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To fill a hole in the drivetrain section, let's start a thread on torque converters. Forum gurus' advice welcomed!

Fundamentally, a torque converter is a donut with two fans inside it. One fan is connected to the engine, the other fan is connected to the transmission. The converter is filled with transmission fluid, and so when one fan spins, it blows fluid against the other fan and causes it to spin. It's a little more complex than that, but that's basically how they work. It's essentially a hydraulic coupling, and that's what allows your engine to keep slowly turning while the drive wheels are stopped at a light. That's also why automatic-equipped cars always want to "creep" while in gear.

In the mid 1980s and on, auto manufacturers started adding a lockup clutch which allowed the two fans to lock together and eliminate slippage while at speed. This lowers the heat of the transmission (slippage = heat), and improves gas mileage. The 4L60E transmission is designed to accept a lockup converter. Be aware that some companies sell non-lockup converters for the 4L60E. They are noticeably cheaper, so be careful you aren't getting a "deal" because you're giving up lockup.

Lastly, the converters in all modern cars also have a "stator" which is a little fan between the two big fans and it helps redirect the flow between the fans. In case you're interested, it is attached to a one-way roller clutch keeps the stator from spinning backwards at low RPMs but allows the stator to freewheel forwards as the vehicle speed and engine speed pickup. Yes, this clutch occasionally fails, but it's very rare to fail on B-bodies. There are several different stators available and they affect the cost and efficiency of the converter.

Why would I want to change or upgrade my torque converter? Your car's engine produces different amounts of torque at different rpms. If you plot it on a graph of torque vs. engine rpm, it's called a torque curve (like the output of a dynamometer test, or 'dyno'). Converters can be constructed in such a way that they allow the engine to rev higher before they begin transmitting the torque to the transmission. This why it's called a "converter" - it converts the engine torque produced at a higher rpm to a lower output rpm.

As an example: The stock torque converter is 298mm (or 12") in diameter. It is designed to "stall" at 1400rpm. What this basically means is that in first gear, the maximum rpm difference between the two fans will be 1400rpm. If you floor it from a stoplight, the engine rpms will immediately jump to 1400 rpm and the car will accelerate. As the car begins to move and the driveline speed starts to increase, you approach the stock LT1's peak torque at 2400rpm and the car will accelerate faster at this rpm than when it was first starting to accelerate. If you were to put in a converter that "stalled higher" - that is, it allowed the engine to reach peak torque sooner, then instead of leaving the stoplight with the engine at 1400rpm, it would leave the line at 2400rpm, accelerating harder because the engine is now putting out peak torque right from the start.

Stall speed is a loose term dependent upon engine torque, vehicle weight, tire height, transmission gear ratio, and rear axle ratio. See the table at the bottom of this post.

The K-factor: Vehicle manufacturers and some of the aftermarket converter manufacturers use the K-factor as a measuring parameter. See post #6 below.

Another way to describe a given converter's behavior is the "Stall Torque Ratio" or STR. This number reflects how tightly coupled the two fans are. A higher STR means a looser converter, which means the engine will rev higher before the car gets going. You get more torque this way, but the drivetrain connection will feel sloppier and the throttle response will feel mushy. It used to be that the only way to get a lot of torque off the line at stoplights or the drag strip was to have a "loose" or "high stall" converter, but...

The aftermarket began selling torque converters with smaller diameters than the stock 12" that could work on a 4L60E behind a V8. 10" and 9.5" are common references for these units for the 4L60E. With a smaller diameter, they have less lever arm than with the larger diameter, so you can keep the coupling very close (and efficient) but still have a higher stall speed. It's the best of all worlds. You also get the benefit of less rotating mass, which means the engine actually revs up more quickly. In fact, most of these converters are GM V6 (245mm) converters with a flexplate adapter ring bolted to the front and a 4L60E pump drive hub welded into the center of the rear. Additionally, companies can customize the internal parts (such as the stator, the fan blade angle, and the clearance between the two fans) or use their own custom parts to provide different STR and different efficiency characteristics. Be aware that modifying a 245mm converter to run on the 4L60E is labor intensive and not cheap; that's why prices normally start around $500 for one vs. $300 or less for a 12" converter. Be wary if you find a vendor that significantly violates this rule. You may also be getting a non-lockup 245mm converter.

For the stock diameter, you have a couple of options. Chevy S10 pickups with the 4.3L V6 used a converter that, when put in a B-body, will raise the stall speed to 2000 rpm. Likewise there is a Corvette torque converter of the same year (94-96) that provides a higher stall speed. Part numbers are included in the posts that follow. Typical stall speed range is 1400 (stock) to 2200rpm. Most reputable torque converter manufacturers will not sell you a 12" converter with a higher stall speed than this because the throttle response just becomes too undesirable.

For smaller diameter, "2800rpm" is usually the lowest quoted stall speed you can get. The sky's the limit with these converters though, as their smaller diameter avoids the drastic efficiency and throttle response tradeoffs you run into with looser stock diameter converters. Be aware that the higher the stall speed, generally the harder the drivetrain will hit the tires. If you don't have good traction as it is, adding a better torque converter will only exacerbate the problem. Lastly, different vendors will put different number labels on their converters and will drive differently. They are the experts at choosing the correct application for your car. Please see the following posts on this thread for more detail.

Be aware that several companies provide their own high-performance parts, including a billet front cover to resist deformation under high torque loads, an anti-ballooning plate at the pump hub to keep the torque converter from ballooning under heavy torque and binding on the pump, upgraded internal bushings, strengthened internal fan blade connections, unique stators, and strengthened one-way roller clutches. Finally, the smaller surface area available in the 245mm and smaller converters makes it harder for the lockup clutch to handle a lot of power. Some companies offer multi-disk lockup clutches which add rotating mass but allow you to lock the converter during WOT. Shop around, look around, research, and do your homework. Also, don't trust any converter vendor that doesn't ask you lots of questions about your vehicle. They have lots of experience choosing the best combination for your vehicle. Let them help choose the right parts combo for your application. Buying parts you don't need is money you could have spent somewhere else.

Rules of thumb on torque converter stall speed for the same exact torque converter as we change one vehicle parameter at a time:
more vehicle weight = higher stall speed
more rear axle ratio (i.e 3.08 -> 3.73) = lower
more engine torque = higher
more transmission first gear ratio = lower
taller tires = higher

For each of these, the reverse is also true:
less vehicle weight = lower stall speed
less rear axle ratio (i.e 3.42 -> 2.56) = higher
less engine torque = lower
less transmission first gear ratio (i.e. 4L60E to TH350 swap) = higher
shorter tires = lower

Finally, if you are planning on buying a torque converter now for your vehicle and then modifying it later, explain this to the vendor so they are aware of your plans. Remember, your goal is to match the torque converter characteristics with the torque curve of your engine to get the best possible performance.

Comments and suggestions welcomed!

Here are some other selected articles and links:
 

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converter

so a 2000 stall converter will give my completly stock 96ss a quicker than stock launch and run my trans cooler
 

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Discussion Starter #3
so a 2000 stall converter will give my completly stock 96ss a quicker than stock launch and run my trans cooler
If the converter is the stock 12" diameter, yes, a quicker launch. No, not cooler than stock. But not much hotter either.
 

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so a 2000 stall converter will give my completly stock 96ss a quicker than stock launch and run my trans cooler

A 2000 stall converter will likely be the same diameter as the stock and will do what you ask in a very marginal sense. It will have the same rotating mass and only increase your stall by 400 rpm (if that) from what is stock. Part of what makes the higher stall converters work is that they are smaller in diameter and lighter in weight and therefore spool-up quicker to their elected stall speed allowing for higher torque multiplication when they get there. That's why it's better to launch from a dead idle then it is to power brake (load the converter). You get a better run (spool-up) at the lock up zone which hits the tires harder.
 

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A 2000 stall converter will likely be the same diameter as the stock and will do what you ask in a very marginal sense. It will have the same rotating mass and only increase your stall by 400 rpm (if that) from what is stock. Part of what makes the higher stall converters work is that they are smaller in diameter and lighter in weight and therefore spool-up quicker to their elected stall speed allowing for higher torque multiplication when they get there. That's why it's better to launch from a dead idle then it is to power brake (load the converter). You get a better run (spool-up) at the lock up zone which hits the tires harders.
So when i take my car to the track, i should launch off idle or load the converter?
 

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As mentioned, stall rpm rating of a converter is not a constant. It depends on the vehicle torque, weight, drivetrain (losses and preload), powerband, launch rpms, etc. The constant value is called a centrifugal K-factor which roughly equals to: rpm * sqrt(torque). This is a basic formula that does not account the other factors.

Only the big companies like Precision Industries, Yank and ATI have access to the equipment to actually measure a converter K-factor. Smaller converter makers guess the K-factor and stall rpm by using approximate mathematical formulas.

It means that not all converters are rated equally. For example a 2800 Edge can stall at 3400 rpms in your car while a 3200 Yank SS converter would stall at 3250 rpms (not the actual numbers). Yank SS 3600 is a very driveable converter despite a fairly high stall rating.
 

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So when i take my car to the track, i should launch off idle or load the converter?
Launch off idle. Not only for the reasons 1FAST94 mentioned, but for desired suspension action/weight transfer too.
 

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moderator comment

This thread needs digital adhesive backing.
It already is. As drivetrain moderator, I decided to change from just stickying every single thread folks liked to just making one big LINKS thread at the top of the section. It keeps the section cleaner and lets me catalog the 25-30 threads that really add a lot of value to this section. This thread is already linked in that list.
 

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Hey Joel!

Great post, i might have to just point people heere when they buy a converter and have questions LOL
 

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Discussion Starter #12
It might save you some time on the phone!

BTW, I welcome any comments on this thread or any other threads I started for the sake of improving the content. As soon as I get some time, I will go back and add the K-factor discussion and several other points folks have made.
 

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Hi Joel,

I have the text of the old Imp SS FAQ.
Not sure if you need it put here the TC section.
Just take what you need and delete the rest..... :p

The stock Impala converter is rated to stall at 1397 RPM (actual stall will be anywhere from 1400 to 1600, depending on engine torque output), while the highest stall converter GM offers is rated to stall at 2025 rpm (actual stall will be around 2000 to 2200 rpm, depending on engine torque output). The higher stall speed means greater torque multiplication which improves acceleration off the line. This also virtually eliminates the annoying creep while idling in drive as well as the clunk you sometimes get when shifting from park into either drive or reverse.

You can tell which converter you have by a 4-digit alpha code found on a sticker attached to the converter.

* DGHG = stock '94+ Impala converter
* DBCF = '86 'vette converter
* DBLF = '95+ L35 S10 truck converter

The code is interpreted as follows:

* 1st Digit: Transmission application
o D = 1984-1/2 and up 700-R4, 4L60, 4L60E
o C = 1984 and earlier 700-R4, 200-4R, 200C, 325-4L
o B = 250C, 350C
* 2nd Digit: K-factor (stall speed)
o K = K-85, 1211 rpm
o G = K-100, 1397 rpm
o F = K-110, 1611 rpm
o E = K-115, 1654 rpm
o B = K-140, 2025 rpm
* 3rd Digit = Clutch and Damper assembly
o C = ? (stock '86 'vette)
o H = ? (stock '94+ Impala)
o L = ? (stock '95+ L35 Vortec S10 truck)
* 4th Digit = Rear cover
o C = 3 lug round
o D = 3 lug round
o E = 6 lug
o F = 3 lug square
o G = 3 lug square

Going to the S10 converter increases stall speed by 628 rpm, which feels very good indeed. The factory 2025 rpm converter is an excellent, low cost upgrade that offers a very noticeable effect on performance without being too radical.

One area that some of the better (and more expensive) high stall aftermarket converters will excel in is durability in high HP applications. While the stock converter can live behind 300 to 350 HP with few problems, going to 400 HP or more can be pushing it. Actually in my experiences the trans will go before the converter, unfortunately when the trans goes the debris usually takes the converter with it (of course the opposite is also true).

Originally, the '85-'86 Corvette torque converter was recommended as an Impala upgrade. As you can see from the 2nd digit code this is a K-140 converter, which is the highest stall factory converter available for the 700-R4, 4L60, and 4L60E transmissions. The original part number for the '85-'86 'vette converter was #8650919, which was later changed to #24201203. GM only sells torque converters as what are called remanufactured units, but depending on the converter application you order, you may actually get a new converter instead of a rebuilt unit. Unfortunately since the '85-'86 'vette is an older application, you will almost certainly get a rebuilt converter if you order under that part number. Not only will the converter be used (rebuilt), but since then some of the internal components have been upgraded.

To get the high stall converter in an updated form with the latest design, order a similar K-140 high stall converter from a 1995+ 4.3L (RPO L35) S10 Truck V6 application (p/n 24202310). This new converter uses the same basic internal components (stators, etc.) as the 'vette converter, and as such has the same high stall speed rating (2nd digit code "B").

GM ranks converters by what they call a "K-factor", which indicates torque capacity and the resultant stall speed. Mark McPhail of GM Motorsports says both the '95 L35 Truck and the '85 Corvette converter described here have the same K-140 internals, resulting in the same stall speed given the same engine torque output. Since stall speed varies with engine torque, GM gives a rating of 2025 rpm, however this converter stalls at around 2200 rpm in a stock Impala.

The L35 Truck converter is listed under a new part number for 1995 and newer applications, so you are much more likely to get a brand new one when you order it instead of a remanufactured unit. The second thing is that the L35 truck converter has damping springs added to the TCC (Torque Converter Clutch) assembly. That may explain the code difference in the third digit. This does not affect performance per se, but apparently reduces noise and possible shudder or vibration when the TCC engages. Note that although the 4th digit code is different in the S10 and 'vette converters from the stock Impala converter (code "F" vs. "G"), the lug shape and mounting pattern is identical,

Finally, this new converter costs exactly the same as the 'vette converter. Both list for about $209.66, and sell for $157.25 trade price. The stall speed is 628 rpm higher than stock but is not in anyway too high for even a completely stock Impala. Remember this is a factory 298mm (11.73") torque converter with a TCC (Torque Converter Clutch) and is a direct replacement for the factory unit. Note there is an additional $35 core charge which you will get back when you return your stock converter to the dealer.

Another company, Precision Industries, is offering a 9.5" 2500 stall converter for the 4L60E transmission called the "Vigilante". This converter, along with the Trans-Go shift kit, are supposed to be able to shave almost half a second off the quarter mile time! Be aware that the 9.5" converter will generate more heat than the stock converter, but the exceptional cooling capacity of the transmission will probably handle it. The Vigilante goes for around $800
 

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so i would decrease my 1/4 time if i launched off a dead idle compaired to loading the converter up ??
 

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How do you know when it's time to refresh or rebuild a TQ?
How do you tell it's wearing out?

I bought my car with a YANK SS3600 in it. First car I've had with any real stall in it. Drives great on the street but you notice the stall on little hills idling around.
 

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I had the earlier (believe it was marked 89) Corvette converter in my SS before the T56 went in. Combined with a shift kit, it was great. I may have to do the same combo in my wagon...
 

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Choosing a higher stall TC does what to the TC "lock up" RPMs? If I decide on a 2,800 stall, and for highway driving, does the TC lock up at say... 2,200 rpm at 70 mph on 3:73 gears? or will there be more slippage?

I still want to be able to cruise at a decent rpm is my concern.

Love this thread!
 

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At low throttle cruise a good TC will lock up well below its stall speed. You should have no problem cruising at 70 with it locked. I doubt you'll be at 2200 at 70 cruising with 3.73s though. My 3.42 gears put me higher than that I think.
 

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As mentioned, stall rpm rating of a converter is not a constant. It depends on the vehicle torque, weight, drivetrain (losses and preload), powerband, launch rpms, etc. The constant value is called a centrifugal K-factor which roughly equals to: rpm * sqrt(torque). This is a basic formula that does not account the other factors.

Only the big companies like Precision Industries, Yank and ATI have access to the equipment to actually measure a converter K-factor. Smaller converter makers guess the K-factor and stall rpm by using approximate mathematical formulas.

It means that not all converters are rated equally. For example a 2800 Edge can stall at 3400 rpms in your car while a 3200 Yank SS converter would stall at 3250 rpms (not the actual numbers). Yank SS 3600 is a very driveable converter despite a fairly high stall rating.
So if i buy the edge converter it wont be as good as a yank or precision industries converter? Does edge guess the K-factor as well?
 
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