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Lowering Usable Rpm

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Paxx, Nov 20, 2003.

  1. Paxx

    Paxx 1/2 ton status

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    I would like to know what modifications I can make too lower the powerband on my truck. Ok this is what I have under the hood.

    350 cubic inches:(.030" overbore for a total of 355ci )
    Balanced and blueprinted by King Balancing
    Isky Camshaft: 262 supercam(P/N201262): .290 lift, 262° duration, 108° lobe center
    Edelbrock Performer #2101 Intake Manifold
    9:1 compression ratio
    Standard Chevrolet Heads
    Quadrajet 4MV, 750 CFM carburetor

    The engine runs great and produces good power but it makes the power starting at about 2000rpm. I would like to see power starting at 1500 or even possibly 1000 if that's not too difficult.

    What do you guys think? I am thinking headers and exhaust will make a difference but I don't know how much and other than that I have no idea.
    Any help will be greatly valued /forums/images/graemlins/thumb.gif
     
  2. SUBFAN

    SUBFAN 1/2 ton status

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    That 108* lobe center hurts....

    I don't remember which, but advancing/retarding the cam is an option....
     
  3. 84_Chevy_K10

    84_Chevy_K10 Banned

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    I believe retarding the cam is what would be done here.

    Changing cam timing seems to move the power band but not change the numbers at all per DD 2000. Basically, just moves the torque curve down or up, but the shape stays about the same.
     
  4. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    Swap in a 6.2...

    Rene
     
  5. Blazer1970

    Blazer1970 1/2 ton status

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    You need less cam. Didya ever notice that the hp/torque curves that are published by cam manufacturers almost always start at 2000 rpm? Personally, I do a lot of driving between 500 and 2000 rpm and I would really like to see what the effects of a performance cam are in this RPM range. I think that even with a mild performance cam (my Edelbrock Performer is 204/214), you probably lose some torque in the 500 - 1500 rpm range, and with an SM465 (essentially a 3 speed), you need all the torque you can get right off idle.
     
  6. rebelk5frk

    rebelk5frk 1/2 ton status

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    [ QUOTE ]
    Swap in a 6.2...

    [/ QUOTE ]
    That looks like a post MJ would make. /forums/images/graemlins/rotfl.gif /forums/images/graemlins/rotfl.gif

    but....for low end power it really is the best option

    /forums/images/graemlins/thinking.gif
     
  7. Greg72

    Greg72 "Might As Well..." Staff Member Super Moderator

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    Here are my guesses....


    Smaller valve heads
    Headers with carefully selected diameter and runner lengths
    A "4x4" type cam
    A stroker crank
    More displacement





    /forums/images/graemlins/thumb.gif
     
  8. Paxx

    Paxx 1/2 ton status

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    [ QUOTE ]
    Swap in a 6.2...

    Rene

    [/ QUOTE ]

    I was waiting for you to say that /forums/images/graemlins/rotfl.gif
     
  9. Paxx

    Paxx 1/2 ton status

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    [ QUOTE ]
    but not change the numbers at all per DD 2000.

    [/ QUOTE ]

    What does this mean? /forums/images/graemlins/confused.gif
     
  10. 75-K5

    75-K5 3/4 ton status

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    Desktop Dyno 2000, it's a mock-up of a dyno for your computer. You punch in all the specs of your engine and it will spit out a torque and HP curve based on the numbers you give it /forums/images/graemlins/thumb.gif
     
  11. fortcollinsram

    fortcollinsram 1/2 ton status

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    Swap in a TBI 454

    What is the duration on that cam at .050"? The cam in my 454 is only like 202* Intake and 210* Exhaust at .050" and like .468" lift Intake anmd .480" exhaust... and my motor pulls the hardest from 1800-3000 RPM..EVEN when I am romping on it, I rarely shift above 4000rpm. So I might spend some time researching cams if I were you...
     
  12. Blue85

    Blue85 Troll Premium Member

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    Your cam is not right for low end torque. I also doubt that the lift is only .290. Everything else on the engine seems good for low end. You might get the biggest difference from lower gears.
     
  13. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    .290" is most likely the lobe lift. Total lift would be multiplied by the rocker ratio, which is 1.5:1 for stock small blocks. Works out to a conservative .435" of lift at the valve. I don't think the cam is all that aggressive...

    Adding 1.6:1 rockers would help a little.

    Rene
     
  14. SUBFAN

    SUBFAN 1/2 ton status

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    The 108 lobe center bleeds off to much low RPM pressure in the combustion chamber....

    Look into the Comp Cams 4x4 series of cams. You will want either of the two smaller ones /forums/images/graemlins/thumb.gif /forums/images/graemlins/thumb.gif /forums/images/graemlins/thumb.gif /forums/images/graemlins/thumb.gif
     
  15. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    When I still had the 350 in my truck I had the Comp Cams 260HE which was a great cam for low and mid range...can't recall the lobe center though. I'm sure it was more than 108...and could have been as much as 112.

    Anyways, it did pull pretty good from 1000 rpm and up which is important for anything with a SM465 due to the wide ratio's.

    Rene
     
  16. rjfguitar

    rjfguitar 3/4 ton status GMOTM Winner

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    swap in a 400, I took my foot off the brake and idled over an overpass once while pulling a 4,000lbs trailer. now thats useable low end torque. /forums/images/graemlins/thumb.gif
     
  17. Paxx

    Paxx 1/2 ton status

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    [ QUOTE ]


    Anyways, it did pull pretty good from 1000 rpm and up which is important for anything with a SM465 due to the wide ratio's.

    Rene

    [/ QUOTE ]

    This is exactly why I need to lower it. When I shift from 2nd to 3rd unless I shift at 5000rpm I come in below my powerband. I don't like to shift at 5000rpm because I start to lose power at around 44-4500rpm.

    So from the sounds of it nothing short of a new cam is really gonna net me the results I'm looking for? /forums/images/graemlins/frown.gif
     
  18. SUBFAN

    SUBFAN 1/2 ton status

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    260HE's have 110* LC's, the 4x4 grinds have 111*....
     
  19. 84_Chevy_K10

    84_Chevy_K10 Banned

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    [ QUOTE ]
    260HE's have 110* LC's, the 4x4 grinds have 112*....

    [/ QUOTE ]

    Mine was 111*.

    Rjfguitar,

    Is a 4,000 lb trailer heavy?
     
  20. SUBFAN

    SUBFAN 1/2 ton status

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