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Is bigger always better?

Discussion in 'General Tech and Maintenance' started by badbreedchiney, Dec 3, 2009.

  1. badbreedchiney

    badbreedchiney New Member

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    it seems i need to change out my exhaust system from headers to muffler. it currently has a 4-2-1header with a 2" piping straight through. i was considering to upgrade to a 2.5" seeing as the change has to be done but a friend is telling me that i will be robbed of power from my lil sohc zc. so is what he true? will i be robbed of power? is bigger always better? if not why isn't it better?

    i've always thought that because its bigger exhaust fumes can flow more freely straight out giving you a little more power.
     
  2. BrutalB83

    BrutalB83 Brutal Moderator Moderator VIP

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    Honestly, the 2" exhaust that's on there currently is probably about right. I wouldn't go any larger then 2.25" if you do upgrade. 2.5" is just too large for a tiny little engine like that that's running N/A...
     
  3. badbreedchiney

    badbreedchiney New Member

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    i found a header to buy, the guy said the inlet is 2.5". does this mean that its 2.5" where it bolts on to the engine and the piping size is different or is the piping 2.5" from where it bolts on to the motor straight to the other end of the header?
     
  4. FLounder

    FLounder power hungry VIP

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    must be referring to the flange size where the header bolts to the cat. most headers for hondas are either 2.25" or 2.5"
     
  5. K2e2vin

    K2e2vin Senior Member

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    I'm going to run 3" on my NA D15 :) The header has a 2.5" outlet though(Bisimoto header).
     
  6. phyregod

    phyregod !!YTINASNI

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    With N/A builds, Too big of an exhaust can hurt cylinder scavenging. Exhaust "pulses", and if you have the right size exhaust and a well designed header, the pulse from one cylinder will actually help suck the exhaust out of the next cylinder when the valves open. Too big and you lose all scavenging, too small and you create a bunch of pressure, which is bad.

    You want to aim for both a high flow AND high velocity exhaust system.

    With turbo builds, from what limited knowlege I have of turbo stuff, Bigger is just plain better. Up to a certain point, then it doesn't matter.
     
  7. K2e2vin

    K2e2vin Senior Member

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    In NA application, optimal sizing makes a larger difference in the header. After that, there's minimal pulses and as long as you have decent flow with zero backpressure, it will not cause any loss in power. Of course, many of the 3" systems on NA cars(including my setup), have noise suppression(muffler) and emissions devices(cat).
     
    Last edited: Dec 3, 2009
  8. badbreedchiney

    badbreedchiney New Member

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    thank you for all the help. from what i understand the 2.5" would rob me of power so now i'll set out to find me a nice 2" straight through.
     
  9. badbreedchiney

    badbreedchiney New Member

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    another question. 1.75 outlet 2" inlet would that be a good option?
     
  10. phunky.buddha

    phunky.buddha Admin with a big stick Admin VIP

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    Match the diameters if you can... if you're crush bending it won't really matter.
     
  11. phyregod

    phyregod !!YTINASNI

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    Bigger to smaller will create a bottle neck in the exhaust and hinder flow.
     
  12. BrutalB83

    BrutalB83 Brutal Moderator Moderator VIP

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    :werd:
     
  13. badbreedchiney

    badbreedchiney New Member

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    which side of the header is the inlet and which is the outlet?
     
  14. badbreedchiney

    badbreedchiney New Member

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    do headers come with inlet and outlet same size or different sizes at each end at all times?
     
  15. K2e2vin

    K2e2vin Senior Member

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    Depends on the design/bends. If you think about it, exhaust gasses will contract when cooled, thus resulting in lower exhaust velocities.

    And no; headers can have the same or different diameters; you just have to do research to find out specifically what each brand has.

    Inlet would be runner-end, outlet would be the flange.
     
  16. phyregod

    phyregod !!YTINASNI

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    That could be a valid arguement, except that it doesn't instantly change. You would have to have a conical length of pipe that was perfectly calibrated to the cooling exhaust for that to make any difference..
     
  17. caboose

    caboose USDM to the bone

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    In NA applications, larger exhaust will cause an increase in power, but that increase will come at a higher rpm. Running 2.5" might give you the highest hp gain, but you will sacrifice performance in the low/mid band. If you run too big, you might not be able to achieve enough velocity in the exhaust even at full throttle, and you will lose power.

    In turbo applications, you want to go as big as possible, because the turbo causes high back pressure before the turbo. The turbo runs off the difference in energy between the higher temperature and pressure gas leaving the motor, and the low temperature and pressure gas in the downpipe. That's why with a turbo, bigger is always better.
     
  18. K2e2vin

    K2e2vin Senior Member

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    Refer to the previous posts. Also, it's not that simple; as stated before the header makes the largest difference; in many instances some notice gains overall with higher gains in certain areas.

    Here's a dyno pulled from H-T(K20A, ref: http://www.honda-tech.com/showthread.php?t=2341715 ):
    [​IMG]
    Dotted is 2.5", Solid is 3"
     
  19. caboose

    caboose USDM to the bone

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    I agree that the primary header tube diameter is going to be driving factor. You will see gains across the board if the header diameter is small enough to be restricting flow at low rpm, but if you go to big you will only be seeing gains in the top end and might lose power in the low to mid.
     
  20. phyregod

    phyregod !!YTINASNI

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    Quoted from the guy that has that posted that sheet:

    This makes good sense.. the header is allowing the pulse effect to take place, along with the first section of the exhaust.. Once the pulse hits the cat, all bets are off, as the cat will kill it.. So from there out, a big gnarly free flowing exhaust would be the right thing to have..

    God damn it why couldn't I have had this info a couple of months ago when I build my exhaust?
     
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