What is a Torque Converter?
Most automotive enthusiasts know that when they run an automatic transmission, they must run a torque converter. They may even consider the converter part of the trans, although the two can be and often are purchased separately. Automotive enthusiasts also know they can see a performance gain in many cases when they switch to a higher-stall-speed torque converter. However, even hard-core enthusiasts often have no idea how a torque converter actually works. After all, it arrives all sealed up. You can't open it and inspect or modify or maintain it. You just choose one, install it and hope you've made the right selection. Fortunately, torque converters don't have to be mysterious. We'll explain right here and now what they do, how they do it and why you might want to change yours.
What does a Torque Converter Do?
At the most basic level, a torque converter connects the engine to the automatic transmission, much like a clutch connects an engine to a manual transmission. The converter housing bolts onto the motor's flexplate (also known as the flywheel), and the housing spins right along with the motor. On the output side, the torque converter's turbine is attached to the transmission's input shaft. Inside the torque converter is a stator assembly. It redirects fluid flow, resulting in torque multiplication and torque multiplication is what allows a converter to provide better low-speed acceleration. Most torque converters today will multiply torque by a ratio of at least 2 to 1. For example, if your motor is making 250 lbs.-ft. of torque and your converter is multiplying it by a factor of two, then the transmission will see 500 lbs.-ft. of torque. This can improve a car or truck's acceleration capability substantially. NASA defines a torque converter as a device for changing the torque speed or mechanical advantage between an input shaft and an output shaft. The goal for automotive enthusiasts, clearly, is to gain a mechanical advantage. The torque converter also serves another extremely important function in a car or truck. A vehicle's engine must be able to connect and disconnect from the differential, so the car or truck can stop moving (i.e., turning the drive wheels) while the engine is still running and the transmission is in gear. In the case of an automatic transmission, it is the torque converter that performs this connect/disconnect function (again, like a clutch in a manual transmission). By slipping internally, the torque converter allows the car to idle while it's in gear.
What's Inside A Converter?
A torque converter is a pump. It shares fluid with the transmission.
Inside the big donut-shaped torque converter housing is an impeller, which is driven by the engine. The impeller's fins (or vanes) create centrifugal force by pumping transmission fluid outward i.e., toward the outside of the torque converter.
Bob Ritzman of B&M Racing likens the car's motor to an electric fan and the torque converter to a pinwheel. When you turn the fan on, if it's facing the pinwheel, the pinwheel will spin because of the air that's flowing across it. You have to look at the pinwheel as being the turbine, or the output member of the torque converter. And that piece is attached to the input shaft of the transmission.
The other key component inside the torque converter is the stator, which we've mentioned. Its located between the impeller (on the input side) and the turbine (on the output side). The stator incorporates a one-way clutch, which has the ability to redirect the fluid flow inside the converter.
As the oil flows through the converter and comes out of the turbine, says Ritzman, it goes through the stator, which redirects the oil flow. And by redirecting the oil flow, that's where you achieve the torque multiplication.
What About Lockup Converters?
Lockup converters contain another part: a torque converter clutch. When the clutch engages, it allows the converter to lock the engine to the transmission input shaft, providing a direct 1:1 communication from the motor to the transmission.
Why is this necessary, or even desirable? In many ways, it comes down to fuel economy. Lockup torque converters have become popular since automakers have been stuck in a sort of Catch-22: Car buyers overwhelmingly prefer automatic transmissions, but automatic transmissions are not as fuel efficient as manual transmissions and automakers must meet government-mandated corporate average fuel economy targets. Overdrive transmissions have been one step along the path toward increased fuel economy from an automatic transmission. Overdrive transmissions allow the motor to spin at a lower rpm during cruising speeds. A higher final drive ratio (numerically lower) does the same thing.
However, when the engine spins more slowly, it creates more slippage within the torque converter, and more slippage creates more heat. Heat within the torque converter reduces fuel economy and can harm both the torque converter and the transmission. The solution is to allow the converter to lock up at a 1:1 ratio. Lockup eliminates the slippage, which reduces heat and improves fuel economy.
Do you want to know if your car has a lockup torque converter or if it's working properly? Try this: Drive along at 50 mph or a slightly higher, steady cruising speed. Depress the brake pedal ever so gently (not enough to actually apply the brakes, but enough to turn on the brake lights). See if you experience what feels like a slight downshift. Then release the pedal very slowly and see if you feel a slight upshift. If you do, the lockup mechanism is working properly. Usually, lockup converters are used in stock applications, but not in higher-performance vehicles. However, some drag racers choose to run lockup torque converters, too. It is possible to lock up the converter at wide-open throttle manually (by using a switch) or automatically (via a racing computer chip). It has been estimated that locking up the converter at WOT in a relatively stock doorslammer, such as a Buick Grand National, can pick up about a tenth in quarter-mile ETs; however, it also speeds up wear and tear on the torque converter clutch.
What is stall speed?
Stall speed is the amount of engine rpm that can be attained at full throttle with brakes locked and transmission in gear before the driving wheels turn. The stall is only attainable if the brakes can hold the vehicle.
Choosing The Right Converter
If your car or truck has been modified to enhance performance, you probably need a converter with a higher stall speed. Most stock converters have a stall speed in the 1500 to 1800 rpm range.
When you make performance modifications, you generally shift the engine's torque curve upward into a higher rpm range. Therefore, you need a converter with a higher stall speed so that you can launch the car in the fat part of the powerband.
If the stall speed isn't high enough, the car won't be making power when you launch and you'll bog off the line. If your setup is really mismatched, the motor may even want to stall whenever you attempt to shift into gear at idle.
So, a higher-stall converter will improve acceleration by allowing the car to launch at an rpm where it is making power.
How high a stall speed is too high? Obviously, you don't want to launch the car at an rpm that is past your peak power output. There's no point in bypassing the engine's powerband.
You also have to consider the car's weight, braking system, engine displacement and engine combination when deciding which torque converter to use. Some converters are designed for high-revving lightweight cars. Some are designed for heavy cars that make a lot of bottom-end torque. And there are converters designed for virtually everything in between.
The key is to match the converter to the motor and to the entire vehicle in question. This is where technical support staff can help. They'll ask you the right questions about your combination including where your car makes power and how much it weighs in order to help you find the right converter for your overall combination.
A quick warning: You definitely don't want to get a higher stall speed than you need. That's because the higher stall speed creates more slippage, which generates more heat, which is hard on the converter and the transmission. If you are going to run a converter with a higher-than-stock stall speed, you'll want to install a transmission cooler to extend the lifetime of your components. (Amazingly enough, 85 percent of transmission failures are due to excessive heat.)
Higher-stall-speed converters also generally are not available with a lockup clutch. Again, this means more slippage, more heat and less fuel economy. But the tradeoff may be more than worthwhile for high-torque applications, since a higher stall speed can substantially improve a vehicle's responsiveness.
Why is the stall speed in most stock vehicles so low compared to the 2000+ RPM stall speed converters recommended for even mild performance?
Most performance engines are modified to produce additional horsepower and torque. The engine's torque curve is normally raised to much higher rpm ranges than stock before they begin to produce real power. The higher stall speed converters allow the engine rpm to enter the bottom range of the power curve and provide the optimum launch of the vehicle.
If more stall speed is good, is a lot more stall speed better?
More stall speed is only good if your engine's torque curve matches it. Too much stall speed is as bad as using stall speeds above 3800 rpm. Increased stall speed increases heat in the transmission. Excessive unusable stall speed is therefore detrimental.
A friend of mine bought a torque converter that was supposedly a 2800 RPM stall speed model but he only got about 2200 RPM out of it. Why is this?
Your friend may have an engine that's incapable of providing 2800 rpm stall speed. Either the engine produces less actual torque than what he estimates or the engines power curve, due to camshaft specs, is set up for a much higher rpm and is producing very little torque in the 2500-3000 rpm range. Many people attempt to check the stall speed by holding the brake and stepping on the gas with the car in gear. This usually will not produce the true stall speed as the brakes will not hold the engine and the tires will start to spin first. The best indication of the true stall speed, is to launch the car at wide open throttle and note the engine rpm.
How does a camshaft change (lift, duration and lobe separation) effect my converter choice?
The camshaft and converter are more closely tied than any other components in the car. If you chose a cam that produces power at higher rpm ranges, you must have a converter that coincides with that range.
How can I be sure that the converter I am buying is a genuine B&M?
B&M stamps our part number on the transmission side of every converter we produce.
Do you recommend the economy priced Tork Master converters for Big Block applications? What about for racing a Small Block?
The Tork Master units are intended for mild performance applications, not big blocks. This would best be described as cars with a manifold, carb change and headers. For higher horsepower applications use the appropriate converter for your application.
If I buy a higher stall speed converter than my stock converter, will my car sit still while the RPM's come up to the new stall speed?
No. A higher stall speed converter is simply less efficient at rpm's below the stall speed. It will act quite normally for around town use.