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Thread: Formula for discs and flow

  1. #1
    BigD Guest

    Default Formula for discs and flow

    Hi, I'm trading my 2010 Limited in on a Screaming Eagle Ultra Classic, and I'm going to have to start all over again tuning-wise. Currently I run a 103 engine with SE 254e cams, V&H Power Duals and Supertrapp Fat Shots mufflers with 18 discs per side and closed end cap. I don't even know if that's the correct number of discs, but I do know that both torque and HP went up significantly when I added 6 more to the original 12 per side. I wouldn't mind it a little quieter, but it's not bad the way it is, I think a happy medium between noise and performance.

    Now my SEUC comes with a 110 motor and SE 255 cams. Man, I really do wish you guys made a matching 2:1:2 header for these bikes, but I'll either go with Fuel Moto or the V&H Power Duals again and a new set of Fat Shots w/closed end cap. What I need to know is will it require more discs for the bigger engine? I know the engine size will probably add to the noise level on it's own, so can I get away with fewer discs? How many to start out with?

    Is there some kind of formula I can use to determine the number of discs based on the potential flow of the engine?

  2. #2
    Join Date
    Aug 2008
    Location
    Blackfoot, ID
    Posts
    120

    Default

    It would be nice if one formula could apply to all situations, but I doubt that will be the case again here. Sorry, there are too many atmospheric and engine variables to factor into a formula. But that's not all bad news, it just means you'll need to experiment with different diffuser combinations until you find your desired power and sound. For starters, I don't see how any fewer than 12 will meet your needs. Your new engine at 110ci has roughly 7% greater displacement, so mathematically you probably want to keep it in the back of your mind that whatever the other bike used, this bike will probably like something 7% bigger; 18 discs *1.07 equates to 19 discs.

    Personally that's not how I'd approach it. I'd start with 14 or 15 and see how she performs, then add 2 if you're using SuperTrapp's FAQ method. You might find a solution quicker if you add 4, ride, then add 4 more, etc, until either the sound is too great or power becomes undesirable. Then go back in steps of 2, then up/down 1 until she's perfect for you.

  3. #3
    BigD Guest

    Default

    Well, I was running 18 discs on each muffler on a 103" engine. The mufflers come with enough for 12 per side, and that 's what the FAQ says to start with. I assume you need to re-tune after changing the number of discs. That really requires unlimited time on a dyno to play with this stuff.

  4. #4
    bschultz Guest

    Default

    "Seat of the pants" tuning usually keeps dyno time to a minimum...

    Every rider is different and every bike is different so the perfect set up may vary for each person.

  5. #5
    Join Date
    Aug 2008
    Location
    Blackfoot, ID
    Posts
    120

    Default

    Quote Originally Posted by bschultz View Post
    "Seat of the pants" tuning usually keeps dyno time to a minimum...

    Every rider is different and every bike is different so the perfect set up may vary for each person.
    BigD, try not to get discouraged with tuning or tuning-to-perfection. This process is a lot easier than you think, especially if you check your spark plug color along with each major disc change. The only thing I can see holding you back would be time in the saddle due to the shorter and colder days. Seriously, I could tune the discs in a couple of hours, easy! And when I was done recording my butt-dyno riding impressions, I'd probably have several different setups that I liked.

    Has the 110 had its throttle bodies synchronized yet? That's an often forgotten tune item that'll help get her started in the right direction too.

  6. #6
    BigD Guest

    Default

    So, for the Fat Shots with a closed end cap, how many discs would be a good starting point for a 2011 110 CVO? If 12 is a starting point for a 103, I would think I need more for a 110. Am I wrong to think that way?

    I'm not familiar with synchronizing the throttle bodies. I do know they changed them for 2011 (surprise) and there's not enough material to bore them out anymore.

  7. #7
    bschultz Guest

    Default

    Start out with 14-15 discs and the closed end-cap (per side).

  8. #8
    BigD Guest

    Default

    See? there is a secret formula somewhere.

    Thanks!

  9. #9
    ajaya Guest

    Default

    Hey, a formula for this seems to be out of question according to me because as someone said here that every bike is different and every rider is also different. So we have only one option left out, which is to experiment with various combinations and get the best setup.

  10. #10
    bschultz Guest

    Default

    That pretty much sums it up; however we can give you an experienced place to start.
    Thanks for your comments ajaya, glad to see you are feeling better.

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