If stall converters multiply your torque, then how come it isnt shown on the dyno that torque has been multiplied? Thanx.
-Brent

 

i was always under the impression that that is how A4's get such good torque #s on the dyno.. they flash the converter and it does show up

 

The flash does show up on the graph if you can flash it without down shifting.

 

It does show up on the dyno.

Often misunderstood is that the STR is not held throughout the RPM range. The STR is measured at the point when the input side is at full stall speed and the output side just starts to move (this is at the exact moment when power is first delivered to the input shaft of the transmission.) After this initial movement, the STR starts to quickly go downward. This is why a higher STR will mean a quicker 60 ft time, and thus a quicker ET (assuming adequate traction) but will show no increase in trap speed over a similar converter that has a lower STR (it may even show a lower trap speed if it's higher STR was derrived at the expense of efficiency and/or shift extension.) The higher STR converter has the advantage only at the beginning of the race.

 

Stall Torque Ratio:

Stall Torque Ratio is one of the most misunderstood aspects of torque converter construction. Our competitors often call stall torque ratio: torque multiplier. The stall torque ratio is the amount of engine torque that the torque converter can multiply at a particular rpm level. By definition, stall torque ratio is when the turbine is at 0 RPMs and the converter is at maximum designed stall. This will produce a positive push on the turbine to increase the torque to the input shaft of the transmission, multiplied by the designed stall torque ratio of the torque converter. For example, a stall torque ratio of 2.0 would multiply 200 lb. ft. of engine torque to 400 lb. ft. of torque at the transmission input-shaft.

The misconception of stall torque ratio is that more must be better. This is not always the case. High stall torque ratio applications, typically are for industrial equipment or engines with limited low rpm engine torque. With high stall toque ratio converters, there are important trade-offs. What you take at one end you give up on the other. Typically, a torque converter with a very high stall torque ratio, such as 2.0-2.5, will be much less efficient above its rated stall speed. There is a sacrifice in higher rpm efficiency to achieve high stall torque ratios. That lower efficiency translates into less horsepower transmitted to the tires over an RPM range.

The problem with a high stall torque ratio converter is that it is only high while the car is not moving. Maximum stall torque ratio occurs at wide open throttle with no rotation of the transmission input shaft. As the input shaft starts to rotate with vehicle forward movement, the stall torque ratio will become non-existent much sooner than a converter of the same stall, with a lower stall torque ratio. A converter with a stall torque ratio of 2.2 for example, would display that at the starting line, but it would drop off much sooner than a converter with a lower stall torque ratio. See graph:



For example: A competitor's converter with a claimed stall torque ratio of 2.5 (red graph line) would typically have an efficiency of around 90% at high RPMs (5,000 plus). That means 300 flywheel horsepower would translate to 270 horsepower at the transmission input-shaft. A Super Yank Converter with a stall torque ratio of 1.6 (green graph line) has efficiency in the 97% range. That means a 300 horsepower engine would transmit 291 horsepower to the transmission input-shaft: A gain of 21 horsepower! For an LS-1 customer dyno sheet showing 97% efficiency press here.

As you can see, the converter with the lower stall torque ratio will multiply torque for a longer period of time than the converter with a higher stall torque ratio. As most of you know, most racing occurs above 3,000 RPMs. That's why the lower stall torque ratio often wins the race:

Lower stall torque ratio is gentler on the tires at the initial launch, but it will pull harder for the remaining 1,305 ft. of the 1/4 mile. Less races will be lost at the starting line from excessive wheelspin.

Lower stall torque ratio will be more efficient and transmit more torque and horsepower to the tires. This translates into lower ETs and higher trap speeds!

Brake Stall vs. Flash Stall:

Flash stall is the maximum your engine's torque can stall a torque converter. In essence flash stall and full stall are nearly identical. If you had a transbrake, you could find full stall by putting your foot to the floor and reading your tach. For argument sake, let's say we're testing a 3500 stall Yank ST 3500. If you had a transbrake, you would see around 3500 rpms. If your motor was at idle and then you suddenly floored the throttle, you might see slightly more (maybe 100 rpm more) stall for a half second as the momentum of the motor's internals "flashed" the converter a small bit above its true stall rating.

Brake stall, on the other hand is a very subjective thing. For most, it's the highest stall you can achieve before your tires spin. This varies greatly based on many factors: Traction, gearing, brake clamping force, and engine torque. With a ST3500, I may only be able to get 2200rpm "brake stall" on the street with street tires...any higher rpm and the motor torque would overwhelm the tires. But if I was at the track with racing slicks on the starting line, I might be able to get 3200 brake stall before the motor torque overwhelmed the tires. See...brake stall is very subjective.

Yank rates their converters based on their intended application. A ST 3500 will stall 3500 rpms in a stock LS1. If you had a 422 and wanted a ST 3500, the converter you received would still stall 3500 because it would be built around the torque of a 422, not a stock displacement LS1. Yank checks the stall of their converters and their competitors by using either a trasmission dyno or a "tranny tricker" in the vehicle tested. With the tranny tricker, you can place the vehicle in 2nd or 3rd gear and stab the throttle to the floor...making it easy to read both flash stall and full stall.

 

For all interested, you can read the above information at http://www.converter.cc/technical_discussion.htm

 

Great info!!!!!!!!!

 

You know, they wrote that a LONG time ago. I think its funny that they have not removed it since they have the highest STR converters on the market right now.

Ryan K.

 

I was thinking the same thing. Some further elaboration needs to made on that page. It makes it sound like a 1.6 STR is going to beat a 2.7 at the track...and that just ain't even close to be the reality of it. It could really confuse someone who doesn't know better. That's why I never quote that page.

Bottom line people, for drag racing purposes, a higher STR is ALWAYS better assuming there is adequate traction to take advantage of it (all else such as stall speed, being equal.) Drag racing is won in the 1st 60ft and that's what a high STR helps you to do, 60ft well.