What you need to know about torque converters | A quick explanation

By Staff Writer
Oct 7, 2022 | transmission, Shop Work, Drivetrain, Torque converter | Posted in Shop Work , News and Notes | From the May 2018 issue | Never miss an article

Photography Credit: Courtesy Dodge

When it comes to handling huge horsepower and drag launches, it’s hard to beat an automatic. Rather than using a solid mechanical connection like a manual or twin-clutch transmission, an automatic transmission transfers power through fluid using what are essentially special fans. This key interaction happens inside the torque converter, a device with three major components: the impeller, turbine and stator.

The impeller is connected to the engine. The higher the engine revs, the faster the impeller spins. It has shaped vanes that fling transmission fluid away from its center. Higher rpm impart more speed, and therefore more energy, to the fluid.

The turbine is connected to the transmission and ultimately the drive wheels. It catches the fluid coming off the impeller. That moving fluid spins the turbine, which in turn spins the output shaft and ultimately the drive wheels. 

The turbine’s shape draws in transmission fluid from its outer diameter and forces it back toward the impeller along both components’ centerlines. As a result, the transmission fluid is constantly flowing from impeller to turbine along their outer edges, and from turbine to impeller near their centers.

It’s possible to achieve thrust with just these two components, but the inherent slushiness of a fluid coupling would make this setup inefficient and slow to react. The engine and impeller would always be spinning quite a bit faster than the turbine. That’s where the stator comes in. 

This fanlike disc has vanes that reroute the transmission fluid’s return trip to the impeller, essentially sending it back along the centerline of the spinning assembly. After being diverted by the stator, the fluid hits the impeller blades with great force. This creates a hydraulic boost for the engine, causing what’s known as torque multiplication, and it’s most effective when the impeller and turbine are spinning at vastly different speeds–like when you hit the gas from a stop, for example.

Modern automatics also have a physical clutch plate that engages when the impeller and turbine are moving at nearly identical speeds. This negates parasitic loss while the car is cruising under minimal load, helping fuel economy.

One more element of torque converter magic is the stall speed. This is the point at which the automatic transmission goes from transferring minimal engine thrust to the wheel to dumping quite a bit of torque into the driveline. On a street car, the stall speed is generally set a bit higher than idle so the car transitions from stop to go without delay. 

Drag racers, though, often play with special setups that allow for very high-rpm stall speeds. They allow the engine to spin up to its maximum torque point before that energy gets released through the torque converter to the drive wheels. A high stall speed is the equivalent of revving a manual-transmission car before dumping the clutch, but automatics benefit from even more pronounced torque multiplication through the stator once the wheels start spinning.

Automatic transmissions get a lot of flak outside of drag racing for their lack of responsiveness or direct control compared to a manual or DCT gearbox. However, there’s no denying their durability even under tremendous torque conditions–it’s hard to snap fluid, after all. And thanks to the torque multiplication effect, an automatic can make the most out of a high-powered launch on sticky tires.

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Jesse Ransom
Jesse Ransom GRM+ Memberand UltimaDork
7/22/22 1:26 p.m.

I think that incremented some of my (reluctant) automatic knowledge, but it needed pictures. Centerline on what axis? What's the overall shape internally? Shape of the impeller? Of the turbine? of the stator? Of their fins? Arrows for fluid flow?

How does torque multiplication work? I assume it's something like converting levers or gears to a hydraulic context, but outside of positive displacement hydraulics that's not intuitive.

It's a topic that's not well covered, and I'm thankful for a stab at it that improved my understanding, but the better-than-average explanation makes the bits I still don't get stand out all the more!

californiamilleghia UltraDork
7/22/22 2:02 p.m.

Please explain the switch pitch converters of the late 60s GM cars , 

These seemed like a great idea to basically have 2  converter styles at once ( if I understand the principles)


Ranger50 MegaDork
7/22/22 6:38 p.m.

In reply to Jesse Ransom :

The stator controls the torque manipulation. It is what drives the fluid back against the impeller. It is always turning until it quits and that is the stall speed because the stator "stalls out" and doesn't turn faster than the impeller anymore.

The "attack" angles of the stator and impeller dictate the desired stall speed. If you don't have these components complementing each other, you end up with a junk converter or one that does not operate as intended.

Ranger50 MegaDork
7/22/22 6:40 p.m.

In reply to californiamilleghia :

There was a triggered solenoid that moved the angle of the stator which gave the 2 separate stall speeds. It's a sucky system that works in theory mostly and just overly complicates everything else.

Pete. (l33t FS)
Pete. (l33t FS) GRM+ Memberand MegaDork
7/22/22 6:42 p.m.

In reply to Jesse Ransom :

If you can visualize a block and tackle arrangement that gives you mechanical advantage, you can get how there can be torque multiplication.

A better analogy, although a little less inside our normal reference, is getting thrust by throwing a ball in a zero G room.  If you throw the ball hard enough to hit the wall, you thrust yourself in one direction and you thrust the room the other way.  If the room geometry is such that when the ball bounces back it hits you, then that is 3x the thrust for the same initial effort.


Now instead of zero G rooms and balls, imagine spraying a water hose at a curve that directs it back at you.  Actually a better analogy would be to imagine the inside of a torque converter...

A 401 CJ
A 401 CJ SuperDork
7/22/22 7:06 p.m.
californiamilleghia said:

Please explain the switch pitch converters of the late 60s GM cars , 

These seemed like a great idea to basically have 2  converter styles at once ( if I understand the principles)


I always wondered if there was a way to have both switch pitch AND lockup.  It'd be like a 3 stage converter.  First 2 stages for banging around the street and then lockup for highway. Probably overkill though since you can get away with a fairly loose (high stall) converter when you have the lock up ability 

Pete. (l33t FS)
Pete. (l33t FS) GRM+ Memberand MegaDork
7/22/22 7:50 p.m.

In reply to A 401 CJ :

Yup.  I have a maybe 4000-4500 stall speed converter in my S60R.  (Drive by wire makes it hard to tell exactly)  It drives normally most of the time.

One thing about loose converters and lockup is that it is tricky to tune the lockup point.  I did an MS3Pro in a Grand National with a 3000rpm converter.  What would happen is when you would lock the converter at highway speeds, engine load would go up, which would unlock the converter... Tuning lockup by throttle position would probably make more sense in that case, plus there was probably some more to be had in the timing curve but I ran out of time for messing with it.

BA5 GRM+ Memberand Reader
10/7/22 10:51 a.m.

This is probably the clearest picture on how a torque converter works (at least to me) that I could find in a quick Google search:

Technically I think the stator is connected to the gearbox as well through a one way clutch.  The stator increases the energy harvested from the fluid, making the torque converter more efficient.  By mostly recirculating the fluid through the vanes you reduce pumping losses.  If you were constantly circulating new fluid through you'd be losing a bunch of energy to just pumping the fluid around, instead of moving the car, like you want to.

madmrak351 Reader
10/9/22 8:16 a.m.

In reply to BA5 :

Thanks for the diagram! Having rebuilt several 4L60e and 1 FWD Mopar auto successful, and even attended a few GM auto trans classes I understand the transmission itself pretty well, but the converter always seemed like voodoo to me. This diagram and the explanation in the OP made it much clearer.

Driven5 UberDork
10/10/22 3:30 a.m.

A simplified diagram...

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