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SR71 Engine Buying Advice

Discussion in 'The Hokey Ass Message Board' started by Powerplant, Feb 21, 2012.

  1. I have read that the Nailheads were chosen as they had the highest Torque rating for Hp level. The start cart is turning a lot more than a Nailhead would out on civvie street!

  2. 69fury
    Joined: Feb 24, 2009
    Posts: 1,416


    I've known about the start cart Buick motors for years, but the recent article in Air& Space clarifies why Buicks were used. The prop shaft connecting the buick to the jet engine used a gilmer belt and the techs wanted a smooth transmission without pulse/shock issues. The article states that that is whey start carts had Buicks with Dynaflow transmissions (i'm not a buick guy and dont know what makes a Dynaflow trans special). The article also states that the later 454s had their transmissions locked in 3rd gear (supposedly to avoid shocking the gilmer drives on the shift).-rick
  3. Relic Stew
    Joined: Apr 17, 2005
    Posts: 1,120

    Relic Stew
    from Wisconsin

    They stay in high gear, unless low is manually selected, with the torque converter doing all the torque multiplication. The later ones, that would have come behind a 401, produced a 3:1 multiplication ratio. They also had a variable pitch stator that changed the stall speed. In the performance position it allowed about 1000 rpm more slippage than in cruise position, so it gave a "passing gear" effect without actually shifting gears.

    (Speculation) In the carts, they could start out with the stator in the high position , with a stall speed of 2600-2800 rpm, giving the 3x torque multiplication, then as the jet started turning and rpm came up, switch it to the low setting, reducing slippage and multiplication, without the shock of changing gears.
    Last edited: Jul 8, 2014

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