Transmission lock-up theory

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  1. #1
    Senior Member Array BlueScreen's Avatar
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    Default Transmission lock-up theory

    First, this is not about the Terminator but I felt this is the most adequate sub-forum for this question.

    I understand what the TCC do. My question is, while drag racing, when should I use it ?
    I have a 4R75W with a Circle D multi-disk 4C converter (3600-3800RPM). I've found that I have to setup the shift point very early because of converter slip. Example, when I get close the rev-limiter in second gear, because of converter slip, I can lock the converter and the revs drop a couple thousands RPM, so I can continue in 2nd gear instead of shifting into 3rd. I think it's faster this way than shifting to the next gear.
    When in 3rd gear, I can pass the finish line without locking the converter. Should I lock it ? When ? Do I aim to lock the converter to keep the rpm as close as possible to the peak torque (not hp) ?

    I understand I need to do trial and errors, but what is the average rule of thumb, to give me a ball park ?
    Thanks

    PS: Bonus question. What is needed to modify in the valve body to allow the converter to lock in 1st gear ?

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  3. #2
    Yes...it can be done Array 03Steve's Avatar
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    I log vehicle acceleration rate all the way down the track with the converter unlocked to get base line data. I then lock it at different points on the track to find what works best. Results vary with converter construction, engine combo, gearing, etc.

    The main idea is that when the converter locks, vehicle acceleration rate data should increase from that point forward (in comparison to the base data).

  4. #3
    Senior Member Array 04Ryan's Avatar
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    ^^^What Steve said...but I'd also like to add that you need to be data logging converter slip. If you are dropping 2000 RPM's on just converter lock something is not right. Sounds like that converter is way too loose for the combo or something is mechanically wrong. When you data log the converter slip you'll be able to see if the converter ever actually catches up with the engine. On my car the converter slip would average around 1100 RPM right after the 1-2 shift...but would quickly fall to ~400 RPM before the converter would lock. You sure the trans is in good shape? Plenty of fluid? Low fluid levels will make the stall speed on the converter much higher then it should. How old is the converter? You the original owner?

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    Senior Member Array BlueScreen's Avatar
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    Thank you both for the replies. I should have given more info about the backstory.
    My car originally came with a T-45, I drove it with that for the past 9 years and I broke the transmission one last final time 1 year ago. I purchased a 4R75W out of a wrecked 04 GT. Over the winter, I fully teared down the transmission, rebuild everything, modified the drums to add more clutch, etc. I also purchased a new Circle D converter. I check all fluid every time I take the car. I once ruined a perfectly good engine because I had an oil leak and went racing with less than 3 liters of oil. I do a lot of mistakes, but I never do them twice.

    Steve, I will double check tomorrow but I don't think I can datalog the acceleration rate. I usually take the difference of speed divided by the difference of time, of different point in the datalog to calculate my acceleration rate. For some reason, the vehicle speed x-axis is always delayed so it kind of makes it hard to calculate correctly.

    Here is a screenshot of my current best time last week-end
    Name:  Log.jpg
Views: 213
Size:  39.8 KB

    1-2 Shift at 6400rpm, vehicule speed 29mph. Without slip I would have been 55mph so 47% slip
    RPM drop to 5600
    5800RPM in 2nd I engaged the TCC, rpm drop to 5200
    2-3 shift at 6400RPM, vehicule speed 84mph. Without slip I would have been 104mph so 20% slip. I know this is not the right data cause is was not slipping. The vehicle speed is offset. This affect the slip % in all gear
    RPM drop to 5600
    6000RPM in 3rd I engaged the TCC, rpm drop to 5300 and ended the race at 5800RPM going 127mph, which is also wrong the time slip show 132mph. Without slip it would have been 140mph.

    So this has me thinking, is my TCC slipping ? I struggle to keep the temperature under 200F doing 60mph on the highway, with a 45k BTU (that get almost no air I must admit). I believe the reported speed is wrong because of the tire size and setting I had to input on the controller is not perfect. For street tire I adjusted everything perfectly with a GPS driving on the highway. I can't do that with my slicks.
    Last edited by BlueScreen; 11-15-2016 at 08:17 PM.

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    Senior Member Array 04Ryan's Avatar
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    What processor are you using? Stock 97 GT processor from your signature? What rear gear ratio? Are you shifting the 4R manually? Manual VB?

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    Senior Member Array BlueScreen's Avatar
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    Forgot that detail, I'm using the USShift Quick4 Controller, commonly known as a Baumann controller. 3.73 gears, 28-10.5-15 slick, auto-shift, stock valve body. Still using the original ECU from my 97, but it doesn't control anything on the transmission.

  9. #7

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    IMO the rpm drop is right for your shift rpm. Locking the converter is just lugging the motor down. Leave the lock up off and try increasing shift rpm to 6600-6700. Do not get to caught up on slip numbers.

  10. #8
    Senior Member Array BlueScreen's Avatar
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    Quote Originally Posted by BNA View Post
    IMO the rpm drop is right for your shift rpm. Locking the converter is just lugging the motor down. Leave the lock up off and try increasing shift rpm to 6600-6700. Do not get to caught up on slip numbers.
    I used to shift at 6900rpm with the manual transmission, I'm slowly getting there a couple rpm at a time with the automatic.

    This is exactly where I was going with the original question, but I'd like to have the "why". I believe it's more efficient to lock the converter and extend a gear about 2000 rpm than to shift in the next one. The torque multiplication of the gear ratio is much higher than the torque multiplication of the converter. Plus, I think it keeps the RPMs closer to my peak torque instead of "lugging the motor down", but I would need a dyno sheet to verify that and I have put the car on the dyno.

    Also, Steve, I checked and I have no PID/DMR for Acceleration Rate.

  11. #9

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    See what it ET's with no lockup. All I am saying is the motor is not recovering as fast as it could when you lock the converter in the middle of 2nd and 3rd gear. If you absolutely have to lock the converter try it at the end of 3rd gear close to your shift point.

  12. #10
    Senior Member Array BlueScreen's Avatar
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    I did the test earlier this summer. I'm 0.2 faster with the lockup engaged in 2nd gear than only in 3rd gear, and that was when I engaged the lockup as soon as it shifted in 2nd. That was on my first day at the track with the new transmission and I was dialing things up, and shifting manually.
    I don't need the lockup even at the end of the quarter mile. I will try to leave it unlocked one more time this saturday.

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    Cool post the results

  14. #12
    Yes...it can be done Array 03Steve's Avatar
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    Quote Originally Posted by BlueScreen View Post
    Also, Steve, I checked and I have no PID/DMR for Acceleration Rate.
    In Livelink it is under MISC. Vehicle Acceleration Rate.Name:  VAR.jpg
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  15. #13
    Senior Member Array BlueScreen's Avatar
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    Steve, for some reason I did not see this every other time I opened the Misc panel. It's there and I log it now. I too have 2 VEHICLE_SPEED and they don't give the same value. I think one is a smoothed version of the other.

    I have a problem with the car doing backfire and not being able to rev over 6000rpm now. That's the reason I was shifting at 6600rpm in the previous log. It feels like I hit the rev-limiter, and now it's a 6000rpm. I have not been able to do a clean pass with the converter unlocked, and one while locking it. I could only make one with the converter unlocked and ran a 10.70 @ 122.5mph. Here's the screenshot. I can send the file if anybody wants if.
    Name:  Untitled.jpg
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    I spoke with a local tuner that was at the track and he told me he always lock the converter as soon as the car shift in 2nd gear, and he keeps it locked until the end of the track. Reason is slip is power loss. On a dyno, they always run higher torque number with the converter locked, because there is less loss. Same thing applies at the track. The track is now closed for the winter, and I need to fix the misfire issue anyway before I return to the track.

    Also, if anybody have an article or a how-to, for the modification needed to lock the converter in 1st gear, I would really appreciate.

  16. #14

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    Send me the datalog file please.

  17. #15
    Senior Member Array BlueScreen's Avatar
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    Try this. Open with SCT LiveLink 6.5

    run5 nolockup good.csv

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