Torque multiplication from a torque converter doesn't do anything that a lower gear in a manual gearbox wouldn't, no free lunch here, no power coming out of nowhere.KP wrote:Don't forget the thing with an auto is the torque is increased by the box as well when it wants it so you can get more torque power out compared to whats put in which helps as well
It multiplies the torque, (up to about 2x) but at the expense of huge power loss which generates heat and wastes fuel. The output torque is greater than input but output power is much lower than input power, the difference goes up in heat.
The main benefit of a torque converter (apart from stall free take off and providing a smoother backlash free driving experience) is that it widens the useful range of each gear effectively giving you an extra gear for free, something that was especially useful in the early days of auto's when they only had 3 gears...
A 4 speed automatic is roughly equivalent to a 5 speed manual, if you look at the gear ratios 5th manual is about the same as 4th auto, 4th manual 3rd auto etc... with 1st gear in the auto being left to cover the speed range that would normally be covered by 1st and 2nd on the manual.
From stationary the torque converter effectively multiplies the gear ratio to give comparable or even more acceleration than 1st in the manual, then as you speed up and the slip reduces, the torque multiplication goes away and now its more similar to 2nd gear in a manual. (Especially in a gearbox like the 4HP20 that tries to lock up the torque converter in 2nd at relatively low speed)
It can also give a useful "half gear change" boost in the higher gears, for example in the 4HP20 if you're in 4th gear at 60mph with a light throttle the torque converter will be locked up, when you press the throttle down about half way it will unlock the torque converter letting the engine revs increase approx 600 rpm, allowing the engine go up to a higher torque part of its power band, plus you get some torque multiplication from the torque converter as well so it gives a good bit of extra torque and acceleration without actually kicking down a gear. The penalty for this extra acceleration is a lot of lost power in heat, as soon as the torque converter unlocks the fuel consumption sky rockets...
Probably the main reason why the Xantia V6 can do respectable MPG figures for a V6 auto is that it tries to keep the torque converter clutch locked up as much as it can to minimize power loss. It's certainly very aggressive about locking up compared to other autos I've driven.