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700R Cooling & gearing questions

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by 1Blazin71, May 28, 2001.

  1. 1Blazin71

    1Blazin71 1/2 ton status

    Aug 23, 2000
    Likes Received:
    Los Angeles
    I have a 700R with a 2300 stall converter in our '71 with 35" tires and 3.73s in the 12 bolt rear end. I have an Autometer trans temp gauge in the pan (passenger side, front corner).

    Around town and on level highway at 70-80 MPH it runs a cool 170 and hardly ever moves above that. On mountain passes (such as the grapevine on I5 north of los angeles) its a different story. Yesterday, with ambient temps in the 80s, the trans hit 230 by the time we crested. Water temp, measured in the head between #6 & #8 cyl, stayed at 180.

    I run a 28000 GVW trans cooler from B&M and do not have the trans lines running through the radiator. The cooler is mounted forward of the radiator.

    I'm concerned about this setups inability to keep up with big grades. I wasnt running more than about 60 MPH and was trying to keep it in 3rd gear, thinking that the pump would be pushing more fluid faster to the cooler.

    I'm contemplating a larger cooler with an electric fan, or perhaps an additional cooler on the frame rail or something. But I suspect that the oversize tires and 3.73s may be contributing to the problem?

    I plan to go to 4.56s once I track down a donor 1 ton and some $$ for a 14FF. Is there something I should consider doing in the meantime? I like the 700R alot but know that its not well matched with the 3.73s and 35"s. Should I consider going to a lower stall converter, add another cooler, or drive 40 MPH up big grades?

    On a related subject, I have a lockup converter with a B&M speed sensitive lockup controller which uses a slide switch to control at what MPH lockup is allowed to come on. I normally dont allow the lockup to engage unless I'm on level ground running at least 70MPH. Any slower or with any grade at all and its impossible to accelerate under lockup without stomping the pedal to get it to kick down. If I try to gradually accelerate, the truck will shudder a bit and occasionally with produce a backfire, I think called spark-knock? I suspect this problem is related to the mismatched (too high, numerically) gearing of the 35s and 3.73s? Would I expect this to be improved once I go to 4.56s?

    As I calc it now, I'm running 1750 RPM in 4th gear at 70 MPH. With 4.56s this would become 2150 RPM. Lockup (at .5:1) makes it 1250 RPM for 3.73 and 1530 RPM for 4.56. Is 4.56 going to be low enough?

    Opinions appreciated.

    Matthew Meyer
    '71 K5 - 4" Lift 35"s
    ZZ4 350 - 700R
  2. NewK5Guy

    NewK5Guy 1/2 ton status

    Mar 2, 2001
    Likes Received:
    Alexandria, VA
    I don't know as much as some about trannies, but you may want to dump that high stall converter. All the rpm/speed points you mentioned were BELOW the rated stall you said you were running. From what I understand, the higher the stall rpm, the hotter the tranny will get. Since less than 100% of engine power is transferred to the tranny during stall, you get massive heat buildup, tranny cooler or not. It's just fluid sloshing around and not doing anything. Once you hit stall and the converter starts working, the fluid isn't sloshing around as much since the internals of the converter are turning now.
    I could be wrong on this, but everything you mention in your post adds up to too high of a stall rating on that converter. At cruise you are BELOW the rated stall. I can't see how you haven't overheated that tranny already as it is.
    I'd seriously consider dropping to a lower stall rating on the converter. The higher stall is only lost power and more torture on the tranny than is necessary.
  3. CaptCrunch

    CaptCrunch 1/2 ton status

    Feb 2, 2001
    Likes Received:
    Yes higher stall converters produce more heat and also the size dia. wise will effect this, but I definetly see a problem w/ the gears. Those RPM numbers pretty much match up w/ 3.08's and 31" tires which is a known 700 killer.

    1987 Chevy K5 Blazer- 350 TBI
  4. coopertwpk

    coopertwpk 1/2 ton status

    Mar 18, 2000
    Likes Received:
    Spring Valley Lake Ca.
    run the trans cooler into the radiator it works better at getting rid of the heat a stand alone doesnt cut it water disapates heat faster than oils

    88k5 silverado w4"lift soon to have <font color=orange>d44 and corp14ff</font color=orange> with 4.56s
  5. Blue85

    Blue85 Troll Premium Member

    Jul 26, 2000
    Likes Received:
    Grand Rapids area
    The higher stall speed will make it hotter, of course, but if you are using the lockup, then that won't matter. The key to making it run cool is to use the lockup. 70MPH is way too fast to delay locking the converter. First of all, the converter should always be locked in OD and you should never run in 4th gear without it locked. The 700 doesn't cool as well in 4th gear. Then, you should have it come in and lock for any cruising speeds in 3rd gear. When you are at 60 on a grade and your lockup controller is set for 70, you are creating lots of heat with the high stall converter and just relying on your cooler to bring the temp back down before it hits the pan. Even if this works, think about the life of the converter. Unlocked is only for accelerating. Locked is for cruising. The higher stall speed can help to make up taller gears under acceleration, but it can't be used as a substitute on the highway.

    Ultimately, the solution may be to lower your axle ratio. It is not great now, but some people have it worse. In the meantime, you should make sure that the TV cable is set right so that the tranny will run in the proper gear. If you are climbing a steep grade, just pull the gear selector to "3". It is no problem to work the engine continuously at 2500 rpm (or whatever it is) for a while. Remember when trucks came with 3 speeds? With 3.73's and 35's, you should be able to get along without the OD pretty much all of the time, with just a small hit in MPG. If your gears are all wrong, the hit in MPG is not very much. Assuming that you are unlocked in either gear, 3rd will generate less heat since there will be less torque through the converter and less torque = less slip.

    But the bottom line is this: get the lockup to work for all cruising speeds. If your 700 is from a non-ECM truck, it should have a valve that will kick the lockup in and out depending on throttle position and vehicle speed. If you don't have that, you can always get a vacuum switch to kick it in and out for accelerating. As a worst-case, you can mount a toggle switch that lets you turn it off just when it becomes a hindrance.

    BTW: your top gear ratio is 0.7:1 when the converter is locked. 3rd gear is 1:1 when the converter is locked. When it is unlocked, you can't really calculate the gear ratio because the torque converter varies from about 2:1 (WOT) reduction to nearly 1:1 (coast), depending on the torque through it. In lockup, the converter is always 1:1

    <font color=green>Why does everyone worry about my gas mileage, but not think twice about an Excursion owner?</font color=green>

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