Banks Talks Tech

The Banks Billet Torque Converter is Diesel Tough

Stock torque converters are marginal and can fail prematurely when used with heavy loads. Here's one that's diesel tough.

By C.J. Baker

Nowadays, most people prefer an automatic transmission mated to the diesel engine in a pickup or SUV. In fact, many such vehicles aren't even offered with a manual transmission. Of course, the automatic transmission used in such vehicles must be up to the job, while still providing acceptable drivability, smoothness, and reliability. Things like vehicle weight, towing capacity, engine speed and torque output, shift quality, and cost are all considered by the vehicle manufacturer, and this is where compromises are made. We've already talked about the compromise between automatic transmission shift quality and reliability of Ford E4OD and 4R100 transmissions in "Why Add a TransCommand" elsewhere on this site. Similarly, it would add extra cost to provide torque converter capability beyond the minimum needed to meet most anticipated vehicle usage. However, for those owners who will subject their diesel trucks to frequent or sustained high loads, the torque converter is another potentially weak part of both Dodge and Ford diesel automatics, and that is what this article is all about.

To most folks, the internal workings of a torque converter are largely a mystery, and we're not going to try to explain how a torque converter works here, except in the most rudimentary way. If you want to know more about how a torque converters work, see "Understanding Torque Converters" and "Understanding Stall Speed". For this discussion, we can define a torque converter as an RPM-controlled fluid coupling that replaces a conventional clutch to allow the engine to continue running when the vehicle is stopped in gear. That's a gross oversimplification for a complex device, but there are only a couple of things we need to know. First, a 100% lockup is never achieved by the fluid coupling in a torque converter. A 100% coupling is achieved only after the lockup clutch, if the torque converter is so equipped, is engaged. Second, we need to know what normally fails when a stock torque converter is subjected to heavy loads and/or high power output from the engine. Let's investigate each of these things separately.

Unlike a conventional clutch that achieves a direct engagement between the engine and the transmission, a fluid coupling in a torque converter never quite achieves a total lockup. Instead, there's typically some slippage. How much slippage occurs is determined by the size and internal design of the torque converter, the load on the vehicle, the viscosity of the transmission fluid in the converter, engine speed, and engine power output. Such slippage is not desirable at cruising speeds because it generates heat in the transmission fluid and it decreases fuel economy. To solve this problem, torque converters are now equipped with an internal clutch that is applied hydraulically when signaled by the vehicle's computer. This normally occurs after the computer senses that the vehicle has reached a cruising speed when slippage of the fluid coupling is minimal, but lockup points vary with different manufacturer's designs. Consequently, the internal clutch in these "lockup" converters is not very large or rugged, as compared to the clutch used with a manual transmission. That leads us to our second topic, torque converter failure.

As we noted at the beginning of this article, torque converters are engineered to do the anticipated job with little safety margin for maximum loads. Making the stock lockup torque converter any stronger than absolutely necessary adds cost to a vehicle, and you know how the rest of that story goes. This is where things get very edgy on Dodge and Ford diesel pickups and SUVs that are routinely used for towing heavy loads or when power is substantially increased, such as with a Banks Power system. Actually, the problem can extend to motorhomes and gasoline engine vehicles too, but because diesels make so much torque, we'll stay with the most demanding circumstances. What happens when there's a heavy load, such as during towing, or a power increase (or both), is that the lockup clutch in the converter slips excessively during lockup engagement. Sometimes this excessive slippage can be felt in the vehicle as a shudder as the clutch tries to engage, then slips, tries to engage, etc. If the load is really high, the lockup clutch can even be over-powered and begin to slip after engagement is achieved. Such slippage accelerates wear of the clutch friction surfaces, and of course, excessive heat is generated. The more the lockup clutch wears, the more it slips. In addition, the more the friction surfaces wear, the more clutch "dust" goes into the transmission fluid, which is shared with the transmission itself. All of this can lead to some very expensive repair work, including a new torque converter and possibly a new transmission too.

Unfortunately, potential torque converter damage isn't limited to just failure of the lockup clutch. Each stamped metal fin inside a torque converter is held in place by several small tabs that protrude through slots. The tabs are bent over to hold each fin in place. When subjected to high loads, the fins can, over time, loosen and become wobbly. They usually don't break loose, but the efficiency of the torque converter diminishes. Sometimes the fins actually do crack and break. When that happens, efficiency is just the first thing that is lost.

Another problem has to do with the stamped steel outer housing of the torque converter, or more specifically, the half of the housing that faces the engine flywheel (flexplate). Inside this housing is the machined metal friction surface of the lockup clutch, and on the outside, six mounting tabs are welded to the stamped steel housing for attaching the torque converter to the flywheel. Under sustained high loads, these mounting pads can cause deflection and distortion of the stamped steel housing, which in turn, causes the lockup clutch metal friction surface to distort and create "high spots". This reduces the friction surface area that actually contacts the friction material, contributing to clutch slippage and "hot spots" on the metal friction surface.

All of the above problems are serious, but help is now available in the form of the Banks Billet Torque Converter. This replacement torque converter essentially solves all the shortcomings of the stock converter. It begins with a solid foundation of a CNC-precision-machined forged steel housing that attaches to the flywheel and provides full circumference support for the lockup clutch internal metal friction surface that now remains completely flat for full engagement. Second, the lockup clutch is modified to increase holding power. Third, the friction material of the lockup clutch has been upgraded to improve engagement and reduce clutch wear. Less wear also means less clutch "dust". The turbine hub is welded to the turbine instead of just riveted. The stator is also supported by Torrington bearings for precise, smooth, durable operation. Topping it all off, the internal stamped steel fins are now furnace-brazed in place to prevent any eventual loosening, and internal fin angles and stator design have been optimized for best efficiency. Most of the Banks Billet Torque Converter parts are CNC-precision-machined. The entire assembly is blueprinted for precision control of tolerances, and balanced for smooth, vibrationless rotation.

We should also mention that there's more to the internal engineering of the Banks Billet Torque Converter than just increased strength and durability. In actuality, the stock torque converters used in most diesel pickup trucks and RVs are units originally designed for gasoline engines. They have a fluid coupling speed, commonly called "stall speed", that is too high for efficiently transmitting maximum diesel torque to the transmission input shaft. Peak diesel torque occurs at engine speeds below the stall speed of the stock torque converter. Under high loads, the creation of the fluid coupling is delayed, generating heat in the transmission fluid and less power to the drive wheels. The Banks Billet Torque Converter includes stator design and internal clearances engineered to match the torque converter stall speed to the torque curve of the turbo-diesel engine by application. Hence, less fluid heating, better acceleration, and more power to the drive wheels during operation prior to torque converter clutch lock-up. The actual workings of this are difficult to visualize unless you really understand torque converters, but understand that in terms of efficiency, the Banks Billet unit is better than the stock torque converter – a lot better.

The Banks Billet Torque Converter is the most practical solution to the stock torque converter problems in diesel pickups and SUVs, and the best way to reduce the likelihood of expensive transmission damage due to overheated transmission fluid or fluid contamination. Of course, replacement of a torque converter won't cure a transmission that's already ailing, but it can help prevent future transmission damage. Applications for gasoline pickups and Jeeps will soon be available too. The refined stall speed and precision motion of the Banks Billet Torque Converter is something you can feel when driving. No longer does acceleration feel "mushy", and in fact, a truck with a Banks converter will accelerate faster. There's another change that you'll feel in the seat of your pants too: it's the extra money in your wallet that you're likely to save in repair bills and fuel expense. The Banks Billet Torque Converter will improve vehicle durability and reliability, acceleration, fuel economy, and your peace of mind. And like all Banks products, the Billet Torque Converter offers the best value for the money. That's a lot of benefits in one package!