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Exhaust Diameter Question: 51mm mid-link?

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  • Exhaust Diameter Question: 51mm mid-link?

    Not expecting much input but thought I would ask anyway. I have seen some after market pipes that neck down the exhaust to 51mm diameter. Unless I am majorly mistaken, that is not the stock exhaust sizing. I thought it was 61mm for a slip on. I am guessing that going to 51mm wouldn't really impact HP or torque but thought I would ask the collective and see what anyone thought. If the answer is no, then my track bike just picked up quite a few more options for a cheap pipe.

    Thanks

  • #2
    If I had to take a guess, its probably right on the sweet spot of exhaust flow.

    (assuming you're talking about the 1st gen bikes) The EXUP valve and stock exhaust collector, even when the valve is wide open, create a fair amount of backpressure. So even with the reduced diameter, the flow is still significantly better with the aftermarket headers/collector

    I can only assume they did testing and found the 51mm is the sweet spot



    I like this analogy

    In a very over simplified way, I like to think exhaust gasses are like water in a garden hose

    Too tight and it takes a lot of energy for the water to flow (restricts the engine)

    Too loose and the water just gurgles out without any real flow (Effects how exhaust gases leave the combustion chamber)

    If its just right, the water is pressurized to flow the most volume with the least restrictions

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    • #3
      The stock slip on is 61mm where it has been expanded to slip over the pipe from the exup. The pipe itself is more like 58.

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      • #4
        Originally posted by DannyDevitoo View Post
        ...I like this analogy: In a very over simplified way, I like to think exhaust gasses are like water in a garden hose
        ..Too loose and the water just gurgles out without any real flow (Effects how exhaust gases leave the combustion chamber)..
        FWIW, agree with the 1st part, but not so much with this last bit. There's pressure trapped inside the cylinder left over after doing some work, & is trying to escape. Once the exhaust valve opens, gas rushes out into the lower pressure area. The 2nd reason that analogy doesn't work, is because of enthalpy. When the exhaust valve opens, the hot gases can expand into the cooler chamber of the exhaust pipe. This force is not insignificant, jet engines operate on that principle for eg.

        Correct Ex. pipe Dia. & pipe length can assist in gas scavenging by resonating at appropriate frequencies - like organ pipes, ie the principle of tuned exhaust. So, if you're looking to pick up 2~3 free hp, pipe diameter & length is something you might want to consider. Google Helmholtz online calculator & play with it. It'll show you "ideal" pipe lengths for a given diameter to get a performance increase around a certain rpm. For a street bike, you probably want that to happen around the rpm where peak torque happens on the dyno sheet. There's a bunch of them posted here - search is your friend.

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