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Old 08-06-2019, 04:41 PM   #11 (permalink)
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Originally Posted by Piotrsko View Post
pretty much you'll eat the friction materials in the planetary gear hubs. the TC should be able to hold 1 1/2 times max engine torque, kinda an automotive rule of thumb.
Exactly what I was looking for, thank you! So the TCC won't slip or sustain damage if locked under heavy load, but it's not good on the trans to shift it while locked. Is that correct?

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Old 08-06-2019, 04:55 PM   #12 (permalink)
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I think it stays unlocked not to prevent failure but because it adds torque multiplication. Adding torque like a reduction gear at the expense of efficiency.
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Old 08-06-2019, 06:32 PM   #13 (permalink)
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Originally Posted by Hersbird View Post
I think it stays unlocked not to prevent failure but because it adds torque multiplication. Adding torque like a reduction gear at the expense of efficiency.
Correct me if I am wrong, but my understanding is that the torque converter basically only multiplies torque from a stop or when trying to accelerate hard in a high gear.

I remember reading somewhere that a torque converter needs at least a 500 RPM difference between input and output speed (slip) to multiply torque, so any less than that just wastes energy from my understanding.
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Old 08-07-2019, 12:20 AM   #14 (permalink)
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Originally Posted by EcoCivic View Post
Correct me if I am wrong, but my understanding is that the torque converter basically only multiplies torque from a stop or when trying to accelerate hard in a high gear.

I remember reading somewhere that a torque converter needs at least a 500 RPM difference between input and output speed (slip) to multiply torque, so any less than that just wastes energy from my understanding.
I believe without lockup and high engine power it almost always will have at least 500 rpm difference, and that will give about a 10% increase in horsepower and torque. Basically free power compared to a mechanical locked up clutch or locked torque converter. Free meaning there were no changes to the motor to make more power, but it comes at the cost of efficiency. Here is one chart I found from a real converter but looks like it might be something of a racing converter.
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Old 08-07-2019, 12:24 AM   #15 (permalink)
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By unlocking the torque converter when you give it the beans it does two things, it flashes the motor into probably a higher power band and also gives you more torque multiplication from the converter. They aren't unlocking it to save it from destroying itself, but to deliver as much power to the wheels as possible.
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Old 08-07-2019, 10:04 AM   #16 (permalink)
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I agree completely with the posts above. For short bursts of acceleration unlocked converters wake up the acceleration. However for cruising you want everything locked up if possible. Back in the pre lockup and pre overdrive days 2.56 gears were popular. The problem was cooling the transmission fluid. I grew up in Southern California and Interstate 5 runs up over the mountains North into the San Joaquin valley. I could always tell which cars and pickups had turbo 400 transmissions because at the top they would shift back into high gear and hot atf would blow out the vent onto the exhaust and smoke like crazy.
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Old 08-07-2019, 06:52 PM   #17 (permalink)
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Thanks everyone for your responses. I have another question now. I boosted my transmission's pressures by unplugging the pressure control solenoids and plugging in extra solenoids so it doesn't set a code and go into limp mode. Is this an okay thing to do? I would think if anything it would extend the transmission's life because it now shifts much quicker and therefore slips the clutches less, but I just want to be sure. Thanks!
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Old 08-08-2019, 09:49 AM   #18 (permalink)
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The slow shifts stock trannies have is because most people dislike the harshness. My experiences don't indicate a life expectancy change on the clutches, but other weaknesses show up after a while when hotrodding and shouldn't be a factor for you
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Old 08-08-2019, 10:47 AM   #19 (permalink)
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Computer controlled transmissions can be tricked into firmer shifts by adding a resistor to the line pressure solenoid wire. Running at high line pressure all the time is not necessary.
4L80 GM transmissions will break the case if the line pressure solenoid sticks high for too long. There is a preemptive fix for this from Gill Younger.

The shift kit in the old days was done with springs and hole sizes on the valve body separator plate. Now it is done with resistors and programming. You have combined old and new technology by working around your computer. I would probably add a “sport/daily driver” switch so you can choose easily. I think your transmission will last longer.

In the previous version of my 700R4 my builder flipped the 2/3 accumulator piston and used a Chrysler spring to firm up the shift. In a performance car that weighs 2500 to 3000 lbs this would be fine but in a 6000 lb truck it was quite harsh. This transmission would shift from 2nd to 3rd with the converter locked and with no accumulator it would snap your neck back and was uncomfortable.

This latest version of my transmission we went back to a stock accumulator setup and it still shifts locked up but it is very nice now.

The previous version of my transmission lost the low planet set and GM has come out with a heavy duty 5 pinion planet since we built the previous version 10 years earlier.

Sorry about the long dissertation but I have been running performance transmissions since the 70s. I hope this gives some old school knowledge.
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Old 08-08-2019, 11:06 AM   #20 (permalink)
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Originally Posted by me and my metro View Post
Computer controlled transmissions can be tricked into firmer shifts by adding a resistor to the line pressure solenoid wire. Running at high line pressure all the time is not necessary.
4L80 GM transmissions will break the case if the line pressure solenoid sticks high for too long. There is a preemptive fix for this from Gill Younger.

The shift kit in the old days was done with springs and hole sizes on the valve body separator plate. Now it is done with resistors and programming. You have combined old and new technology by working around your computer. I would probably add a “sport/daily driver” switch so you can choose easily. I think your transmission will last longer.

In the previous version of my 700R4 my builder flipped the 2/3 accumulator piston and used a Chrysler spring to firm up the shift. In a performance car that weighs 2500 to 3000 lbs this would be fine but in a 6000 lb truck it was quite harsh. This transmission would shift from 2nd to 3rd with the converter locked and with no accumulator it would snap your neck back and was uncomfortable.

This latest version of my transmission we went back to a stock accumulator setup and it still shifts locked up but it is very nice now.

The previous version of my transmission lost the low planet set and GM has come out with a heavy duty 5 pinion planet since we built the previous version 10 years earlier.

Sorry about the long dissertation but I have been running performance transmissions since the 70s. I hope this gives some old school knowledge.
Thank you for your help. I don't think I am going to break anything due to the high line pressure because I didn't modify anything mechanical (like springs) and from my understanding, the PCM already commands maximum line pressure at full throttle to keep the clutches from slipping. Maximum line pressure is also commanded in "limp mode" to prevent the clutches from slipping and burning up if there is a problem. So it's designed to handle maximum pressure, at least intermittently. The shifts are never "harsh" by my standards as long as the converter isn't locked, so I don't think I'm going to break anything from hard shifts either.

My concerns would be if I am working the pump harder it may possibly shorten its life. Also, I imagine that holding a high pressure might require more power from the transmission's oil pump, reducing MPG and creating more heat in the transmission. Am I right?

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