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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:
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:
