I found this comment interesting as I had always assumed that the gearbox controlled the TC lock-up point. My '87 will lock at 40mph at all but a heavy throttle application, whereas the 3.9 I had years ago wouldn't do so until the car reached 50. Back in those days a mate had his 3.9 reset (?) so that the lock-up took place at 45.
I have read that, should my own ZF ever need replacing, it would be a good idea to have the factory-fitted TC replaced by a larger unit.
Sorry, I misspoke, you are correct that the transmission valve block, electric or hydraulic controls the torque converter lock-up clutch, while the larger torque convertors "stall" at lower RPM than a smaller one. To me semantically the effect is the same, the reason for that is that "stalling" in the context of a torque converter refers to the point at which the torque converter fluidic elements stop "slipping" relative to each other, they move as one, and thus can be perceived as locked up.
So, now I've clarified that the larger converter doesn't lock up earlier, but instead stalls earlier, I'd better explain why this is something I perceive as desirable... When the impeller and turbine are "slipping", transmission fluid temperatures rise because the fluid is being put under pressure without doing any useful work, and that energy has to go somewhere. Because smaller torque converters stall later, and are thus spending more time slipping, this will result in higher ATF temperatures for the same workloads than you would see with a larger converter. Because less of the energy from the engine that's being put into the fluid via the impeller is being converted into propulsion and instead that energy is going into waste heat, you should also see (slightly) better mpg with a bigger converter than you would when using a smaller conversion.
The driving characteristics of an automatic box are as equally dependent on the converter as they are the ratios... You know how automatics were derided as "slushboxes" because of their horrible nature of accelerating out of a junction. In that situation, the initial 1500rpm above tickover doesn't really result in much propulsion, but you keep on the loud pedal up to say 40mph for this example. As the speed increases the box shifts up, so by the time you are at speed, you're up a couple of gears and the revs have dropped accordingly, lets call it 3rd gear at 3000rpm, when you lift off to coast, the torque convertor goes into its sloshy mode and engine revs drop to 1500rpm, and you can now play tunes on the loud pedal between 1500rpm and 3000rpm, without affecting vehicle speed. Whereas a modern say 6 or 8 speed auto, hell, even a decent 5 speed auto like the one in the hippo, once the fluid is warm, will be very closely tied between engine RPM in coast and engine speed when accelerating at that road speed.
Going with a bigger torque convertor reduces the size of the disparity of engine vs transmission revs, so in that case, with a larger torque convertor, the revs might fall to 2250rpm, and be offering some propulsion between that speed and the 3000rpm engine to road speed. I know that's a lot of words, but it's a difficult thing to describe, but if you something more visual, compare these two videos you'll see how little the later 4.6 compared to the earlier range rover:
Timestamp: 13:46
Timestpamp 03:27
You'll also find that the smaller the torque convertor is, the smaller and less capable it's lock-up clutch is, and from what I've read, the smaller stock torque clutches seem to struggle with standard loads, never mind tuned engines / larger displacements. Apart from the lost efficiency of an overwhelmed and thus slipping torque convertor clutch, and the detriment to the driving experience this creates, the slipping TC clutch also contaminates the fluid, the debris of which then attack other clutches and or band brakes within the gearbox, hastening it's demise.
So, in summary, the benefits of going with a large torque convertor would be less transmission sloshing about resulting in a more relaxed driving style, because the vehicle will "pick up" at lower engine RPMs. It will do so with less energy being converted into waste heat, ergo lower transmission fluid temperatures and a slight increase in fuel efficiency. The larger TC also has more capacity for the transmission of torque from the engine into the gearbox. That increased torque transmission capability also means the TC clutch is less stressed ergo less likely to slip and fail, so the bigger TC has reliability advantages as well.
