Techtorial: Torque Converters 101

(Article begins on the Tech, Tests & Installs page HERE)

A torque converter is merely a reaction to one of two things; a force applied (being the engine) or a resistance (being the car). The torque converter harnesses the engine's power so it would make sense that the more power you make, the tighter the converter needs to be and the less power, the looser. The same principle applies to the vehicle. If it is heavier, it presents more of a resistance and therefore requires a tighter converter; lighter equals less load and hence, a looser converter and so on. Installing a larger camshaft and a better flowing set of cylinder heads can actually require a looser converter even though you've made more power. What you have done is moved the power further up in the RPM range. Now you need to get the car to leave in that range and therefore, need a looser converter. If you were to add stroke to the engine and increase cubic inch instead, you'd be making more torque as well and the same converter could need to be tightened up.

ATI’s “Outlaw” Lock Up converters utilize a clutch pack that locks the converter up solid, resulting in 0% slip. It can be activated with fluid when used in ATI’s SG4 or compressed air when used in a Bruno or Lenco drive.
In this picture we can see the individual parts that make up a torque converter. To the right is the “pump” side of the converter. The angle of these fins help determine the stall of the converter. Left shows the turbine sitting inside the outer cover. This is what your input shaft goes into. Bottom left is a stator. The number of blades and the angle of the blades is the biggest determining factor in the stall. This piece locates on the stator tube of the front pump in your transmission.

Something as simple as changing gear ratio or tire height will change where the converter flashes. This leads us to another point; where does a converter flash? Or the most common question "What's my stall?" While you can pull the chip in the two step and matt the gas for a second to see where it flashes too, this isnt the best way or the most accurate. This process shows where the converter goes in regard to the force applied, but it does not take into account the resistance. The best way to check the stall speed or "flash" is to watch the tach when you leave the starting line. The next best way is to perform the following test in a safe environment: put the transmission in high gear (assuming you have a manual valve body) and let the vehicle begin to roll. Keep it about 5 mph or so and whack the throttle. Watch the tach and you will see it jump up and hold at a certain RPM before accelerating. That number is the actual flash or 'stall'. You have now put the converter in a realistic environment with both the force applied and the resistance.

Matching a converter to a combination is a critical key to making your vehicle perform to its full potential. For instance, ATI recently had a customer with a car that ran very well. The customer took the engine, transmission & converter and put it into a similar car in all respects except weight. It was the same body, same tire & gear and he put the same engine & transmission with the converter in the new car. However, the new car weighed 400 lbs heavier. What he discovered was that in the new car the converter whaich had worked so well previously now slipped substantially. By creating more resistance, he pushed through the converter showing that he required a tighter converter. We see a similar situation in nitrous cars. People want one converter to run both normally aspirated and on a heavy nitrous hit such as 300-400 horse power or more. While it can be done, it can't possibly be efficient both ways. It needs to be looser on motor and tighter on nitrous. They need two different converters.

All ATI converter parts are made in house. Here you can see the inside of a stator being machined to accept the patented sprag assembly. In some cases, sprags are replaced with a solid hub making them “spragless” but the stator stays the same.
Stock Eliminator cars require looser converters to get the weight moving as quickly as possible. It is very common to see low ETs with slower MPH out of these cars.

So with this information we can deduce that every combination requires its own converter. If you make a change in the combination, you will most likely benefit from a change in the converter as well. Again, converter efficiency is key. Something too efficient without enough power to drive it will slow a car down and something not efficient enough simply keeps the power from finding the pavement. The percentage of slip that is considered "acceptable" really depends on your power level and vehicle weight. Starting out with a horsepower level in the 500 range would require a converter to have a certain amount of slippage to work properly. As you begin to make more power, you can begin to bring that percentage down. Its like having enough power to pull a certain gear ratio. Look at Stock Eliminator cars, they typically have a higher rate of slippage in the converter and a lot of gear due to the fact that they make very little power comparably but look at how they 60ft. Now look at a Pro Mod making a substantial amount of power ... it has a lot less gear and a larger tire all the while the converter has a very limited rate of slip if any at all. Most Pro Mods with ATI Super Glides these days are using lock up converters providing zero percent slip.

In short, choosing the right torque converter may not be as easy as one may think, however it’s certainly something worth taking the time to learn about and definitely something that needs to be right in order to be at the front of the pack. That's why ATI requires a detailed information sheet when building a custom TreeMaster Converter for each and every customer.

David Caine
Sales & Tech
ATI Performance Products

ATI Performance Products, Inc. is a family-owned and operated company producing quality race and performance parts since 1961. For more info call 800-284-3433 or visit