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how big a pipes should i run on a dual exhaust?

Discussion in 'The Garage' started by jekbrown, Apr 23, 2003.

  1. jekbrown

    jekbrown I am CK5 Premium Member GMOTM Winner Author

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    ok gang, i am havin trouble deciding what to do about my exhaust. Basically, y-pipes suck azz so I wanna go true dual. Problem is i dont wanna buy 2 more cats. the one i have now is relatively new and a 3" model. behind it is a 3" flowmasta 50. With my mild 350, I dont wanna run dual 3" pipes... no backpressure. Would it work to buy another identical 3" cat... and then neck the piping down to 2.5 or even 2.25 after the cats and send it through 2.25-2.5" mufflers? any ideas appreciated. I really dont wanna pay to have a trick custom y-pipe made... nor do I wanna buy 2 new cats! /forums/images/graemlins/wink.gif /forums/images/graemlins/grin.gif

    j
     
  2. sled_dog

    sled_dog 1 ton status

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    thats what I would do, step it down to 2.5 or 2.25. If its a stock 350 2.25 may be a better idea.
     
  3. BowtieRed

    BowtieRed 1/2 ton status Author

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    i got 2" no cats, no mufflers i like it /forums/images/graemlins/laugh.gif
     
  4. Brian 89KBlazer

    Brian 89KBlazer 1/2 ton status

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    Flowmaster tech line told me that for a stock 350; you could get away with dual 2.25" exhaust. Since I've got a cam, headers, and a mild performance rebuild; they told me to go with a dual 2.5" so as not to restrict the engine's HP but still have good low end torque.

    Hope that helps some /forums/images/graemlins/tongue.gif
     
  5. jekbrown

    jekbrown I am CK5 Premium Member GMOTM Winner Author

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    yeah, the only perf upgrades my 350 has is an edel alum intake and edel headers. /forums/images/graemlins/frown.gif guess 2.25" would be best for duals eh? my only question now is... are there any problems inherant to necking the tube down in the middle of the system? dunno what those probs might be... i guess thats y i am asking. lol! thanks guys!

    j
     
  6. SUBFAN

    SUBFAN 1/2 ton status

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    Did your Edelbrocks come with the Y pipe? If so how did it work out?
    I was considering that setup, as I live close to work, and rarely get the chance to burn all the moisture out....
     
  7. jekbrown

    jekbrown I am CK5 Premium Member GMOTM Winner Author

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    yeah, it came with a y-pipe... problem is once i have my flexy front springs on my driveshaft is gonna hit the pipe and I cant adjust it out of the way any more than it already is.... even if I could, it means losing clearance in a place I'd rather not lose clearance. For most rigs it would prolly be fine... but my rig is gonna be pretty dang flexy and the 205 is tucked up nice and high... i dont wanna screw either of those traits up just to have a y pipe.

    j
     
  8. K5MONSTERCHEV

    K5MONSTERCHEV 1/2 ton status

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    doesn's smaller pipe make more torque?
     
  9. Beast388

    Beast388 1/2 ton status

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    I have the opposite of what you are going run exhaust wise. I have 2.5" pipe from the collectors to the 40 series Flowmasters, and then 3" pipe from the mufflers straight out the back. I went with bigger pipe after the mufflers so that the bends up over the axle wouldn't restrict flow.

    I wouldn't put anything less than 2.5" after the initial 3". The drastic change in diameter would hurt flow IMHO.

    FYI, I figured up the different volumes for the common exhaust pipe sizes:

    3.00" pipe = 84.823 cubic inches per foot of pipe
    2.50" pipe = 58.910 cubic inches per foot of pipe
    2.25" pipe = 47.710 cubic inches per foot of pipe
    2.00" pipe = 37.700 cubic inches per foot of pipe

    As you can see, going from 3" to 2.25" almost reduces the volume in half....not good for flow.
     
  10. K1Orion

    K1Orion 1/2 ton status

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    I asked a similiar question a while back. I decided to run 2.5" with a crossover pipe, 2 2.5" cats, and no mufflers. The crossover is supposed to help with torque.I haven't done it yet so I can't tell you how it sounds, sorry. You could do the same thing with 3" cats and use 2.5" pipe and probably not be hurting anything.
    I decided not to use mufflers originally and if it was too loud I could add them later.
     
  11. MudNurI

    MudNurI 1/2 ton status

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    I run 3" all the way.........

    with a 305, I'm not searchin for tons of power from my exhaust set up.
     
  12. jekbrown

    jekbrown I am CK5 Premium Member GMOTM Winner Author

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    [ QUOTE ]
    As you can see, going from 3" to 2.25" almost reduces the volume in half....not good for flow.

    [/ QUOTE ]

    it would cut it in half... but it'd be dual exhaust as opposed to the single i have now. Seems like if I wanted to maintain my current performance I'd want to cut flow in half... /forums/images/graemlins/confused.gif /forums/images/graemlins/confused.gif /forums/images/graemlins/confused.gif

    j
     
  13. Lonnie

    Lonnie 1/2 ton status

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    Keep in mind that although 2 smaller pipes can have similar area to 1 larger pipe, they will not work as well. Flow restriction is based on surface friction of the gasses moving through the pipe. Measuring the circumference of a pipe will show how dual exhaust has more contact surface area with the same overall area of a larger single pipe. A single pipe will also offer more torque as you essentially have all exhaust pulses in 1 pipe....the reason people use H or X pipe balance tubes to increase torque. I recently started using a single 4" exhaust on some Camaro buildups in place of dual 3" systems that I previously used on high HP cars. The result was similar HP even though the area was slightly less. Throttle response seemed better & noise was also reduced. Keep in mind that this was with mandrel bent tubing as crimp type bends do not work well on large pipe & substantially reduce flow on all sizes.
     

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