Torque Converters are like a magician’s black box. We know basically what is going on, but we are not really sure how it is doing, what it is doing. The torque converter plays a critical role in how the truck performance. The wrong converter can make the truck feel like it has no power, gets horrible fuel economy and it is as slow as molasses. On the flip side of that, the right converter can make all the difference in the world. The truck will perform well, it will be easy to light the turbocharger and fuel economy is good.
The torque converter really is one of the most misunderstood components on your truck. It is first and perhaps most important link between the engine’s power and the ground. Torque converters work the same, no matter what type of application it is in. So, whether it’s a gas or diesel they operate the same, but tuning them is as different as night and day. Before we get into how to choose the right torque converter for your application, it is important to understand the components that make up a torque converter.
Front cover (the pump):
The pump (or series of veins) is welded to the cover which is bolted to the fly wheel. This turns at the speed of the engine at all times. The veins inside of the cover take the transmission fluid that is in the center of the torque converter and sling it to the outside, across to the turbine.
The turbine’s center section is splined and rides on the transmission’s input shaft. The turbine has veins that face the opposite direction of the front cover. As the fluid is slung from the front cover into the turbine, it creates pressure in a clockwise direction. Once the pressure is high enough, it starts to send power back to the transmission. (Think of this as two electric fans facing one another. If one fan is turned on, the air it pushes will start to spin the fan that isn’t on.) The fluid that is slung from the front cover to the turbine is then redirected back to the center of the pump through the stator.
The stator is directly responsible for the torque converter’s characteristics. As the fluid leaves the turbine housing it is directed back to the stator vanes. These vanes then direct the fluid to the front cover.
The position of the vanes is a major influence in the stall speeds, how much load is put on the engine, and how much of a torque multiplication the torque converter has. Like the turbine housing, the stator is, also, mounted on the transmission’s stator support shaft and is positioned between the turbine housing and the front cover.
Behind the turbine housing are the clutches and plates. Depending on the builder of the torque converter the amount of clutches and plates will vary. Many factory torque converters on gasoline applications have only one clutch, while most OEM diesel applications have been moving towards 3 clutches and in the next few years that should be the norm for diesels.
The clutches apply the clamping pressure that locks the back cover to the turbine housing. This makes the torque converter fully locked and the transmission’s input shaft will spin at engine rpm.
The back cover is the final piece in the torque converter assembly. On the inside of the cover is the surface area for the last clutch to press against. The inside of the cover is grooved which the pressure plates ride against. The outside of the back cover is welded to the front cover in most applications to keep the fluid inside. In some extreme race applications, the back cover is bolted to the front cover.
So, that is the basic components that make up a torque converter. To describe the characteristics of a torque converter, there are common terms used. Those are:
Lock up is when the clutches in the converter are engaged. When the torque converter is locked up, the transmission’s input shaft will spin at engine rpm.
Stall is the point at which the converter has built up so much pressure against the transmission that the engine cannot overcome it and the engine will not rev any higher (with the brakes applied). To find out what the stall speed is on your vehicle, you can put your vehicle in gear, hold the brake and apply the accelerator pedal. The point that the engine cannot increase rpm is the stall speed. The stall is not the point at which the tires break loose, but when the engine cannot rev any higher.
Tight Converter or Low Stall:
A tight converter is where the engine cannot get enough rpm’s to light the turbo. Many times at the drag strip, you will hear or see a truck take forever to build its’ rpm’s enough to light the turbo. That’s because the engine is fighting the torque converter and cannot spin fast enough to create the heat and volume of air needed to get the turbo lit.
Loose Converter or High Stall:
A loose converter is where an engine can “rev to the moon” before anything happens. The converter is not producing enough resistance to hold the engine back. When driving down the road and the accelerator pedal is applied, the engine will rev up, but it takes a while for the vehicle to start increasing speed, this means you have a loose converter.
What’s the difference?
The stator is usually the difference between a converter that has a low stall, high stall or the right stall. The stator is the single most important piece within the assembly. It gives the converter most of its characteristics. The angle, pitch, length and position of the blades are the important aspects.
So, how do you know what to ask or look for when choosing a converter? Well, we went down to Sun Coast Transmissions and talked to Ron Wolverton asking just that.
“Without looking inside of a converter it is hard to know what you are getting,” says Wolverton. “Price is one indicator. It is hard to add billet parts, heat treated splines and clutches at a discounted price.”
Another item to be aware of is the “One Size Fits All” torque converter. Wolverton mentioned that many people are using a dodge style stator in all of their applications. “Well, that just doesn’t work,” he continues. “The Fords are a great example of that. They are close to the same displacement (6.0 vs 5.9), but the Ford stators and Dodge stators are totally different. The Ford’s love to rev and if anything, are close to a gasoline style stator, while the Dodge engines love to be loaded and we use a very aggressive stator on them.”
In addition, to watching out for one size fits all and value priced torque converters, check to see if the company you are purchasing from can fine tune and make the appropriate adjustments for your exact applications. Many companies are reusing or slightly modifying the factory converter. The factory converters are designed for the 350 to 400 horsepower that the factory rates the engine at. With the addition of chips/modules, air intakes, exhaust, turbochargers, injectors, and don’t forget nitrous, many people are pushing 500 to 800 horsepower on their daily drivers. A custom torque converter is the only way to get the proper stall speed, and to be able to load the engine enough to get the most out of it.
Also, ask about refreshing and changing the converter as you continue to build/modify your truck. Some companies, like Sun Coast Transmissions offer a NDR (non defective return) where they will disassemble the converter, clean it up, and change it to meet the new requirements of the engine for a fee less than the purchase of a new converter.
So, whether you are about to spend a few thousand dollars on a new transmission or have been building your truck up for a while and need a new torque converter, calling around and asking the right questions can make all the difference.