is the on ramp to the highway which is power.

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    Haven't seen @DBFIU in a bit. He's been busy with life.
    Stage 2 or 2.5 E9X M3 S65 V8 supercharger kit for sale: http://www.boostaddict.com/showthrea...r-kit-for-sale

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    Click here to enlarge Originally Posted by Sticky Click here to enlarge
    Haven't seen @DBFIU in a bit. He's been busy with life.
    bummer he was one of the most informative posters on here... However i did some digging and i found this http://www.kengineering.info/header%...%20article.htm

    according to this with 1.25 inch primaries 3 of them merging into 1 pipe needs to have an area between 2.1 and 2.6. Turns out a 1.75 inch pipe has a area of 2.4 so that would be perfect.

    so far i have that i want to do a front and rear 3 cylinders with 1.25 inch primaries merge into 2 1.75 inch pipes and those 2 pipes go up and meet a t4 divided flange. the 2 banks will never actually touch until after the manifold.

    Now after reading that article i would like to calculate my primary ideal length. For that i need this info P= 850 x ED divided by RPM - 3 where P= primary length, ED= 180 + the number of degrees before bottom dead center that the exhaust valve opens, and RPM = the RPM you want to tune for.

    so can anyone chime in on the degrees before bottom dead center that out valves open?


    quick edit turns out the flange i will be using is made for schedule 40 1 1/4 inch butt weld pipe. according to mcmaster carr the ID is actually 1.38 inches which means the area of the collecting pipe needs to be between 3.2 and 2.6 which would better support a 2 inch collecting pipe so i will still use 1 1/4 inch schedule 40 but it turns out the actual ID is a bit bigger than 1.25 which is weird so i will just adjust my collector size to be 2 inches
    Last edited by WDBi; 08-15-2013 at 08:36 PM.
    Turbo lag is the on ramp to the highway which is power.

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    any updates to this? I'm liking the engineering effort going into the design!

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