Thank you sir! You answered my question based on your experience. A question I have been trying to get answered for over a year. especially this part... “But it was made clear to me that this unit would not result in an "underwhelming" or awful launch as compared to other similar stall speed units with higher STRs, it would just be a little less aggressive in this regard with the trade-off of better efficiency at higher rpms (which is what I wanted most).”

I love the idea of the low STR route. It fits my needs.... On paper, but I was worried I would regret it I'm practice and was hoping to avoid a re-stall. No need to be a professional in this. Your experience helped me.

thanks. Never thought I would find an old school SY3500 user. Too cool.

 

I posted in another thread a few months ago that STR is almost never discussed in this forum anymore, whereas back in the early 2000's that talk was all the rage. I think your explanation (which I've abbreviated above) gives a good reason for that. There are too many parameters involved for that one factor to have standalone significance.

But back to the original topic. In my experience, like RPMWS6, I don't see advertised STR as having a noteworthy effect on shift extension, at least when it comes to similar popularly available converters.

 

I found this on another site copied from Yank's website years ago. It mentions the SY3500 so it must be pretty ancient. Good info nevertheless:

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.

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!

 

Great find, looks like something from the internet wayback machine! I remember reading all that info on their site long ago, when this model was still a current offering.

Glad you found that helpful. Frankly, I would probably still be using that SY3500 today in something or other if I hadn't sold it along with the car it was in at the time.