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Thread: Back to basics....L vs. HD Pulley Combo's...Explained in Chart/Graph Form

  1. #31
    World's Fastest Street HD
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    Quote Originally Posted by BlairTruck View Post
    my boost test was done the same days only hours a part
    i love your typically and usually gains, its guessing
    It's not guessing. Do you realize these trucks have been out for a DECADE now, and the "typical" and "usual" gains are exactly that. Those numbers aren't my guesses lol. Start reading into the archives on here, NLOC, and F-150 online and you'll see a heaping amount of posts that back those up directly. You know how many dozens and probably hundreds of people have done the exact same "same day tests" where they go WOT, then add an intake and go WOT and see where the boost increased to? Or added an exhaust and go WOT and see where the boost decreased to? Heck its been done thousands of times, pretty much every time a L gets a mod and hits a dyno the boost level increase/decrease is checked. That part certainly isn't rocket science! Heck do it yourself, throw an overdue intake on the truck and see what you gain in boost. I bet you its 1.5-2 lbs

    Apparently you have a freak magical truck that makes more boost without increasing blower RPM, with a precision calibrated boost gauge thats not off by even a tenth, and with exact ideal conditions during your one time test, to create 12 lbs with a 6 lower and 100% bone stock everything else, even though most of the other guys on here with a 6 lb'er also only make 12 lbs along with intake mods. Funny coincidence there huh?

    The accepted boost values for the lightning with various sized pullies could be wrong....although the boost sizes for the pullies have been the accepted values for the last decade without dispute, and why 9" lowers are called 6 lb lowers, 8.5" lowers called 4lb lowers, etc.. My table is based upon that, but you can even strip away all talk of boost levels, you could call it X, Y, and Z levels if you want and it wouldn't make a difference with that table.

    You have to have a basic understanding of physics to understand the table, which I can't think of how I can possibly dumb it down any more for you, but I'm sure you can agree its pretty common sense that increasing the pulley ratio by a larger lower or smaller upper directly increases the supercharger RPM. Your entire arguement saying a 6 lb lower adds 6 lbs on a L or HD, when the L and HD have different size uppers defies the fundamentel laws of physics. What you are saying is that the HD supercharger doesn't need to be spun as fast as a L supercharger to create an additional 6 lbs of boost over stock, and that is just absurd lol Its the exact same supercharger.

    If the uppers were the same, and say the lowers were what came different to decrease the boost on the HD from the factory, THEN adding a 6 lb'er on a HD would put the blower RPM equal to the same blower RPM on a L with a 6 lb lower. But since the HD has a larger upper, the supercharger is always spinning proportionally slower no matter what the lower, so its impossible that the HD blower spinning slower can create the same or more amount of boost than a L blower spinnng at equal or more RPM's.

    Quote Originally Posted by BlairTruck View Post

    im sure graphs can be wrong sometime and maybe even you could be wrong for once
    I have no problem admitting when I'm wrong, but I'm not wrong on this one, if I'm wrong on this, then Newton and the Laws of Physics are also wrong
    Last edited by Harley#356; 05-28-2010 at 11:13 PM.

  2. #32
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    C'mon guys, heres a formula for theoretical boost. Does not account for variances in efficiency of intakes and exhaust. So this will take the intakes and exhaust out of the equation.
    (blower volume in liters x pulley ratio) / (engine volume in liters / 2) x 14.7 - 14.7 = boost
    to get pulley ratio divide lower pulley by the upper pulley

    For the mile high handicapped in Denver, substitute "x 14.7 - 14.7" with "x 12.2 - 12.2"

    And for those of you that are REALLY spinning your blower it will be less efficient. Here is the formula that will help you to calculate.
    http://www.modularfords.com/forums/s...ower?p=1088426

    Josh - all of a sudden that Eaton boxed up in my garage isn't so magical.
    Last edited by Shady Al; 05-29-2010 at 08:09 AM. Reason: Air density

  3. #33
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    Hey guys,

    I just picked up a cog system for cheap, its and 8# lower with a harley upper 3.25. My current SC is an MP....I've deleted all 4 stock cats and replaced them with 2 High flows, I have a silver bullet catback. I also have the JLP intake and Thunder Pipe.

    My question is will the 8# and 3.25 and the MP be too much for my stock bottom end? How much boost will I make according to the chart provided by harley#358?

    Thanks!

  4. #34
    World's Fastest Street HD
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    the cog isn't going to make any more boost than a non-cog pulley combo of the same size. And it'll actually be closer to the actual values on the chart since it won't lose boost due to belt slip.

    8 lower with a stock size 3.25 upper is 15346 blower RPM which is 12.26 theoretical lbs of boost. With the stock manifolds still the high flow cats won't really lose much noticible boost, and you'll pickup an easy 2 lbs from the intake and thundertube, so you're looking at around 14.25-14.5 lbs of boost with an eaton. Now the MP I'm assuming its like the ported eatons where you can pickup boost just from the smooth blower case and improved inlet of the MP, so if that gains 2 lbs you're probably looking at 16.5 lbs

    It's doable with a good tune, but it is on the edge of that safety margin. Now that said I know guys with 20 lbs on their stock block with a MP for racing and running perfectly fine for the last few years, but personally its more than I'd feel comfortable with. I'd drop to a 6 lower if you can get one in a cog.

  5. #35
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    I've read that you want to spin the MP to around 16#'s in order to take advantage of it. So it sounds right where I'd like to be, offcourse I will upgrade fuel pumps not sure that I need 60lb injectors with my hp. I also have a Mafia waiting to be installed. Thanks Harley#356.
    Last edited by hemithis; 06-02-2010 at 10:14 PM.

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    WOW!!! Good stuff...really helps to explain an issue that when I asked others was somewhat nebulous to them in explaining!
    Thanks...Jim

    "Experience hath shewn, that even under the best forms of government those entrusted with power have, in time, and by slow operations, perverted it into tyranny".

    ~ Thomas Jefferson

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    Quote Originally Posted by Harley#356 View Post
    the cog isn't going to make any more boost than a non-cog pulley combo of the same size. And it'll actually be closer to the actual values on the chart since it won't lose boost due to belt slip.

    8 lower with a stock size 3.25 upper is 15346 blower RPM which is 12.26 theoretical lbs of boost. With the stock manifolds still the high flow cats won't really lose much noticible boost, and you'll pickup an easy 2 lbs from the intake and thundertube, so you're looking at around 14.25-14.5 lbs of boost with an eaton. Now the MP I'm assuming its like the ported eatons where you can pickup boost just from the smooth blower case and improved inlet of the MP, so if that gains 2 lbs you're probably looking at 16.5 lbs

    It's doable with a good tune, but it is on the edge of that safety margin. Now that said I know guys with 20 lbs on their stock block with a MP for racing and running perfectly fine for the last few years, but personally its more than I'd feel comfortable with. I'd drop to a 6 lower if you can get one in a cog.
    Nice chart and makes since...that being said I am putting an 8pound lower on my 03 HD because i have faith in your chart and I'm in florida so its also hot. Am i going to be okay running an 8 pound lower stock upper and CAI with the canned pulley tune from diablosport?????????????????????????

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    Great information. Quick question, would it make sense for someone with a Harley truck to start with upgrading to a lightning upper pulley?

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    Quote Originally Posted by Spoolx View Post
    Great information. Quick question, would it make sense for someone with a Harley truck to start with upgrading to a lightning upper pulley?
    Well yes and no...do you plan to do a lower pelley? If yes i would stick with the larger upper pulley so you have more belt rap and put a 8lb pulley on the bottom to make 14 to 15 pounds of boost which is about as much as you want on stock block and eaton....if i had to do it over again i would just throw a whipple twin screw on and be done

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