All torque converters have a specific stall speed, which is the maximum rpm at which the engine will turn over before the converter will allow no more slippage. When the converter hits the stall speed, the impeller starts to spin the turbine wheel, causing the converter to grab and launch the vehicle.
A modified torque converter with a higher than stock stall speed can shave anywhere from three to seven tenths of a second off a base line elapsed time at the drags. A modified converter lets the engine wind up faster and launch harder.
The stall speed of a torque converter is determined by its size and the stator wheel. Generally speaking, the smaller the diameter of the converter, the higher the stall speed. A 10-inch torque converter, for example, has a higher stall speed than an 11-inch converter. Stall speed can also be increased by shaving down the stator wheel to reduce the depth of the blades.
Some people confuse stall speed with torque multiplication. A higher stall speed generally gives more torque multiplication. The highest amount of torque multiplication is achieved when the engine reaches its maximum stall speed just before the car launches. It peaks out as the transmission starts to churn, and then diminishes as the speed of the turbine wheel catches up with the impeller.
Power braking is a trick drag racers use to increase torque multiplication off-the-line. It is also a way to check a converter's stall speed. With the transmission in Drive, one foot on the brake and the other on the gas, the engine is revved until it's strained to the limit. The rpm at which the engine is held corresponds to the stall speed of the torque converter.
Power braking can also be used to diagnose a bad converter. If the stall speed (which is usually around 1500 to 2200 rpm for a stock converter) is roughly a third lower than it should be, it means the stator wheel isn't holding at slow speed. The one-way roller clutch is slipping, allowing the stator to turn when it shouldn't. The result is loss of torque multiplication and super sluggish off-the-line acceleration.
Sometimes a stator wheel will hang up and remain locked at high speed. If this happens it will act like a speed brake, causing the torque converter to fight against itself. The car may start out and drive normally at low speed, but as speed starts to build the engine will strain harder and harder to overcome the resistance. If the engine won't rev much beyond 3000 rpm in neutral, the stator is definitely hung up.
HOW PERFORMANCE TORQUE CONVERTERS ARE MODIFIED
Modifying the stall speed of a torque converter is not a do-it-yourself undertaking. It requires special equipment and know-how to slice open the converter, change the angle of the impeller fins, and/or shave down the fins on the stator wheel.
Installing a modified converter, on the other hand, can be a do-it-yourself job. Usually it is just a matter of dropping the transmission, unbolting the old converter from the flexplate (flywheel) and bolting on the new torque converter..
But there can be complications. With some applications, the replacement torque converter has a larger diameter pump hub than the stock one it replaces. This requires machining the pump opening in the transmission case to accept the larger hub drive. You'll find this to be the case with some aftermarket 9 and 10-inch converters for Chrysler Torqueflights. On some older GM applications ('65-'67 Buick, Olds and Cadillacs with "variable pitch" transmissions) the pump and input shaft must be changed over to a fixed design to accept a performance torque converter. On Ford C-6 transmissions the bolt pattern on the engine's flexplate may not match that of a smaller converter, requiring a different flexplate to be used.
CHOOSING A PERFORMANCE TORQUE CONVERTER
Don't fall into the "bigger is better" trap. It's true that the higher the stall speed, the higher the torque multiplication. But the higher the stall speed, the higher the engine has to rev before the converter grabs hold and launches your car.
A high stall speed doesn't mean a car won't go until the engine reaches hits 4500 rpm. With a light touch on the gas pedal, it'll start out at 1,000 rpm like a stocker. But tromp on the gas and it'll wind up like a Suzuki in low gear. High stall speed torque converters are great for the strip, but for everyday life on the street they can be really annoying, not to mention horrible for gas mileage.
Some general rules on choosing a torque converter:
* The stall speed rating of the converter should be equal to but not exceed the rpm where the engine develops peak torque.
* The heavier the vehicle, the more it will benefit from a higher stall speed. The lighter the vehicle, the more it will benefit from a lower stall speed.
* Always follow the recommendations of the torque converter supplier. If a supplier recommends a certain unit for a particular application in their catalog, don't try to second guess their advice. They know what works.
DIFFERENT KINDS OF TORQUE CONVERTER APPLICATIONS
Applications for performance torque converters can be broken down into six distinct categories: street, combination street/strip, all-out drag racing, off-road, towing and mileage.
The typical street performance vehicle needs good off-the-line acceleration but also has to provide reasonable driveability. The type of torque converter that's best for this type of application is probably a 10 or 11-inch medium-duty unit with a stall speed 200 to 500 rpm higher than stock.
A street/strip performance car can sacrifice some everyday driveability in order to blow the doors off the competition when things get serious on the weekend. For an unblown small block or a slightly modified big block engine, a medium to heavy-duty converter that offers stall speeds 800 to 1,200 rpm higher than stock would be a good choice. For a blown engine or a big stroker motor, a beefed-up converter with a stall speed 1,000 to 1,500 rpm higher than stock will provide the necessary punch.
For serious drag racing, race converters come in 8, 9, and 10-inch sizes, although 11-inch models are also available. A race converter is built to withstand the abuse of 500 ft. lbs. plus of torque. The stall speed will depend on the engine's requirements. If a torque converter supplier doesn’t offer what you want, they can probably custom make a converter with what ever stall speed you want.
Off-road racers usually run big block engines that are built for low rpm torque. A high speed torque converter would be a detriment in such applications so stick with a unit that offers an increase in torque multiplication below 2000 rpm but has only a slightly higher than stock stall speed.
If you want to tow a speed boat or race car behind your pickup, a torque converter that allows a little more slip than stock (say 200 to 300 rpm more than stock) should make you happy. And if you are worried about fuel economy, an economy converter that offers increased efficiency and/or lockup capability would be your best pick.
OTHER MODIFICATIONS THAT MAY BE NEEDED
A transmission shift kit would be a good add-on for street performance. You should also consider increasing the fluid capacity of the transmission with a deeper pan, and increasing the cooling capacity of the ATF cooler. Raising the stall speed of the converter generates more heat in the transmission fluid. Unless the extra heat is gotten rid of, it can cook the fluid and ultimately the transmission. Stock transmission oil coolers are typically mounted inside the radiator where the coolant runs close to 200 degrees. Some cooling! Transmission fluid starts to break down when temperatures exceed 300 degrees, and many stock transmissions behind an unmodified engine can run that hot. Some means of auxiliary cooling is an absolute must, therefore, when extra horsepower and a higher stall speed feed power into the transmission. An oil cooler mounted in front of the radiator will keep the ATF temperature within safe limits.