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  1. #1
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    Exhaust bore question

    Hi all, I've been searching the threads on here about bore size and back pressure but can't seem to find the answer I'm looking for.

    I've bought a 888 coupe back box and want to put it on my z18xe coupe. I know the bore of the exhaust is bigger than standard, but my question is this.

    If I put in a non silenced centre section (remus, sportex cybox etc.) will that create enough pressure in the system to counteract the loss of back pressure (if any) by fitting the 888 back box?

    Hope someone can help.

  2. #2
    fdrightup
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    a non silenced centre section makes more sound also upto 1600cc exhaust inside diameter 50mm to 62.5mm and 1800 cc upwards 62.5mm to 91mm
    inside diameter but this depends on what other tuning has been done to the engine , if you have a loss of back pressure then you to rebalance the system by having more air and fuel entering the inlet,
    when i put a different back box on my car i lost performance so i had to
    to rebalance the system (eg) inlet side

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    OK.

    I haven't done anything yet, just want to make sure everything is right before I do.

    Might be easier to have a centre section made up?

    I've got a derestricted airbox with a K+N filter and thats it at the moment.

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    Also, would I lose back pressure by fitting the 888 backbox?

  5. #5
    fdrightup
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    Quote Originally Posted by MarkW View Post
    OK.

    I haven't done anything yet, just want to make sure everything is right before I do.

    Might be easier to have a centre section made up?

    I've got a derestricted airbox with a K+N filter and thats it at the moment.
    you could use the cat for back pressure also you could put bigger inlet and out let pipes on the cat, if you have a centre box and the baffle is 50mm inside diameter the longer the baffle the quieter the sound and no you do not lose power and the shorter the baffle more sound

  6. #6
    fdrightup
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    Quote Originally Posted by MarkW View Post
    Also, would I lose back pressure by fitting the 888 backbox?
    because every engine is different all you can do is try the back box then go from there
    (eg) some loses more and some loses less

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    The 'need' for back pressure is a myth. What you need to maintain is exhaust gas velocity in speed of the gas in the pipe. so it's a balance between getting the right bore - go too small and the back pressure increases (a bad things) and go too large the velocity decreases (a bad thing).

    If you're not planning on doing too much to the engine then 2-2¼" will be fine. The 888 back box won't really affect anything, assuming you've got a standard centre section

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    Ffoeg, thanks mate, what I wanted to do was have a stainless centre section mated up to the 888 rear box, so to confirm, I can have a 2.25" centre section made up to connect to the rear box and it shouldn't make any difference?

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    Quote Originally Posted by MarkW View Post
    ...it shouldn't make any difference?
    oh I'm not falling for that one


    It will make a difference, and hopefully a positive one

    The miltek pipes for NA cars are just a 2½" turbo pipe with a reducer section to mate to the 2" OE front pipe, which IMO is too big a dia for otherwise standard NA Astras. I'm running 180 BHp and only just think that size is right for mine.

    2¼" should be perfect for yours

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    Awesome, thanks mate.

    I'll get a centre section made up in 2.25" tube that then expands to mate up to the 888 box.

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