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Remember when we were talking about 700R4s and high gears?

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by 84_Chevy_K10, Aug 7, 2001.

  1. 84_Chevy_K10

    84_Chevy_K10 Banned

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    About a week ago we were talking about 700R4s and how they don't like high (numerically low) gears due to stall speeds and engine RPM at cruise speed.

    But I got to thinking today when my torque converter clutch didn't lock (fixed it already).

    If a 700R4 has a lockup torque converter, (mine does and so does every other factory installed one to my knowledge) then why does it matter? I guess if you had really high (higher than 3.42 for a stock truck and depends on your tire size otherwise) gears then you'd have your foot in it all the time and the vacuum switch would constantly be unlocking your torque converter, but if you had somewhat decent gears and the torque converter locked at cruise speeds, doesn't that eliminate all your problems with stall speed? Isn't their supposed to be no converter slippage at all when it's locked up?

    My line of thinking here is, that to a point, 700R4s MIGHT be able to stand high gears if the torque converter stays locked most of the time. I have 4.10s and 35s, and could use a little more gear, but if my line of thinking is correct, then me and many others might be safe, at least for a little while.

    I know I'm swapping my 700R4 for an NV 4500 as soon as I can get the linkage for the clutch and the tranny and figure out what kind of transfer case I want to use. But this isn't about me--this is about people that are still stuck with too high of gears for their tire size, at least for now. I'm good to go after the latest swap to 4.10s.

    What do you guys think?

    Tim
    '84 Chevy K10, lifted, loud, fast, and 3/4 ton axles
     
  2. Leadfoot

    Leadfoot 1/2 ton status Premium Member

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    Your right in saying it will keep converter slippage from happening and their will be less heat from converter slippage, but also keep in mind with high gears (lower numerically), when at cruising speed your foot is not into the throttle as much (which to a point gives you better mileage), BUT (and this is the killer of tranny's) the 700 is controlled by the Throttle Valve cable. The more it's pulled (from the GO pedal being pushed toward the floor) the higher the pressure to keep the clutches engaged. When cruising your foot is not into the pedal as much (especially when converter is locked), now higher gears make the tranny work harder to keep it going a given speed (especially towing), and due to the TV cable not being pulled as far, you have greater resistance with lower line pressure, which causes slippage, heat, and dead tranny.
    Now I know you will say that with higher gears, you have to put your foot to the floor just to get it moving or to accelerate (and this is true), but you eventually take your foot off the throttle (and on and off, and on and off, etc. etc.), the clutches start to slip, you hit the gas, pressure is brought back up, and the slip goes away (but a lot of heat is generated), this keeps going on, and eventually the fluid fails and so does the tranny. With lower gears the tranny does not work as hard and the throttle position for a given cruise speed is higher, therefore more pressure to keep clutches engaged.
    Now you might say (which I did at one point), why not just fool the TV cable into thinking the throttle is more than it actually is to boost the pressure. This can be done in a small increment (TV adjustment), but if it's done too much it can cause seals to blow at low RPM (the front seal was notorious for blow out as it was), and will cause overly harsh shifts (hard shifts are good, but there is overdoing it).
    Now I am no expert on 700's, but I have done extensive research and talked with people who build performance units and this is what I have learned.

    If you didn't build it yourself, how can you call it yours.......?
     
  3. 502Burban

    502Burban 1/2 ton status

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    Leadfoot said it better than I ever could, and I have to agree. My understanding of the 700R4 problem with higher (lower numerical) and towing, is that the 4th gear (overdrive) clutch pack isn't up to the task. This causes the clutch pack to slip, burning up the clutch material and overheating the fluid, eventually causing failure. Only what I've been told by my trans guys, but I don't think that the problem stems from the converter lock-up as much as from the OD clutch pack. I also like Leadfoot's position regarding the TV cable.

    Just my $.02

    Robb
    '73 GMC Suburban - 502DFI
    <a target="_blank" href=http://community.webshots.com/user/robbrj>community.webshots.com/user/robbrj</a>
     
  4. pcorssmit

    pcorssmit 1/2 ton status

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    I thought you were into mud running, if so, why do you want a stick? I have an NV4500 in my Dodge, and while it's a good trans, its not a very quick-shifter. In the sand, you're more or less stopped by the time you're done changing gears. I would think it would be even worse in the mud.

    Pete

    '83 K5, 350 TBI (ex 6.2), 700R4, NP208, Dana 60/14 bolt, 4.56s, Detroits, 3" lift, 15-39.5x15 TSLs
    '97 Dodge 2500 4x4 CC LB Sport, Cummins 5 spd
     
  5. dyeager535

    dyeager535 1 ton status Premium Member

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    Throw a vacuum gauge on a 700R4 equipped truck and drive it. With a Q-jet, the higher your vacuum, the better your economy is going to be. As well, with a 700, since TV cable is directly controlled by throttle position, higher RPMs via the gas pedal are going to keep the tranny "locked up" tighter from fluid pressure.

    Basically, if you can keep the vehicle in a high vacuum situation, it appears you will have the best of both worlds. (I'm using vacuum as an indication of RPM vs. throttle position)

    I've noticed that driving my SM465, the best economy is going to be when I have the RPM's up around 25-3000RPM on flat ground. Anything less than that (700R4 with the TCC locked up is going to be the same) RPM wise is going to result in low vacuum, and the carb dumping fuel into the motor, while the tranny is not seeing a higher TV cable position. Both conditions are bad, and when I learned to shift my 700 later than it wanted to, performance went way up, with no loss in MPG.

    You would think 3.08's with a 305, vehicle with something like 40 SQ FT of frontal area, and OD would be a recipe for horrid mileage and tranny life, but I put quite a few miles on that truck, and could always average 17mpg freeway with the carb. The gears killed me in town, but again, if I had held it in gear, and kept the RPM's up (and thus vacuum) I'm sure that would have gone up as well!

    Just some observations...

    Dorian
    My tech/links page: <a target="_blank" href=http://www.dorianyeager.com/index2.html>http://www.dorianyeager.com/index2.html</a>
    No anti-theft measures on your truck? No pity when its stolen
     
  6. 84_Chevy_K10

    84_Chevy_K10 Banned

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    Yes, I forgot about the almighty TV cable that controls everything on the 700R4.

    I think I have enough gear, 4.10s and 35s isn't the best but it could be worse.

    I want an NV 4500 because I can't stand automatic transmissions period. I am willing to give up some mud performance for the safety of knowing that I'm not going to burn up my tranny, EVER. I also tow a 25' trailer a couple times a year, more problems, more heating up my 700, etc.

    I am a 700R4 fan and I know how to adjust the TV cable, make sure that the TCC clutch works properly, etc. I have taken good care of mine with a flush and pan drop as well as a big B&amp;M cooler.

    Thanks guys, I'm glad I started this thread.

    A 700R4 with the TCC locked up is not going to be the same as an SM 465 unless you have 30% lower axle gearing as 4th gear in a 700R4 has an overdrive.

    Tim
    '84 Chevy K10, lifted, loud, fast, and 3/4 ton axles
     
  7. dyeager535

    dyeager535 1 ton status Premium Member

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    700 isn't going to be the same as an SM465 in final drive ratio, however, my point was SUPPOSED to be that an automatic can still have higher vacuum than a manual with the same throttle input because of torque converter "slippage"

    You bog down a locked 700 or a 465 and there is no difference, both are locked to the engine, and require greater amounts of gas to get moving instead of just downshifting.

    Dorian
    My tech/links page: &lt;a target="_blank" href=http://www.dorianyeager.com/index2.html&gt;http://www.dorianyeager.com/index2.html&lt;/a&gt;
    No anti-theft measures on your truck? No pity when its stolen
     
  8. BlazerGuy

    BlazerGuy 3/4 ton status

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    What about a TH400?? Not many people burn those up [​IMG] but you do lose OD [​IMG] but you could install a aftermarket OD [​IMG].....

    <a target="_blank" href=http://www.k5.rockcrawler.com>Click here to see my K5</a>
    *<font color=red>under construction</font color=red>*
     
  9. 84_Chevy_K10

    84_Chevy_K10 Banned

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    I'm not interested in a TH 400. I think that'd be stupid to put a TH 400 behind a 305, even if it is 250 hp/330 ft-lbs.

    I hate automatics anyway. My favorite automatic is a 700R4, but I'm really a stick-shift guy. My friends have 4 speeds in their trucks (both use 44" tires). They've basically learned how valueable 4 low is, and that you need to select the correct gear before you hit a deep pit. In 4 low you can pretty much start off in whatever gear you want anyway, so it doesn't really matter.

    Tim
    '84 Chevy K10, lifted, loud, fast, and 3/4 ton axles<P ID="edit"><FONT class="small">Edited by 84_Chevy_K10 on 08/07/01 09:11 PM.</FONT></P>
     
  10. 84_Chevy_K10

    84_Chevy_K10 Banned

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    Sorry I missed your point. What I was seeing in that comment is, "more RPM, more vacuum" but I was thinking in my head, "overdrive, less RPM, therfore less vacuum by his thinking" when I read it. I understand your point now--more vacuum when your foot is out of it more and TCC is locked.

    Tim
    '84 Chevy K10, lifted, loud, fast, and 3/4 ton axles
     

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