tech question about automatics and torque converters and torque multiplication

[dclee]

Been thinking about this, I guess wheeling automatics is a little different than what is considered "traditional" (though maybe not so much anymore). In my old 80 Series, I had adopted a two-footed driving method, for a variety of reasons, but one of them was to be able to rev the engine while braked, before tackling a steep climb, in order to lock up the torque converter and get more efficient power transfer from the crank and less heat buildup. Recently, though, I have heard that some people will include the torque converter's stall ratio (1.88:1 on my old A442F tranny) in the equation for crawl ratio. However, I am wondering if that is really the case (and what happens in reality)?

Crawl ratio is determined by (diff ratio) x (trans ratio) x (transfer case ratio). So in the case of the 80 (in stock form), it is:

4.11 x 2.95 (1st gear) x 2.488 (Lo range) = 30.17:1 crawl ratio

However, some people have said that I need to include the torque converter stall ratio in that equation as well. In the case of the A442F on my 80, that introduces a 1.88:1 stall ratio. So the end result is:

4.11 x 2.95 x 2.488 x 1.88 = 56.71:1 crawl ratio

That is actually a fairly significant difference! But is it true and do you actually see that kind of reduction when wheeling? Given my admittedly limited understanding of how torque converters work, I was under the impression that, while you may be utilizing that 1.88:1 increase in torque, that is made up for by the fact that the converter is slipping (to keep the engine from dying), causing power loss and heat buildup. I'm not sure if it is a one-for-one relationship between torque lost to slip and torque gained (as if through gear reduction) by the converter, but for the purposes of the argument, I'll assume they are relatively equal. Now, the A442F (and I assume every Land Cruiser automatic tranny since) utilizes a lockup torque converter, so if you rev the engine high enough, the converter locks up and you effectively get a solid mechanical connection between the crank and transmission, so no more loss of power to slipping. However, I believe you have now also eliminated the torque multiplication (or "gear reduction") effect of the converter, so wouldn't that mean that you should actually not figure in the converter's stall ratio in the equation for crawl ratio in any situation?

Sorry for the long-winded post and the "Auto Transmission 101" questions, just couldn't find a clear (to me) answer searching on MUD or Google.

Thanks,

 

[Sandroad]

The torque converter lockup function is a function of engine and transmission sensors, not engine speed. The FSM for my ’04 has the specs:

For the engine, the throttle valve has to be 5% or less open to get the converter to lock up. So, if you accelerate the engine speed (like to pass or go up a hill), the converter unlocks.

For the transmission, in the D position, the vehicle speed in 5th gear has to be greater than 42 mph to get the converter to lock up. Once locked, it doesn’t unlock until the speed drops below 37 mph.

The only reference I can find to adding the torque converter to the crawl ratio is Jim Allen’s “Four Wheeler’s Bible”, which does add the torque converter to the calculations. However, the torque converter's ratio is variable. It is greatest a idle speed and gets less as engine rpm increases.

This is apparently where the 2-foot driving method has an effect.

The change in converter ratio with engine rpm for the A750 transmission in my '04 is not in any info I have.

 

[OregonLC]

Been thinking about this, I guess wheeling automatics is a little different than what is considered "traditional" (though maybe not so much anymore). In my old 80 Series, I had adopted a two-footed driving method, for a variety of reasons, but one of them was to be able to rev the engine while braked, before tackling a steep climb, in order to lock up the torque converter and get more efficient power transfer from the crank and less heat buildup.

Completely agree with the two-foot method. Much better control than throttle modulation alone and is one of the nice luxuries of having a autobox in an off-road vehicle. However, I'm not sure if I'm tracking with your statement about it being locked up. Should be the exact opposite. At slower speeds the TC is completely unlocked (and consequently is developing more heat). It would be the MT equivalent of slipping the clutch. By revving the engine, you were most likely hitting the stall point of the TC.

 

[100 TD]

Torque converter lock up is at "specified parameters" controlled by the ECU, revving the engine does not necessarily lock the converter. It normally locks only when at higher speeds than 40 km/h etc IIRC with certain accelerator pedal input, gear input, speed sensor input etc.

 

[dclee]

Thanks guys. OK, it's a little clearer now. So what I take from all this is:

The converter doesn't lock up based on engine speed (or maybe even throttle position) but vehicle speed.

However, there is a relationship to converter stall ratio and engine speed, which make sense since the converter is there to prevent the engine from dying at idle.

So when revving the engine, I may not be locking it up, but I am getting a progressively more solid connection between crank and transmission, reducing heat and getting more efficient power (as I originally thought), as the converter "firms" up with the increasing engine speed.

Do I have this right so far? So, based on this, would it be correct to say that the converter stall ratio probably doesn't come into effect, or at least fully come into effect, unless the engine is at idle, in which case you are presumably standing still.

Thanks,

 

[OregonLC]

Good link for stall speed: Banks Power | Understanding Stall Speed