If you have one converter that has a 2.0 STR and change to a converter that is identical in every respect EXCEPT this one has a 2.5 STR, will you see an increase in RWTQ of 25% ? Or any increase at all? What would the dyno show?

 

STR applies when the trans is in the situation/condition of maximum stall(full throttle w/driveshaft not rotating).As driveshaft rpm goes from 0 rpm up to operating rpm,STR goes from advertised STR down to a very low(if a torque convertor was 100% efficient,STR would be 0 as with a M6) ratio value.
RWTQ would be higher for a brief instant at the starting line and and soon as the vehicle starts moving,the stall ratio starts it downward plunge.
A set of dyno tests with different ratios would be interesting.

 

Thanks for the reply. I understand what you are saying, but wouldn't the STR be 1.0, not 0.0??
Dyno tests would be nice. I find it strange that no one seems to have done this. Maybe since STR is really only an instantaneous event and then all STRs end up being equal at higher RPMs.

 

A Torque converter consists of a pump, turbine, and stator. The fin design of the stator determines the STR of the torque converter.

As fluid enters the converter, the pump (which is turned by the flexplate/crank) moves the fluid against the walls of the tc, the fluid then hits the turbine (which has fins facing opposite the pump) which turns the transmission. The fluid must then exit the turbine, which means the fluid will be traveling in the opposite direction of the pump. This is where the stator comes in. The stator very quickly disrupts the flow of the fluid to match the direction of the pump to minimize the amount of energy lost between the engine and the trans. Eventually, the pump and turbine are spinning at nearly the same speed. At this point the fluid exits the turbine and enters the pump already in the same direction as the pump, so the stator is just free-wheeling and isn't needed anymore.

STR isn't really just an instantaneous thing, but it's value slowly decays from whatever the rated max is, say 2.2:1 to 1:1.

 

Rate of decay is an important factor. Typically, a high STR converter's torque multiplication will decay more rapidly than a low STR converter's will. CoonToot, using your example, your 2.0 converter will show less initial torque than the 2.5 would, but the 2.5's multiplication would not last as long as the 2.0's. There are some conservation laws at work here.

 

Right, as I understand it, the stator must redirect the fluid before it hits the pump. So, a more agressive fin desing on the stator means more energy is lost when it hits the fins as opposed to a stator with a less aggressive fin arrangement. Imagine running through an extremely loose turnstill and a loaded turnstill over and over again. Which one is gonna wear you out faster?

That's why higher str converters are less efficient at transferring power from the motor to the trans than lower str converters. Take a look at some dyno graphs of similarly modded vehicles. High stall, high str converters will consistently dyno significantly lower than a similarly modded vehicle with a stock stall.

Just my 0.02

 

thanks for the info guys!

 

It will make more torque that fades earlier, at least
this seems to be the rule within a given design style
or family. See this chart, off Yank's tech pages: