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Why Use 4-Lo?
Contributed By: Richard Hager
You're in 4-hi....climbing that hill, your foot's kinda 'in it'...notice how the rpm climbs a bit without the tank...errr...fsj...going any faster? That's your torque-converter, 'slipping' in order to convert torque! Except in this case, it's converting a pretty good percentage of that torque to -heat-. Where? In your expensive tranny.
By the way, if you assume for this discussion that you're throwing maybe 15-25 HP into heat in this situation, that's a good 10,000 watts or more. Like....take 10 1,000 watt radiant electric heaters and stack 'em into a wall of heat on your desk, pointing at you, and turn 'em all on at once....yep, that's a LOT of heat! Now stick all that -extra- heat inside your little 727 case.
Now, your temp-gauge is sensing -engine coolant- temp, NOT tranny temp. And even if you have a tranny-cooler, it's a long skinny run from the tranny to the cooler and back. Also, in order to transfer any great amount of heat from one medium to another (fluid to air), one medium has to be considerably hotter than the other. In other words, the fluid in your tranny is a -lot- hotter than the fins on your tranny-cooler.
So, while you might be happy with your less-than-steaming temp-gauge reading, you can bet your bottom clutch-plate that your tranny is running -very- hot on those hills/trails in 4-hi.
Your T-case is -after- the tranny, so shifting into low-range has a direct effect on the amount of -torque- that the engine/convertor/tranny has to supply. Power= torque X rpm. Low-range = higher rpms, so lower torque for the same HP to the wheels. The amount of heat generated in a torque-convertor is proportional to the -delta- rpm between input and output, -not- the absolute rpm it's spinning at. No delta, no heat. Big delta, big heat; even if the engine rpm is low.
A 2.6 to 1 low-range is reducing the heat-load in your convertor and tranny by 260% !
As they say, that's why Jeep put the lever there in the first place...
I do a lot of trailing in 4-hi too, but as soon as the terrain starts causing me to notice that I'm pushing the pedal a lot more without getting much change in vehicle speed, then I switch to 4-lo and give my truck a break.
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