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Old 10-22-2012, 08:55 AM
TransEngineer TransEngineer is offline
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Quote:
Originally Posted by sself510 View Post
I would like some more advice if you don't mind. With the current TC it stalls from 3000-3200, but in first gear from a dead stop with WOT, it jumps to 2500 rpms, bouncing there refusing to go to the new stall. I also tried WOT no brake from a dead stop, it also jumps to 2500 rpms bouncing until my truck reaches a certain speed, then jumps up to the new stall and takes off like its supposed to.

With this issue I'm not sure what I can do to help my problem, I'm not sure I can get this issue tuned out, or just change the stall. In your opinion what would you reccomend? Look for someone to adjust the parameters, or change the stall? I can get a 2500-2700 stall, and a 2800-3000 stall. I'm looking for the best, and also safest choice, I'm really frustrated with what is currently happening and would really appreciate your advice.
I'd recommend using the lowest stall speed (as close to stock) as you can get. The stock converter stall speed is about 2300 RPM.

I think your problem (which gives you the "bouncing") is the PCM trying to control the trans input torque. As engine RPM increases, so does the engine torque. But more importantly, the stall torque required increases with the square of the engine RPM. That is, the engine torque that's required (to spin the converter when the output is stopped [stalled]) goes up with the square of the engine RPM.

For example, let's suppose that at your normal (stock) stall speed of 2300 RPM, the engine torque is 340 ft-lbs, and the turbine torque (converter output) is 640 ft-lbs. Now let's also suppose that the PCM wants to keep the trans input torque (at stall) below 620 ft-lbs. So it cuts back the engine torque to 329 ft-lbs. That's a reduction of 3.2%.

But suppose your converter has a stall speed of 3200 RPM. As the PCM sees the engine speed rise (above 2300), it recalculates the trans input torque. At 2400 RPM, it would take about 370 ft-lbs of engine torque, which would give 696 ft-lbs of trans input torque. So at 2400 RPM, the PCM would be telling the engine to cut back its torque by 11%. At 2800 RPM, the stall torque (engine torque) required would be 500 ft-lbs, giving 941 ft-lbs of trans input, requiring a 34% torque reduction. And at 3200 RPM, stall torque required would be 655 ft-lbs, giving 1233 ft-lbs of trans input, requiring a 50% torque reduction!

So I think what's happening is.... your engine speed is going above the normal stall speed, which causes the PCM to drastically cut back the engine torque, which causes the engine speed to fall, which causes the torque to drop, which causes the PCM to allow less torque reduction, which causes the engine speed to rise, which causes the PCM to crank the torque management back up, etc., etc.

Now your converter (with higher stall speed) has different characteristics than the stock converter, but the PCM thinks it has a stock converter, so it calculates excessive torque management requirements, which gives you the odd behavior. So I think the closer you get to a stock converter (stall speed) the better off you'll be.
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