How much power will a Torque Converter Clutch hold?
 

Hello everyone, I am wondering how much power a fully applied torque converter lockup clutch will hold before it starts to slip or suffers other damage.

Long story short, I installed a manual lockup switch in my 05 Civic, mainly for performance reasons. With my old trans, I would lock the converter at full throttle for better acceleration. However, I have my doubts about if doing so is good on the TCC.

The TCC is (on my car anyways) only designed to lock under light to medium loads in 3rd or 4th gear, so I don't know how good it is on it to hold the engine's maximum power. And to make matters worse, I added around 30 HP above stock to my engine.

So how much load can the TCC safely handle? I imagine it can't handle maximum power since it was never designed to be applied under heavy loads let alone full throttle, and especially not with the extra power I added to my engine. Is this something I need to worry about?

Thanks in advance for your opinions!

I'm curious, what was the performance gain with it locked?

I have a torque converter that was ordered with a low stall and a high performance lock up clutch. I have it in my one ton Gmc with a 6.5L Diesel and a very custom 700r4. All mechanical with no computer controls. I can pull my 10,000 lb Avion travel trailer in overdrive locked up. The converter never slips, if the tranny is not right the overdrive band will slip first.

Well I don't have any numbers (yet anyways), but from my experimentation, I feel like the car pulls a little harder once I lock it once I get above 5000 RPM or so. The other benefit is that locking the converter allows me to stay in that gear for a little longer because the RPM's drop a little. Normally I can stay in 2nd gear until around 70, but locked it will go to 76 in 2nd.

The most fun (and possibly most destructive) way to use the manual lockup though is to floor it while it's locked going about 55 or so. The trans slams into 2nd gear hard, the RPM's jump to around 5000 (about where my engine makes peak torque) and it takes off so hard! The way it dramatically slams into 2nd gear and immediately pulls makes the car feel a lot quicker than it is LOL.

However, 2nd gear was the first gear to start going out on my old trans (quickly followed by the rest), so I'm not sure that shifting the trans with the converter is locked is healthy for it. Maybe it was just a coincidence since the trans had 243K miles on it when it died, who knows.

Nice! :thumbup: Would you expect that a stock Honda TCC would be strong enough to not slip under maximum throttle? And is there a way to know if it is slipping?

You are forcing the second gear clutches to carry all the load when you drive your car like this, the torque converter is a cushion when unlocked. The converter is not the weak link in this chain. Remember there is always a fuse in a circuit, you may be moving the point of failure to second gear from somewhere else. When I was a young man my pickup went through universal joints, entirely my fault due to abuse. I discussed this with my father and I suggested installing larger and stronger joints. This is when the fuse discussion occurred, if we move the point of failure to a more expensive part we really have not gained anything.

I put a converter lockup switch on my 1996 F250 diesel. I only use it when I'm in the mountains and using an exhaust brake while towing a travel trailer. I don't purposely leave the switch on / converter locked during gear changes because of the possibility of transmission damage due to hard shifting. I don't know if the transmission would actually be damaged, but it isn't worth the chance, IMHO. The hard shifts have to be putting more strain on something somewhere.

could be that the next fuse is 40 amps instead of 20 amps though, in which case you doubled your available current/torque, though via destructive testing. Might just be a sheared key, might be a cracked case.

That's kind of what I figured, thanks. But is the problem the harsh shifts, or is the problem the TCC slipping from not being able to handle that much power?

Let's remove the shifting while locked from the question for now and focus on the actual lockup clutch. Would I be able to push enough power through the TCC to actually slip or damage it?

For example- When I moved to Memphis, I pulled a 2500 pound trailer full of my stuff through the mountains. When I was climbing mountains, the I had the trans in 2nd or 3rd gear under a lot of load. Would climbing with the converter locked cause the clutch to slip, or would it hold the power okay? Thanks

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?

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.

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. https://www.w8ji.com/images/Cars/Tor...er/torque1.gif

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.

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.

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!

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

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?

Oil gets hot going over relief in a hydraulic system, So if there is a mechanical relief in your transmission that may be a problem. I think the computer controlled transmissions just control the pressure with no relief. High line pressure is only commanded when needed so fuel mileage is out the window at full throttle anyway. It may cause extra load when not needed for normal driving.

How much power will it hold? It depends...on size/diameter of the clutch, how worn it is, line pressure, fluid condition, and engine torque. A good one can hold a surprising amount of power before slipping, but a worn one might slip all the time.

That makes sense, thank you. My TCC is a ring about 9 inches in diameter and 1/2 inch wide. I calculated that the surface area of the clutch is about 13.3 square inches.

In my opinion the TCC seems quite small to me considering the size of a manual transmission clutch for the same car, especially since it's a wet clutch. Unless the hydraulic pressure holds the TCC with way more force than a manual trans clutch is held?

It really depends on clutch material, line pressure and how much surface area there is for the fluid to push on.

Clutch material can vary quite a bit. You can buy "soft" clutches for manual transmissions or you can buy a ceramic clutch that you cannot slip and it grabs instantly(like in the big trucks).

More line pressure equals more clamping force. The same goes for fluid surface area. The more area there is for the fluid to clamp the clutch together, the more it will hold.

You can buy torque converters for pickups that have multi-disc clutches that can hold a ton of power.

A smaller tcc or smaller car will have less holding power, but they make less power to begin with.

If you want to find out for sure, use a tcc switch and give it the beans in each gear, watching for slippage.

That makes sense, thank you! I never feel any slipping when the TCC is locked, even flooring it at peak torque. However, my concern is that I may not necessarily feel if it is slipping. The PCM on many cars (including mine) slips the TCC quite a bit by default and it is never felt. If it is in fact slipping under load am I likely to feel it, or will it slip itself to death with no warning that it is self destructing? Thanks in advance!

Do you have a tach or a way to see your rpm? If your rpm's never change when you goose it, you're fine.

Thanks. When I floor it with the converter locked the RPM never changes until the car starts accelerating, like a manual transmission. But Is it possible that it is slipping and not showing on the tach? Like if it is only slipping a tiny bit?

If your tach doesn't move, then it's most likely not slipping. Being a wet clutch, a little bit shouldn't hurt it like it would on a dry manual clutch. Still not good, but not as bad.

Static friction is higher than sliding friction so if your TCC slipped, you would notice it because it would probably slip quite a bit. Because of that differential, it's very unlikely to slip a tiny bit.

This same situation is why it's very difficult for a human to ride the optimum pressure while braking. Tires have the most stopping power when rolling but once they lock up suddenly you need a lot less brake pressure to keep them locked. Modulating that by feel is very difficult. Computers are much better because they can keep the tire slipping just a bit which is an indication that tire friction is highest.

PS. My TCC and trans are just fine after ten years of using a lockup switch so all those nay-sayers can eat crow now. :D

I've been using a TCC lock switch on my Chevy 1500 2wd for close to 15 years now. (4L60E). Initially I discovered that the PWM valve (electronic pressure modulation valve) would reduce the pressure to the clutch and slip it, so I also had to disable that. I tow large loads all the time....... far larger than the pickup is rated for. I lock up once I get moving, and I shift manually, as I also have a switch wired in that prevents the 3:2 downshift and the 4:3 downshift, though I NEVER use 4th when pulling. As I know when my transmission will downshift, I can let off the throttle momentarily during the shift... again, I'm manually shifting while towing, and often in normal driving. This relaxing of the throttle before a shift is far different than the violent shift one gets when a vehicle downshifts under full throttle.
I have had zero problems with my tranny, which has right at 200K on it (1997 year). I do a flush every few years, by just changing the filter, and refulling, then pumping fluid out a cooler line through a clear plastic tube until I see bubbles, shut off, refill, etc... until I runs clear new fluid....... About 4 gallons total.

H.W.

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.

Great info, thank you so much! I find it amazing that such a small clutch can hold that much torque when it is surrounded in oil! As stated above, the actual surface area of the friction material is not very large.

I'm not doubting what you are saying, but why would a manufacturer design the TCC to be so much stronger than necessary? In stock form, the TCC will not engage on my car when the engine is under heavy load, it will only engage under light load in 3rd or 4th gear, so it would not need to be able to handle peak torque.

Also, what is your opinion on shifting the trans with the converter locked? It definitely shifts harder when locked, but will any damage be done over time if I regularly shift with the converter locked? Thanks

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.

H.W.

I see where you're coming from but there are a lot of factors involved. For example, in a 4l60 (which really means anything based on a 700r4 i.e. 4l60, 60e, 65e, 70e) the only difference between 3rd and 4th is that a band (band brake) applies. The band STOPS the drum it interacts with. So in that case there is actually less weight spinning in 4th then there is in 3rd. I doubt the weight of those spinning parts is a major factor, just illustrative of unintuitive it is trying to picture how a planetary-gear-based auto trans actually works.

It's undeniable physics that a 1:1 ratio is inherently more efficient than an overdrive ratio, but how those ratios are achieved can easily outweigh that. One problem with making your top ratio 1:1 with planetary gearsets is that you can make 3 ratios with a planetary geaset. An underdrive, a 1:1, and an overdrive (and a reverse but that's irrelevant unless you are applying one reverse to another reverse to get back to a forward gear). So already you realize that in order to get 4 forward speeds like a 4l60 you have to have more than one planetary gearset (a 4l60 has two sets). Then, you realize that if you wanted to have 4 gear ratios BUT with the top ratio being exactly 1:1, you actually need THREE gearsets with all their associated spinny bits. Now you can feed one gearsets underdrive ratio to another gearsets overdrive ratio or vice versa and end up with 1:1, but that's not giving you the benefit you were referring to anyway. So with planetary based automatics you can always just bind 2 of the 3 elements of all the planetary sets and end up 1:1, but to have sufficient gears under 1:1 (called the trans's 'ratio spread') would in some cases require more parts and weight and friction and loss then having the same number of ratios, but with the top ratio being overdriven.:eek: