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?


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?


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.

In this example, LinCo Diesel Performance is showing us the brake stall speed of a brand-new L5P Duramax. No tuning, an untouched factory Allison, and the stock VGT are all still in the mix. Stall speed checked in at 2,400 rpm.
Testing the same, brand-new L5P, LinCo demonstrated what happens when additional engine torque output is introduced. Adding a Banks Power Derringer to the mix and cranking it up to level 6 brought the truck’s brake stall speed up 150 rpm to 2,550. It serves as a perfect example of how torque directly effects stall speed.
Because stall speed is torque-based, any time you add a larger turbo (especially a fixed geometry) without swapping to a higher stall converter, your stall speed will drop. When this happens, drivability suffers. Retarding your injection timing can help bring the charger to life quicker (or adjusting vane positioning if you went with a larger VGT), but there is only so much your tuner can do to make an improperly-spec’d converter work with a larger turbo.
Turbine Blades
In high torque applications, due to the high force they see, it’s not uncommon to lay the fins over inside the turbine or impeller. To improve their strength and durability, the turbine and impeller blades are often furnace-brazed, and sometimes even TIG-welded individually in place in aftermarket converters.
Suncoast Billet Drive Hub
Because the drive hub drives the transmission pump, Sun Coast welds a billet drive hub in every converter it builds. Thanks to the upgraded material, you get stronger stator and turbine splines for maximum durability in high horsepower applications.
SunCoast’s L5P-1074-3D converter drive hub and splines
Peering into the drive hub on SunCoast’s popular L5P-1074-3D converter (a 2,200 to 2,400-rpm stall, triple disc unit for the L5P Duramax), you can see the billet splines present within it. The first set of billet splines are for the stator. The second set of splines are for the turbine, which is what turns the transmission’s input shaft.
Suncoast Billet Front Cover
Billet front covers have been a selling point for aftermarket converters for forever, but do you know why we have them? Factory torque converters are notorious for growing when exposed to added pressure, often expanding to the point where the front cover pushes against the flex plate. This is called converter ballooning. A stronger, billet front cover strengthens the converter as a whole, keeping it from ballooning under higher pressure. It’s important to note, however, that even with a billet front cover in the mix the impeller can still balloon when exposed to excessive pressure. According to SunCoast and LinCo, it’s best to limit pressure to 330-psi or less in street applications.
Allison 1000 stock converter, L5P Duramax
This is a shot of a stock converter in the Allison 1000 that’s parked behind the L5P Duramax. Though slightly different than what is found in ’01-’16 applications, the latest Allison converter is still notorious for its lugs present on the front cover, which are prone to distortion. Shorter, stronger lugs are the norm on aftermarket Allison converters.
SunCoast’s triple disc 1074
In this comparison between a stock Allison converter and SunCoast’s 1074 (positioned weld to weld), you can see how much deeper the front cover side is. Not only is this due to Sun Coast’s use of a thicker, billet front cover, but because more hardware is stored inside (i.e. triple disc). Beyond being larger, Sun Coast’s triple disc 1074 is also significantly heavier. In the case of the converter employed in L5P Duramax applications, it tips the scales between 75-80 pounds (vs. 65 pounds for the stocker).
Going above and beyond the industry standard of static balancing, Sun Coast treats all of its converters to dynamic balancing as well. The company’s dynamic balancing process is so precise that each converter is balanced to within 0.001 of a gram.
Allison 1000 SunCoast Billet Stator
Factory stator failure is common in high torque applications, but OEM units have even been known to crack at low horsepower levels also. This is especially true for the Allison 1000. Cataclysmic in nature, when the stator fails, debris and shrapnel is circulated throughout not only the converter but the rest of the transmission—even plugging up the trans cooler. As you can imagine, it makes for one heck of a racket inside the converter as well.
Billet Allison Apply Piston from SunCoast
As energization increases, fluid is permitted to travel down the center of the input shaft, and applying the pressure that’s required to engage the lockup apply piston. A billet Allison apply piston from SunCoast is pictured here.
For utmost quality control and validation, Sun Coast pressure tests all of its converters. The test consists of each converter being exposed to water temperature of 175-185 degrees F, and subjected to 200-psi worth of pressure. Among other parameters, this validates the strength of the converter’s welds.
In this photo, the cast factory Allison 1000 stator is on display up top, Sun Coast’s billet, 17-blade version is on the bottom left, and its billet 16-blade stator is in view on the bottom right. The angle, amount of blades, and the length of the blades all effect the torque converter’s efficiency. As a general rule of thumb, a higher blade count equates to a higher stall speed. Additional selling points for the billet stator is that they are virtually indestructible, Sun Coast’s having been specifically designed to hold up to boosted launches and huge torque loads.


LinCo Diesel Performance

Sun Coast Performance

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