Torque Converter 101

A torque converter is basically made up of a pump and a turbine. Think of a hollowed out bagel and the way bagels are cut. Now imagine the two bagel halves being connected together through a bearing that allows each half of the bagel to spin independent of one another. The pump can be thought of one half of the bagel while the turbine is the other half. The pump half is connected to engine, and the other half (turbine) is connected to the trans. The trans fluid inside the torque converter (inside the hollow bagel) is is "flung" by means of little fins that are inside one half of the bagel (pump) to the other half of the bagel (turbine). The turbine also has fins attached to it on the inside, and these fins spin resist the fluid flung over by the pump and cause the turbine to spin. Since the turbine is connected to the trans, the car will start moving. (This is an oversimplification of course - there is a stator inside there as well.....)

The faster the engine spins, the more force the fluid gives to the transmission, until a point where the difference is so great, either the engine cannot spin any faster, or the car moves. That is the so-called "stall" point where, in the xfire, is about 2400 to 2600. It is at this point that the torque multiplication of the torq conv is the greatest, and in a good design, this multiplication is close to 2. Yes, the torque of the engine is mult by 2 or so, and this is the advantage a torque converter has over a manual, since a manual does not have a torque converter.

So, when you floor it off the line in the auto, the engine revs up faster than the car moves, and the cars speed "catches up" with the engine (the differential of the pump and turbine speeds) and the auto has this torque multiplication advantage over a manual.

So it sounds like the auto is superior to the manual, right? No. This 2x or so multiplication of torque does not make up for the parasitic losses in the auto trans. To make a long story short on the internal workings of the auto, the "overhead" of the complex gearing (planetary and sun gears) in a auto trans is far higher than a manual, and is, very roughly, about twice that of a manual.

To sum all of this up, the horsepower lost spinning the auto trans is more than spinning a manual (full race-prepped autos with super high stall converters excluded-the super high stall really multiplies the torque). Combine that with the fact that there is an extra gear in the xfire manual, and the auto CANNOT be as fast as the manual, provided the manual is shifted properly... and that is the auto's advantage. A missed shift by an overly excited driver (yours truly on occasion) is impossible to recover from. The auto is all about consistency....but it is NOT faster.

One last point regarding the lock up feature of torque converters. The efficiency (the difference in spinning speed of each half of the bagel) of the converter starts off at around 90 or so percent at the stall point, to about 95-98% at about 500 rpm higher than stall (in the xfire, probably around 3000-3200 rpm). Note that 96-98% is not 100%, and this equates to lost power (in the form of heat) and resultant loss in MPG. The lockup feature does just that - it forces the two halves to be "locked together". But the overhead of the gearing in the auto is still higher than a manual, and that is why in all cars (except for the xfire), the manual gets better MPG. (I still do not understand why the MPG number is better for the auto in the xfire).

So there!, you automatic loving xfire owners!!