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Old 09-07-2019, 02:51 PM   #33 (permalink)
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Originally Posted by Vigo View Post
By and large any converter clutch a car comes with with hold the full stock engine torque. Adding peak hp to an engine doesn't necessarily change that scenario very much because a clutch only cares about torque and the torque your engine makes at peak power even after mods is probably less than what the stock engine made at its torque peak.

Aside from that, there is a big difference between the clutch holding peak torque and engaging during peak torque. This is highly related to where the whole 'don't tow in top gear' thing comes from. Going back to my first statement, in almost all cases every gear in a car will hold that engine's peak torque, including the top gear. Some newer cars may not fit this because the electronic controls can be used to completely disallow you from making full torque without shifting out of that gear, so they could build the trans so that some gears may not hold full torque. But the not towing in overdrive thing predates electronic controls completely anyway. So, using the 4l60 someone mentioned as an example, 3rd and 4th gear actually use the same clutch pack, and it's this clutch pack which typically burns up first in a 4l60. Same clutch pack with the same capacity whether you are in 3rd or 4th gear. The PROBLEM comes from shifting back and forth between gears during high engine torque. If you simply manually control the shifts and reduce throttle during shifts, the accelerated clutch wear that comes from high-torque downshifts while towing pretty much goes away. At that point you can both tow in any gear you want, and also not suffer much effect to the lifespan of the trans.

Minor caveats to above paragraph. On some transmissions there can be a situation where you're at a low enough throttle to stay in X gear, but a high enough throttle to unlock the converter clutch (again assuming you haven't taken manual control of it through mods). In that scenario putting high engine torque across an unlocked converter for long periods like climbing that hill in California someone mentioned, will put a bunch of heat into the fluid. So that's another scenario to watch for and another reason to take manual control of the TCC if you know when it should be on and off.

Long story short, avoid engaging holding elements like clutches and bands under full engine torque if you want a stock or better-than-stock lifespan from the components. OEMs now do this automatically with torque management during shifts by retarding ignition timing or skipping ignition or injection events during full throttle shifts. That's part of why modern sports cars make such distinct noises from the exhaust during shifts.
My situation involves a 4L60E. 4th gear is overdrive.... about 20% I believe, and while it basically is strong enough, it is not a very strong gear, intended to be used under very light load cruising.... Of course the computer shifts out of it under pretty light load. I have a prejudice against overdrive....... It's a stupid thing to have IMHO. You are running through a gear to speed up the driveshaft, then through another gear set to slow the axle rotation speed down. They are normally light and weak, nor is it rational to speed up so you can then slow down... it costs energy unnecessarily. In third gear in my 4L60E, and the highest gear below overdrive in most vehicles, the power is not running through ANY gears in the transmission.. It is locked up straight through... It's most efficient configuration. As a result there is no discernible fuel efficiency gain from overdrive that I have ever been able to observe. It would be far better to have a faster rear axle ratio, and 1:1 in top gear, and the rest of the gears steps down in RPM, but "overdrive" sounds "cool", and sells, irrational though it is.

On my 4L60E, and most automatics these days, the torque converter is used as if it were a gear. The first downshift you feel is not a downshift at all, it is the torque converter unlocking........... This makes heat, and costs fuel economy as you are churning oil to allow the engine to rev more. When I'm shooting for maximum economy, I always lock the TCC, and manually shift..... because the shift points are set with the idea of the torque converter being an intermediate shift. This NOT done for efficiency, but for that smooth feel everybody seems to want. On my pickup (C1500 1997 2wd), the computer will tell the PWM pressure control to slip clutches to make it feel smoother......... something I strongly object to, and the reason I disabled the PWM for the TCC.
My highway fuel economy on a trip in decent weather conditions tends to run 21 - 22 mpg.... 4.3L vortec V6, which for a full size extended cab pickup carrying hundreds of pounds of tools, is not bad at all.
I've overhauled numerous transmissions over the years, and though the 4L60E is not one of them, I've seen them apart. My choices are based on trying to extend transmission life as well as economy, and at 200K, it has far outlived most 4L60E transmissions. They are not a strong transmission.

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