Exhaust: 2 into 1, or 2 into 2?
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Exhaust: 2 into 1, or 2 into 2?

This is a discussion on Exhaust: 2 into 1, or 2 into 2? within the Vintage Motorcycle Racing forums, part of the Caferacer.net Forums category; Just because I think its gonna get funny: At the risk of personal injury and altering existence as we know it, I pose this question. ...

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  1. #1
    Junior Member RichM's Avatar
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    Exhaust: 2 into 1, or 2 into 2?

    Just because I think its gonna get funny:

    At the risk of personal injury and altering existence as we know it, I pose this question.
    Is there an advantage of having a 2 into 1 exhaust system over a 2 into 2 system? More importantly, why do you say so?

    Go ahead - have fun.

  2. #2
    Senior Member craig641's Avatar
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    For the most part it depends on the motor in the bike. Is there a particular bike you're talking about?

    Two cylinder two stroke motors tend to need two separate exhausts.

    Craig

  3. #3
    Senior Member craig641's Avatar
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    For the most part it depends on the motor in the bike. Is there a particular bike you're talking about?

    Two cylinder two stroke motors tend to need two separate exhausts.

    Craig

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  5. #4
    Moderator jbranson's Avatar
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    Supposedly the 2-2 makes more power on a 180 degree crank twin, and the 2-1 makes more power on a 360 degree crank twin. I've never seen any conclusive evidence to support either. I built 2-1 systems for 175 twins because they were lighter and less prone to crash damage and sounded really cool. At the state of tune of most vintage bikes, even race bikes, I'm not sure it makes enough difference to worry about.

    I will say that 2-1 pipes on a 360 crank twin seem to make a noticeable difference in mid-range power on the race track, on a small displacement twin (which always need help in the mid-range). But I've never dyno tested them back to back with a 2-2 system.
    JohnnyB

    PS. I'm not sure if my reply was humorous, but everyone has a different opinion of what's funny.

  6. #5
    Moderator jbranson's Avatar
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    Supposedly the 2-2 makes more power on a 180 degree crank twin, and the 2-1 makes more power on a 360 degree crank twin. I've never seen any conclusive evidence to support either. I built 2-1 systems for 175 twins because they were lighter and less prone to crash damage and sounded really cool. At the state of tune of most vintage bikes, even race bikes, I'm not sure it makes enough difference to worry about.

    I will say that 2-1 pipes on a 360 crank twin seem to make a noticeable difference in mid-range power on the race track, on a small displacement twin (which always need help in the mid-range). But I've never dyno tested them back to back with a 2-2 system.
    JohnnyB

    PS. I'm not sure if my reply was humorous, but everyone has a different opinion of what's funny.

  7. #6
    Junior Member RichM's Avatar
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    OK JohnnyB -

    Why? Why does that "happen", or is it just a weight thing?

  8. #7
    Junior Member RichM's Avatar
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    OK JohnnyB -

    Why? Why does that "happen", or is it just a weight thing?

  9. #8
    Moderator jbranson's Avatar
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    Why does what happen? Mid-range power?

    Theory is that with a 360 degree twin with a 2-1 system exhaust pulses from one cylinder create a negative pressure wave in the system that helps evacuate gases from the other cylinder, and vice versa. The effect varies in efficiency with rpm, cam timing, primary header length, collector pipe length etc.
    JohnnyB


  10. #9
    Moderator jbranson's Avatar
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    Why does what happen? Mid-range power?

    Theory is that with a 360 degree twin with a 2-1 system exhaust pulses from one cylinder create a negative pressure wave in the system that helps evacuate gases from the other cylinder, and vice versa. The effect varies in efficiency with rpm, cam timing, primary header length, collector pipe length etc.
    JohnnyB


  11. #10
    Senior Member tabby's Avatar
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    With a 360 degree crank, all of the pulses are 180 degrees apart, or even firing, so one pulse can help scavenge the next, and the next, and so on. With a 180 degree crank, the lead pulse may scavenge the following pulse a little, but then both cylinders have a big wait as they both have to intake on top of each other. JohnnyB is right. It can work real well, as long as all of the other factors are optimized too. That usually means getting really lucky{not likely} or spending a boatload of time with a dyno, and lots of tubing.
    Think of it this way. I have a fairly simple brain, so I have to do things like this sometimes.

    360 degree crank twin = 1 bang-2 exhaust, 2 bang-1 exhaust, 1 bang-2 exhaust, 2 bang-1 exhaust,,,, See the symmetry?

    180 degree crank twin = 1 bang-2 bang, 1 exhaust-2 exhaust, 1 bang-2 bang, 1 exhaust-2 exhaust,,,,,
    If I wasn't sooo lazy, I'd do it out with drawings and proper degree spacings.
    Aging fart on a soon to be antique. 86 GSXR750, 883 Cosworth kit, head milled .015, 5 angle valve job, Yosh Stg 2 cams and spring kit, Extrudehone porting, Mikuni RS38s, Hindle exhaust, Falicon crank work, Carrillo rods, 17\" Dymag wheels, EBC rotors and pads, SS lines, Ohlins shock, \"Ti\" rear spring, Raceteched forks, 127 rwhp, 10.82 @ 130.7 in the 1/4. Not the bestest or fastest, but pretty frikkin tough.

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