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

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

    Been a lot of discussion lately with pulley sizes and some people are still a bit confused on how the pulley ratios work.

    HD's come stock with a 3.25" upper and L's come stock with a 2.93" upper. Both trucks come with the same size lower.

    HD's stock are 6 lbs of boost and L's stock are 8 lbs of boost.

    Aftermarket pullies are named for the boost increase they add on a Lightning, NOT a harley truck. A 6 lb'er adds 6 lbs to a Lightning, it does NOT add 6 lbs to a harley truck. The rough general rule of thumb has been 2 lbs less for a HD, but since its a ratio, this isn't fully accurate. From a generalized standpoint, boost is a linear function of blower RPM. More RPM = more boost. While up high this is not true because hot air is less dense therefore less boost, and low RPM its a bit different as well, the generalized ideal boost level vs RPM for the middle of the "s-curve" of boost vs RPM can be linearized.

    http://www.nhtoc.com/vbforum/newthre...newthread&f=18
    That thread was our sticky for a while, but with everything in text form for some its a bit harder to interpret. I took Ryan's approach and threw it into a chart and a table that should clear it up for everyone.

    The first table is the calculations for pulley combos & net boost for L's vs. HD trucks

    The second table is the pulley combos for least to most boost on any truck depending on the combo of uppers & lower's. So a 4 lower & L upper makes more boost than a HD upper and 6 lower, etc.

    The chart is blower RPM vs net boost for the known L pulley combo's, and where the HD pulley combos fall into place at.

    This chart and tables are up for discussion as I may have overlooked something, but PLEASE READ THE ENTIRE LIST OF ASSUMPTIONS & EXAMPLE ON THE RIGHT OF THE CHART BEFORE BRINGING UP DISCUSSION POINTS!!!



    This will soon become a new sticky

    M.HYDE: 2000 HD F-150 #0356 of 8197, Built 05/18/00, Bought 07/03/00, 700/784, 09.89@138.78
    *Worlds Fastest Street HD: '08/'09/'10/'11/'12/'13****Worlds Fastest Eaton Powered HD: 12.19@112.48 11/17/07*
    D.JEKL: 2000 HD F-150 #1552 of 8197, Built 06/20/00, Bought 06/25/09, 372/450, 13.75@099.46 (*SOLD* 10/15/11)
    REPTAR: 2012 SVTF-150 #2893 of ????, Built 01/19/12, Bought 08/26/11, ???/???, 14.93@091.30
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    BADGER: 2013 Explorer Sport 3.5L TT, Built ??/??/??, Bought 05/14/13, ???/???, 14.53@096.25
    GEN.0L: 1985 XL F-150 w/'94 Lightning Donor Engine Swap, Holley Projection EFI, 03 HD Paint

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    Looks like you spent a lot of time making that. It looks good and hopefully should explain better. Thanks for taking that time.

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    That has to be the best and most useful explanation I have seen yet. Yes I read the old thread and understood it. But with the charts you have almost dumby proofed it. That is so much easier to understand and comprehend than anywhere else I have read. Amazing work

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    i aint even gonna argue that, what can i say
    well put together

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    Quote Originally Posted by Ryan@RhynoTune View Post
    Looks like you spent a lot of time making that. It looks good and hopefully should explain better. Thanks for taking that time.
    Helped out having your post in the other thread to give the gears in my head a kick start on putting the chart together I'm an engineer so I like numbers and graphs haha. What better way to spend a sunday afternoon right!?

    Quote Originally Posted by mdan575 View Post
    That has to be the best and most useful explanation I have seen yet. Yes I read the old thread and understood it. But with the charts you have almost dumby proofed it. That is so much easier to understand and comprehend than anywhere else I have read. Amazing work
    Thanks! I've been meaning to throw a graph together for it for years, but just never got around to it. Since I've been slacking on how-to's lately, figured I'd do something useful for once in a while other than pic whore haha

    Quote Originally Posted by BlairTruck View Post
    i aint even gonna argue that, what can i say
    well put together
    lol well thats no fun


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    Great info josh once again pulley concerns covered.

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    Josh,

    What you did is absolutely great collaboration of all the stuff that has come before! I hope your effort finally puts this issue "to bed" ... speaking of which? ... you probably need to spend more time in bed getting laid instead of doing "truck stuff"!!

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    World's Fastest Street HD
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    healthy balance of both Lee

    My gf was actually helping me out on it a little because she's an excel whiz, and excel was screwing me up a little on the intercepts for a bit. Thats what I get for doing it on somebody else's computer with ancient Office 2000 instead of 2007

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    Great stuff Josh... amazing how a HD with a 6# lower is spinning at nearly the same ratio as a L with a 2#. Your numbers and assumptions are right on. Thanks for taking the time to do this.

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    World's Fastest Street HD
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    not a problem. its been long overdue lol. better late than never I suppose though haha

    me doing my s/c conversion on Jekyll really got me thinking about it more. I was only going to do a 4 lb lower to keep the boost lower since I only wanted around 10 lbs, but with a HD upper and 4 lb lower thats only around 8.5 lbs. The full exhaust is going to offset the boost gains of the intake mods, so I'm still going to end up around 8.5ish. Just got off the phone with LFP, they're getting a 6 lb lower to me by Wednesday for my Dyno on Thursday!!

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    i still think your wrong but i have no arguement.

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    Sweet!! Sticky this!!

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    Quote Originally Posted by blairtruck View Post
    i still think your wrong but i have no arguement.
    +1

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    World's Fastest Street HD
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    Quote Originally Posted by BlairTruck View Post
    i still think your wrong but i have no arguement.
    Quote Originally Posted by tpr103 View Post
    +1
    Well thats no fun! Discuss! Whats the reasoning for thinking its wrong? I do agree it is hard to argue though LoL!

    Blower RPM = Boost. I know some guys want to say a 6 lb lower still adds 6 lbs on a HD. And the HD upper just yeilds 2 lbs less net boost than a L, and therefore a 6 lb lower on a HD is equivalent to a 4 lb lower on a L, but then you'd be saying that a 6 lb lower on a HD turning 14,538 RPM's makes the same net 12 lbs of boost as a 4 lb lower on a L turning 15,230 RPM's. 692 RPM difference in blower RPM is certainly worth some amount of boost! So a HD can't make the same 12 lbs net turning 700 RPM slower! Nothing magical there The L combo with a 4 lower is turning 11.5 times more per second than the HD combo with a 6 lower, that most certainly creates more boost.

    Quote Originally Posted by havinnoj View Post
    Sweet!! Sticky this!!
    I definitely will! Just giving it a few days in the "discussion" area before its stuck and forgotten about lol

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    Quote Originally Posted by Harley#356 View Post
    Well thats no fun! Discuss! Whats the reasoning for thinking its wrong? I do agree it is hard to argue though LoL!

    Blower RPM = Boost. I know some guys want to say a 6 lb lower still adds 6 lbs on a HD. And the HD upper just yeilds 2 lbs less net boost than a L, and therefore a 6 lb lower on a HD is equivalent to a 4 lb lower on a L, but then you'd be saying that a 6 lb lower on a HD turning 14,538 RPM's makes the same net 12 lbs of boost as a 4 lb lower on a L turning 15,230 RPM's. 692 RPM difference in blower RPM is certainly worth some amount of boost! So a HD can't make the same 12 lbs net turning 700 RPM slower! Nothing magical there The L combo with a 4 lower is turning 11.5 times more per second than the HD combo with a 6 lower, that most certainly creates more boost.



    I definitely will! Just giving it a few days in the "discussion" area before its stuck and forgotten about lol
    I just got into this little argument the other day....it is hard to argue with numbers.

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