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Torque converter behind transmission

Vary torque converter stall speed by gearing input torque
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This idea came from considering various manual-automatic crossover arrangements for street/drag applications. In the course of investigating manual-converted automatics using torque converters I got the idea of running the torque converter behind the transmission.

The advantage is that this would allow the use of an effective stall speed much higher than one would normally consider practical for street use - but that effective stall speed would only apply in the lowest gear. The effective stall speed falls as the gear ratio increases, but in an exponential proportion. The effective stall speed in the second gear (of five, say) would be "mild street", in third it would be stock-like. In high gears the converter might actually be stalled at idle.

The reason for this is that the converter's stall speed varies exponentially with the input torque according to a thing called the K factor. The torque converter used here will be a very low-K unit as usually used with big diesels, because the input happens at a geared proportion of engine speed, and a proportionately multiplied torque.

For instance, if the engine makes, say 500lb.ft @ 4000rpm, the torque converter might see 1250lb.ft @ 1600rpm when the transmission is in first gear. Even a low-K converter might still not be quite stalled under those conditions.

The math seems to work best with a fairly high first gear, so that the converter does a lot of the whole business of getting up to speed, with the first shift happening relatively late, as would be the case with a racing converter. The difference is that pulling away in second is a much more civilized process.

I looked at this for a while and in the end wondered if it was worth the bother.

Ned_Ludd, Jan 18 2007

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       I get you Idea but it has a lack of a purpose   

       Drag cars use several short throw gears and all use manuals there more efficient and can handle the high torque, the top fuel dragsters tend to weld the clutch plates together as it is.   

       If the point is to start with a vary low gear then put to 5 speeds in line then you also have a second overdrive increasing you top speed.
dev45, Jan 18 2007
  

       //worth the bother//   

       Nope!
jhomrighaus, Jan 18 2007
  

       bad idea [-]
evilpenguin, Jul 04 2007
  

       Very interesting. It would have the effect of reducing the total slip percentage, but only in a vehicle with full overdrive, and only in the top gear. Lockup converters eliminate slippage entirely and are fully baked.   

       The output of the tranny is of such a low speed in the lower gears that it may not be sufficient to get the converter to impart any useful velocity to the driveshaft. If I understand the workings of a torque converter correctly, there is a minimum speed it must see on its input end before it reallys starts to "grab."   

       I wanted to bun it for the first attribute mentioned, until I realized the last one. In order for this idea to work, the tranny would have to be one with mostly overdrive gears, in order to keep the converter in its working range, and then there would have to be a gigantic final drive to bring those fast driveshaft speeds back down to useful axle speeds.
elhigh, Jul 04 2007
  

       This is off topic, but in the new Buick Lucerne V8, first gear takes you to 57mph. The thing still manages a 0-60 time of a little over 7 seconds somehow, and one editor noted that the torque converter jumps you off the line until about 7mph, then you have to sit back and wait until about 4,000rpm for the engine to actually start pulling. Sounds ridiculous to me, seems like if they geared 1st gear a little differently they could easily shave a second off the thing's 0-60 while not hurting fuel economy. Perhaps this idea would help.   

       And I have a transmission related question for anyone who might know the answer. If it is true that CVTs can only handle a certain amount of torque before they break and that is why they aren't used in high performance cars, couldn't you simply lower torque by gearing the engine before the power goes through the CVT, then re-gearing it afterwards to make up the torque?
acurafan07, Jul 04 2007
  

       acurafan07   

       Yes CVT have trouble with high torque. I do not see how to do the gearing you are suggesting with out some big problems. The largest problem will be the huge a mount of Heat you will make doing that.   

       I should mention the CVT's are getting better with torque it there was a point they could not Handle a 4cilender now they are working on small 6 cylinders
dev45, Jul 22 2007
  

       Worth the bother? Lack of purpose? I'm voting for it.
normzone, Jul 22 2007
  
      
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