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Thread: what is you exhaust size?

  1. #1
    Regular Member Jaspa's Avatar
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    Vehicle : Vectra

    Trim : GSi

    Engine : Z32SE

    Year : 53

    Mileage : 57,000

    Default what is you exhaust size?

    hi all,

    well been giving this some more thought and was wondering what sort of exhaust diameters people are using?

    i have a 3.2 v6 and plan on getting the whole lot done at some point next year.

    i know the scorpion systems are 2 1/4'' but to me that doesnt sound enough??? i can imagine you turbo boys using 2.75-3'' but i dont know what is right for me im thinking at least 2.5'' possibly 2.75

    there is a local exhaust company near me willing to make all this for me but i have to request the diameters....bigger the pricer obv.

    so i have been reading up and at the moment i stand as follows...


    6x1.75'' headers (2x 3 into 1)>>>> 2x 2.25 or 2.5'' downpipes>>>> 1x 2.75 or 3'' centre pipe incl. sports cat and middle silencer>>>> 2x 2.25 or 2.5 '' rear pipes and back boxes

    anyone have any input on this....cheers!

  2. #2
    Regular Member accveccysriturbo's Avatar
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    Default

    Depends on who you speak to mate.

    When I was looking round I phoned 5/6 different exhaust places and was told these three things.

    1. You don't need to increase exhaust diameter beyond that which you have using an example of a 500 bhp BMW they had running standard exhaust.

    2. 3" is too large as the exhaust won't build up enough pressure and the gases will just float around inside the system.

    3. 2.75" into 2.5/2.25 back boxes is too much again - stating pressure won't build up.


    Anyhow I went to MIJ and spoke with them, they said 3" system would be over kill and used their Skyline as an example so ended up with 2.5" system into 2" back boxes.

    Hope that helps, it confused me for ages. Given Maptun promote a 2.5" system with 3" down pipe made me feel happy about my decision

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    Default

    I'd be speaking with a proper exhaust fabrication place, they should be telling you what the difference in sizes makes. I.e bigger will loose torque but gain bhp etc.

  4. #4
    Regular Member hagga's Avatar
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    Default

    Quote Originally Posted by wazpaz View Post
    I'd be speaking with a proper exhaust fabrication place, they should be telling you what the difference in sizes makes. I.e bigger will loose torque but gain bhp etc.
    As wazpaz says.. you can't just knock them up... important too is your header dia/length.. they're worked out to your cc/rpm limit etc... http://www.custom-car.us/exhaust/header.aspx
    .. mate had worked out what they should be for the v6.. i'll get on to him and find out this weekend if he's about.

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    Default

    NA exhaust tuning is quite tricky (as mentioned above). Us turbo guys do have it a little easier as turbos, generally, hate back pressure therefore going a bit overboard on the bore doesn't necesserily kill performance but it can with a NA, although V6s are a little more tollerant.

    I had a 2.5 V6 with a 2 1/4" cat back zorst and it was fine but it was on the limit once I threw 70 brake of nitrous through it. I would say 2.5" would be ok for a modded 3.2 though.
    That said, my biggest gain came from replacing my log headers with tubular items. They totally free'd up the 2 1/4" zorst when added to a 2.5" collecting down pipe the biggest gains were coming at the top of rev range where before it was falling off. e.g. I was using the V6's natural torque to pull me to 5.5Krevs then changing up but with the headers you still had the torque but with added power all the way to the 7K red line.

    I don't know what the availability is for tubular manifolds on the 3.2 but if you like revving your motor I would defo look into running a set. They make a V6 sound awesome and knocked half a second off my quarter mile time too.

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