Talking the torque: do you really know how torque converters work?

One of the most important components in an automatic gearbox-equipped car is the torque converter. It’s often one of the first things changed but, alas, one of the least understood. That’s probably because it’s a steel donut that you can’t easily see inside or pull to bits to gain an appreciation of what makes it tick. 

Everyone has heard of high-stall converters but a lot of people have no real idea how they work, what makes them a high-stall or even how to effectively check the stall speed in the first place.

If you have a car that has had the engine upgraded and is making noticeably more power than stock, and perhaps you’re taking it to the drag strip, it might be time to start thinking that the stock converter isn’t up to the job. You will most likely be told you need a high-stall converter, and if you’re in the above situation, there’s a high chance that is correct. 
Ideally your converter should be set up so that the stall speed is pretty close to the rpm at which your engine makes maximum torque. This will give you the greatest initial acceleration and, if you’re drag racing, that is important, which is why drag cars use high-stall converters.
Before we dig too far into the high-stall part of it we need to have a basic understanding of the components that make up a converter and their functions.

The four main components are the impeller, turbine, stator, and front cover. The impeller or pump is made up of a circular array of radial vanes that form the rear half of the converter housing — the end closest to the gearbox.

The turbine (which looks a lot like the impeller) is fixed to the spines of the gearbox’s input shaft and spins freely inside the front half of the converter housing — the end closest to the engine. The example pictured above has been fitted with a 4340 billet steel mounting cover, which will then be welded to the impeller, pressure tested, and electronically balanced. 

There is no mechanical connection between the impeller and the turbine. Sitting in between these two is the stator (or reaction member). While we are talking about the stator we need to mention the sprag, which is a one-way mechanical clutch inside the stator. It locks the stator in place while the converter is in stall mode. It also allows the stator to spin with the rest of the converter after the turbine speed nears the pump speed. This makes for a more efficient and restrictive fluid flow.

Lastly there is the front cover, which bolts to the flex plate. Once the internals are assembled, the front cover and impeller (rear half) are welded together.

The converter is bolted to the flex plate so it spins as soon as the engine starts. As the impeller is rotated by the engine (remember the impeller forms the rear half of the converter housing), oil is pumped in through its centre before inertia forces it out through its outer edges with considerable force into the turbine’s vanes. Then to the stator, which forces oil onto the impeller in the direction of the motor, and increases torque until speeds reach the same on both halves and it freewheels. These vanes are curved oppositely to the impeller, forcing the fluid to change direction as it enters the turbine. It’s this reversal of fluid flow that forces the turbine to rotate.

Think of paddle wheels and water. Rotating a paddle wheel in a stationary body of water makes the water flow, while flowing water can make a paddle wheel turn. A torque converter works by combining these two actions. The engine drives one paddle wheel, which pushes fluid onto the other paddle wheel causing it to turn. The other simple visual example is two electric fans facing each other, one running and one not, the fan that is running will force the other fan to spin.

One of the characteristics of a torque converter is that it multiplies torque, by a factor of as much as two to two-and-a-half times or even more, depending on the design. Maximum torque multiplication occurs at rest, as the vehicle just starts moving. As speed increases, the torque multiplication decreases. Once the impeller and turbine speeds approach each other, torque multiplication decreases to basically zero. 

A non lock-up converter absorbs between two and seven per cent of total engine power due to the ‘slippage’ that exists between the impeller and the turbine. Lock-up converters use a clutch that links the impeller and turbine and allows near 100 per cent efficiency at high speeds.

To quickly test the stall speed of your standard converter, put your car in drive, then while standing on the brakes quickly floor the gas pedal and watch the tacho. The revs that it reaches tells you the stall speed. Often in this situation the trans will overpower the brakes and either start spinning a driven wheel or pushing through the front brakes. Be careful not to hold the revs for long as damage can ensue.

The downside of this quick test is that any decent converter will multiply torque so much that it will easily overpower the brakes. If you can get to the rated stall in this way, you’ve probably got a pretty sloppy converter that will never reach maximum efficiency. 

Torque converter specialists have unique machines for determining stall. However, the best way to test the true stall speed of your converter is to do a flash stall test. This involves rolling down the road at about 15–20kph and then standing on the throttle. The rpm the tachometer immediately jumps to when the car locks up and takes off is your flash stall speed. 

As you’ve gathered, stall refers to the maximum speed the motor can achieve against the converter when the turbine is locked and prevented from rotating. The rpm achieved (stall speed) will be a function of the engine torque and the converter design.

In general, the higher the stall, the less efficient the converter is at high speed. So why would you want a high-stall converter? To allow the engine to get into the core of the powerband quicker. A converter optimised for drag racing will have a stall speed much higher than a street converter. Allowing the engine to get into the power band quickly more than compensates for the disadvantage of lower efficiency over such a short distance.

On a street car the penalty of having a high-stall converter over a factory one is poor economy and extra heat generated. A high stall can also be annoying to drive on the street because it degrades throttle response.  
When you stab the throttle there is a delay while the engine revs before the car starts to accelerate. To put this into perspective, you can achieve up to a half a second or more improvement in quarter-mile times by switching to a high-stall converter. The improvement all comes in the first part of the run and, interestingly, trap speeds may decrease due to the inefficiency of the higher stall converter at high speeds. 

For a street/strip car the perfect converter stall speed is a compromise. For example, a given combo may need a 4500 to 5000rpm stall for the best quarter-mile times, but that’d be annoying on the street due to excessive slippage. A converter that stalls at 3000 to 3500rpm will allow tolerable street driving without too much effect on quarter mile performance. Stock converters typically stall in the 1500 to 2000rpm range. 

Of course what is tolerable to one person may not be to another. To get a converter that stalls at the right speed for your application you need to speak to a converter builder. For the best drag strip results you want a converter that stalls at close to peak torque.

Claiming a converter has a specific stall point without making reference to the motor it’s behind is misleading, as there are too many variables. The factory converter, which may stall at 1500rpm behind a stock motor, may well stall at 3000rpm or more behind a blown big block. Technically stall speed is not just a function of the converter. It is also a function of engine torque. 

This can perhaps be easily described by defining the ‘K’ factor. ‘K’ is simply the constant in the equation K = rpm/sqrt {torque}. The equation describes the observed behaviour of the converter behind a specific engine. What this allows us to do is determine what the stall speed of a given converter will be if put behind a different engine.

For example, if a motor has 400lb·ft (542Nm) of torque and stalls a particular converter at 3000rpm, K = 3000/√{torque} = 150. Since we know K = 150, we can predict the new stall speed if the torque is say 500lb·ft (677Nm) by changing the equation to rpm = K*√{torque}. In this instance the new stall would be rpm = 150*√{500} = 3350. 

This formula isn’t perfect — it won’t work if the engines have wildly different torque curves, for example. It also won’t tell if a particular converter will hold together under a marked increase in torque. But it gives a decent ballpark estimate, and serves to illustrate a basic aspect of torque converter function. 

For most street and street/strip cars you probably want a stall speed in the 2500 to 3500rpm range. Don’t buy an off-the-shelf converter thinking it will give you the exact advertised stall unless it has been proven to do so in an identical set-up. Speak to the manufacturer or retailer first to be sure you’re getting what you need for your particular vehicle and requirements.

A race-quality converter can be expensive. All the parts, including the case, need to be strengthened to absorb the high loads. When a trans brake is used, for example, the converter has to absorb the full power of the engine with turbine locked. This generates enormous heat and pressure, and is why such converters are furnace brazed and have anti-balloon plates welded into them. This prevents the converter from distorting and bursting.
 
You also need a very good transmission cooler if using a high-stall converter. Good race-quality converters for a high-power application start in the $2000-plus range, and if you go ex-USA, a top-of-the-range A1 could cost close to $5k.

A good place to start for street/strip cars is with one of the off-the-shelf high-stall converters available from most good speed shops. These will include brands like B&M with its Holeshot and Super Holeshot range, and the TCI range of Saturday Night Specials and Street Fighter converters. These converters are good for stall ranges up around 3000 to 3500rpm, and you could expect to pay somewhere in the region of $1500.

If you want a higher stall than that, basically you have to get a full race custom eight-inch converter built, and you can then have your stall as high as you want it. 

There is no 100 per cent exact result with a custom-built converter as there are many variables, and it’s not a precise science. For a converter builder to get it right or as close to right as possible first time, you need to supply as much information as possible. Engine size, compression ratio, cam specs, head and flow figures, carb, inlet manifold and so on, along with peak torque and the power that your engine makes and at what rpm. A dyno chart is far better than plucking some figures out of the sky based on what a mate’s car supposedly did measured by his seat-of-the-pants-o-meter.

The converter builder will also need to know the weight of the vehicle, the gearbox details including first gear ratio, the diff ratio and tyre size, along with all the information from any current time slips.

There are a number of capable converter builders here in New Zealand, and Auto Trans in Auckland immediately springs to mind. Dominator brand converters are built in Australia and are common in New Zealand, or if USA is the economy you wish to spend your money in you can go the whole hog and get an A1 converter, which many people rate highly.

Just remember that bigger is not always best, especially when you’re talking stall speeds. A 5000rpm converter in a heavy street car is going to be a bitch. Granted it will be fun taking off from the lights, but that’s about it. Hill starts on a steep hill would be interesting, especially in the wet, as would any attempt to tow. Whereas a healthy big block in a heavy car would get the best of both worlds with, say, a 2800rpm converter.

If and when you buy a new converter, ensure you follow the instructions that come with it, not only regarding installation but also use. Many come with very specific warnings about burnouts going on for too long to the point where the bite of the track starts to pull the revs of the engine down. This can damage the converter’s one-way mechanical diode clutch (sprag) and void any warranty. So purchase, read the instructions, install, read the instructions, have fun. 

Huge thanks to Tim at Auto Trans for technical advise and images used in this article. 

This article was featured in NZV8 Issue No. 49 (June 2009). You can grab a copy here.