The articles below have been reprinted by permission from Precision Industries.
STR has been talked and argued about for many years. The following information is what we at Precision Industries have found by doing our own testing to be helpful for you to try to understand this subject.
Our competitors must have a crystal ball to figure the STR of their torque converters. There are a couple of companies in the performance torque converter business that do have the capability to test STR, MOST DO NOT! Precision Industries happens to be one that does have.
Our test dyno has the capability to test from 200 ft. lbs. of torque up to 900 ft. lbs. of torque. The other companies use dynamometers that vary from 100 ft. lb. of torque to approximately 300 ft. lb. of torque. There is an old GM printed sheet that has floated around our industry for about 10 years showing the STR of stock GM torque converters that were built back then. Our competitors either use this sheet or are guessing because they have no way of knowing what the STR really is. They also advertise STR's of 2.7, 2.9, 3.0, 3.2, etc. this is pure BS. All torque converter companies use the impellers, turbines and stators that come in the factory torque converters. In our tests we have never seen an STR over 2.55. Precision Industries do have torque converters with STR over 2.55 but these units have specially machined stators in them and are not worth the high cost for a street/strip application.
If our competitors try to tell you differently just ask them to show you a picture of their test machine, not just a printout. The formula for STR is EXACT OUTPUT TORQUE ÷ EXACT INPUT TORQUE = STR. This requires a known power source and a data recovery system. STR is just what the name implies. The ratio of torque multiplication at stall. As soon as the turbine rotates (car moves) the ratio starts dropping rapidly until enough RPM has been reached for the ratio to drop to 1:1. The RPM that the ratio reaches 1:1 varies depending on other factors in and out of the torque converter such as impeller exit angle, stator design, impeller to turbine clearance, input torque (engine), etc. A fact that most everyone overlooks is that a torque converter does not make torque! It takes the torque the engine produces and multiplies it for a very short period of time. This is why some cars perform better than others with the same torque converter.
Some car enthusiasts put all kinds of money in the engine, transmission, torque converter, NOS, rear end gears, etc. then want to cripple the torque converter by lowering the STR to try and prevent wheel spin on take off. This is as foolish as pulling off one of the plug wires.
Why don't they finish the project and work on the rear suspension? Trying to tune your car by using STR is probably not very smart seeing how our competitors don't have a clue about what STR their converters have, after all the stock torque converter you took out of your car has a STR of 1.9-1.94. The best way to get the right torque converter for your car is to buy it from a company that has a large database of previous sales for your kind of car. Then tell the sales person what mods you have made and what mods you intend to make and let that person help with the decision. The key words here are large database. The "Johnny come lately companies" have no large database or experience with what works with these late vehicles.
Why try to reinvent the wheel with your hard earned money when there is bound to be someone in our database that has the same mods as you do and their torque converter/engine combination ROCKS! At Precision Industries we found out 10 years ago that STR is not the most important consideration when deciding on a torque converter it is the whole COMBINATION!