Torque Converter 101

The Basics Behind The Fluid Coupler We All Depend On

We all know our automatic transmission relies on a torque converter to move our truck, and many of us have even upgraded this vital component if not the entire transmission, but beyond the basics most diesel owners have very little understanding of the converter’s functionality and importance. That’s for the experts to know, right? Not anymore! Teaming up with SunCoast Performance and one of its largest dealers, LinCo Diesel Performance, we’re de-mystifying the torque converter this month, as well as arming you with some of the knowledge you’ll need to spec out the perfect converter for your unique parts combination.

In the following pages, we’ll discuss how the torque converter works, breaking down the parts and pieces that make it operate. We’ll also explain the difference between brake stall and flash stall, discuss how added power effects converter stall and performance, and tip you off as to the kinds of upgrades you should expect to find in a reputable aftermarket converter. From billet front covers to high blade count, billet stators to furnace-brazed turbine fins, we’ll illustrate why these are more than merely industry buzz words used to sell products—they’re offered because we need them.

Suncoast Allison 1000 Billet Torque Converter — The torque converter is the fluid coupling device located between the flex plate and the transmission. It’s responsible for transferring torque and horsepower from the engine to the transmission, and ultimately the wheels. The torque converter’s key components are the impeller (or pump), turbine, and stator, along with the front cover, drive hub, lockup clutch and apply piston.

Torque Converter Impeller — The impeller, positioned on the flex plate side of the converter, acts as a hydraulic pump. To an extent, it determines fluid velocity within the converter. The impeller throws transmission fluid outward, and as fluid leaves its outer blades by way of centrifugal force it enters the blades of the turbine.

Suncoast Billet Torque Converter — Without a doubt, the torque converter’s most advantageous aspect is its ability to multiply torque. Torque multiplication occurs when the impeller’s speed is faster than the turbine’s. For example, when you’re accelerating (and during the period before lockup), torque multiplication is taking place. Then as the turbine reaches roughly 90-percent of the rotational speed of the impeller, coupling occurs—making the converter a simple fluid coupling. In lockup, the impeller is effectively locked to the turbine and you have maximum efficiency (i.e. peak power making it to the wheels). During lockup, there is no mechanical loss, other than the rotational mass of the components involved.

Automatic Torque Converter Turbine — The turbine blades are curved, which forces the fluid it receives from the impeller to change direction (opposite the impeller). In particular, the turbine’s outer blades see tremendous fluid force (sometimes even laying over or breaking), which causes the turbine to turn. And because the turbine is mechanically connected to the transmission’s input shaft via splines, when the turbine turns the vehicle moves. Fluid leaves the turbine through its center, traveling on to the stator.

Diesel Torque Converter — Though many different types of torque converters exist, for our purposes we’re focusing on converters that make use of a sprag and that are of the lockup variety. While non-lockup and/or spragless converters have been used in the diesel segment (think A727 TorqueFlite or all-out race applications), a lockup unit with a sprag is most common.

Suncoast Billet Stator — Located in the center of the converter, the stator redirects transmission fluid returning from the turbine toward the impeller. The stator will hold stationary when the converter is in stall mode (instances of high turbine slip), but spins with the rest of the converter once turbine speeds approach impeller speed. Blade angle, blade count, and blade length all play a critical role in stall speed and coupling efficiency.

SCT Livewire Custom Flash Drive — Always remember that your stall speed goes up when you add power (i.e. torque). Looking to throw a tuner on your truck? The stock converter will no longer stall at the same rpm it used to. Full disclosure: testing your own brake stall shouldn’t be done for long periods of time, or often for that matter. The process creates excessive heat in the torque converter, not to mention the fact that it also places added stress on the transmission shafts.

Lockup Clutch Plate — A lockup converter means that a lockup clutch and a lockup apply piston are employed. During lockup, turbine speed matches pump speed and the engine is essentially coupled to the transmission’s input shaft (spinning 1:1). Lockup can be engaged either mechanically or hydraulically, but most lockup converters use hydraulic fluid pressure to engage the lockup apply piston.

Torque Converter Know How: What is Brake Stall?

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This is the stall speed most of us are familiar with. If you’ve ever crammed the brake pedal to the floor, dropped the selector into Drive and held the accelerator down until the engine could no longer build rpm without moving the truck forward, you’ve tested your converter’s brake stall. While many converters are labeled for having a certain stall speed, it’s important to know that there is no set stall for any one converter. The same converter that stalls at 2,000 rpm on a stock truck could stall at 2,300 rpm on another one where the engine is making more torque.

Torque Converter Diagnosis: What is Flash Stall?

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Flash stall refers to the amount of rpm observed when you initially hit the throttle with the vehicle under load, the opposite of sitting up against the converter while matting the foot brake during the brake stall test. This initial hard acceleration can cause the converter to flash at an rpm higher than its brake stall speed. Flash stall is directly linked with drivability.