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  1. #1
    Join Date
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    Aitkin, MN
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    Default Cooling Upgrades Techie Question

    Good Day!

    Why are dual thermostats part of the cooling mods?

    If memory serves, the cooling upgrade GM introduced to our beloved 6.5TD in 97 (?) includes a higher-output water pump (85 (?) gpm to 130 gpm) & a new, dual thermostat housing. These components are fortunately available for retrofit from several top-quality DP advertisers.

    Again from memory, this upgrade increased flow through the block ~ 75%, & flow through the radiator ~ 9%. This makes sense, as some of the cylinders (6 & 8?) seem to fail from overheat before any others; it seems like increasing coolant flow through the block might stop this problem.

    My question comes up because if flow through the radiator is only increased ~ 9%, why do we need dual thermostats?

    TIA & Blessings!
    82 6.2NA K15 4X4 pickup, 4spd man w/ OD, 335K+ "In Rust We Trust" (parked)
    95 6.5TD 2500 4X4 pickup, Gear Vendors Aux. OD, > ¼ million miles - gone
    95 6.5TD 1500 4X4 3/4T Suburban, Kennedy exhaust, > ¼ million miles
    93 6.5TD 3500 4X4 1T crew cab LB pickup, 230k miles

  2. #2
    Join Date
    Sep 2001
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    Lubbock TX, USA
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    4,194

    Default

    IIRC, the higher GPM waterpump was to increase coolant flow through the block, to help eliminate "hot spots" in the heads and elsewhere where the coolant would start to boil. Increasing the flow across the surface of the interior block parts helps to transmit more heat to the coolant and out to the radiator.

    Why would flow only increase 9% through the radiator? Hmm, the outlet of the dual stat system didn't increase in size did it? Exactly.

    97 was the first year to be standard on all normal production 6.5's.

    It was on mid to late 96's as well.

  3. #3
    Join Date
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    Default

    Good Day!

    "Why would flow only increase 9% through the radiator? Hmm, the outlet of the dual stat system didn't increase in size did it? Exactly." Which makes me wonder all the more - why the dual 'stats?

    Blessings!
    (signature in previous post)

  4. #4
    Join Date
    Nov 2003
    Location
    Sun City AZ
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    Default

    Maybe in case one fails?
    Or to allow a combo - 1 180 and 1 195?
    Or - obviously I don't know - but I went with option 2....
    OOps sold my "baby" - 96 K-2500 6.5TD - ~047,068mi, Isspro's Boost, Pyro, and Tranny temp; Heath Plate Remote mounted FSD; Amsoil Fluids and Air; Bilsteins, BD Down Pipe-Gutted CAT-& B-D'ized 3.5 Kennedy exhaust; '97 cooling upgrade; CAD upgrade; new IP at 34,700 under warranty; Homemade: Boost fooler, Hi/Lo idle, OPS relay, TCC Lockup ( TFT), Glow extender for AC60's, & "GMC H..'s" 6 position Shift modifier .

  5. #5
    Join Date
    Apr 2001
    Location
    CA
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    13,573

    Arrow

    Quote Originally Posted by moondoggie
    Good Day!

    "Why would flow only increase 9% through the radiator? Hmm, the outlet of the dual stat system didn't increase in size did it? Exactly." Which makes me wonder all the more - why the dual 'stats?

    Blessings!
    (signature in previous post)
    The thermostat has a flow area considerably less, and restriction considerably more, than the outlet, hose, and radiator core. Duals closes the gap between them, and also allows more of a progressive and responsive engagement. Redundancy would be a bonus, but not by design.
    1985 Blazer 6.2
    2001 GMC 2500HD D/A
    dmaxmaverick@thedieselpage.com

  6. #6
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    Default

    Good Day!

    Thanks everyone - just what I had hoped for. Further clarification of course gratefully accepted.

    Blessings!
    (signature in previous post)

  7. #7
    Join Date
    Oct 2003
    Location
    Maine
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    Default

    While I lived in Washingotn, I had the oppurtunity to visit Bill Heath. I was waiting for him to get off the phone so, I tooled around the shop. There was an engine on the stand with the dual thermostat crossover. I had mine off (I have a single crossover, the newer single thermostat style) at one time and noticed that there is a restrictor disc built into the the crossover, where the coolant enters the recirc path back to the water pump. We'll, the dual stat didn't have one.

    They added another stat with reduces restriction and increases flow through the radiator, but removed the restrictor disc, which reduced the restriction for the recirculation path, increasing recirc flow, but reducing the radiator flow what it would be with the disc and dual thermostats, Hence, only 9%.

    Now, if I can only get stationed in Wisconsin.....

  8. #8
    Join Date
    Mar 2020
    Location
    Los Angeles, CA
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    Default

    Related question, I’m building an ‘87 6.2 to put in my ‘84 suburban, and have been offered all the accessories and serpentine belt system including water pump off of a 6.5 out of a hummer h1. Anybody know if that will work?
    Thanks
    Niel

  9. #9
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    Feb 2000
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    Montana
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