When I was shopping for my torque converter, an issue came up with STR or stall torque ratio. It was explained to me that basically what STR means is how much the engines torque is multiplied at the rated stall speed by the torque converter. I'm at a loss here because I don't understand how a converter that is slipping can multiply engine torque.

Can someone explain this concept, and how it works?

Thanks
John

 

Here's a fairly good explanation:

http://auto.howstuffworks.com/torque-converter.htm

Interestingly a torque converter (TC) can multiply torque only when it is slipping...a lot.

Basically, if you understand the path of the oil thru the torus (donut) shaped converter, imagine the driven side (or turbine) which is connected to the trans gears, not moving because the drive shaft isn't turning. Now the oil coming of the pump side of the converter which is connected to the engine, hits the outside of the turbine and leaves at the inside of the turbine. It now hits the stator which directs it toward the back side of the pump. The oil has a lot of momentum, so it gives the pump a little(?) push in the direction it's already going. This extra push is extra torque. Given the correct size and geometry of the TC parts, this push can double the torque or more. The ratio of multiplication, when the turbine (and driveshaft) are not moving or "stalled" is called the stall torque ratio.

As soon as the turbine starts to move, and the drive shaft starts to turn, the multiplication drops like a rock, heading to 1:1 ratio. Of course, initial launch is where you want the most driveshaft torque.

The things that make for high stall speeds (the pump/engine speed when the throttle is wide open and the turbine is stalled) generate lots of heat, and usually continue to slip a few % even at a high cruise speed. Ther's really no free lunch, but STR or torque mulltiplication is close.

My $.02

 

One of the reasons I swapped the T56 for a TH400 was to take advantage of torque multiplication to launch my car, which is quite heavy. Haven't had enough track time to tell if it's going to work as planned. But I learned that converter manufacturer's make all sorts of claims about STR, which are impossible to verify.

Here are questions about STR that I would really like to know more about.

1. What is the relationship between stall speed and STR?
2. How are STR and "efficiency" related?
3. Do different ATF formulations have a significant effect on STR?
4. Does tranny fluid temp have a significant influence on STR (and efficiency)?

Rich Krause

 

The question that I’ve always had that I’ve never seen an answer for is if you’re given the choice, and the torque multiplication only happens below the stall speed (at your launch), why would you want to choose say a 1.75 vs a 2.5 ratio? It would seem to me that the higher multiplication will come of the line marginally harder assuming that you’ve got the traction, but in most cases, you’d never know the difference assuming the stall speed is optimum for the car anyway…

 

I think that' s a circular argument. One the main variables that would determine "optimum" stall speed would be the STR at that speed. Engine torque @ stall rpm*STR*overall gear ratio = torque at lauch, right?

Rich Krause

 

Well, that’s the thing… I don’t think so. Stall speed is the maximum speed that the converter can slip significantly at, and the point where you loose torque multiplication. The way that most serious racers launch their cars they’re instantly at stall on the line, so as far as I can tell they don’t spend any real time actually using ‘torque multiplication’

 

My thoughts, some of which are conjecture:

1. Higher stall speeds should give higher STRs, but maybe not in all cases. There are various ways to achieve higher stall speeds. One way is to use a smaller diameter converter, originally designed for a wimpy 4-banger, or even an S-10 V6, and input about double the design torque. Stall speed increases, but, because STR is probably already 2.0+, it doesn't increase a whole lot. Remember, this TC was designed with blade and stator angles for that high STR to get the 4000 lb SUV moving.

2. Internal modifications like blade bending or reshaping, stator replacement or modification, and some I'm not familiar with may allow the TC to slip more, and therefore give a higher stall speed. There is a fairly good chance that the increased stall speed will be due to inefficiency and STR may go either way. IMO, these mods are difficult to test unless one has a TC dyno. There aren't too many of these, AFAIK.

3. Trans fluid: IMO, there shouldn't be much difference in STR among fluids, unless their properties, like viscosity, change dramatically with temperature. I've never seen any data, however. I'd limit the time at stall as much as possible. Because my reactions are so slow, I stage last, and leave when my staged light comes on. Well, almost.

4. A fully stalled converter, absorbing hundreds of hp, heats the oil at an alarming rate in the TC. Here's where a big loss of viscosity might increase stall speed if the system is held at stall for more than a few (?) seconds. IMO, if this happens, the "thinner" oil may decrease the STR. Again, a TC dyno would really be required to tell.


On WS6 TA's thought:

Stall speed is the point of MAXIMUM torque multiplication. It's where the TC output(trans input) is zero. As soon as the TC outut/trans input shaft starts to rotate, the torque multiplication starts to drop off, usually very rapidly.

Torque multiplication is happening in real time. You don't "store up" torque like you store up energy in a rotating flywheel which is then released when the clutch is engaged. Oh, of course the TC itself has mass, so it does store flywheel engergy, but not torque.

My $.02

 

Here is some good reading on what is STR.

WHAT IS STR?

 

Excellent info, in spite of all the marketing hype. I wish I'd found this; I wouldn't have typed so much earlier.

 

So that brings us back to my original question, assuming that you choose the coorect stall speed and are not traction limited, why would you choose anything but the highest STR that you can get?

 

Well, I guess you would want the highest STR possible provided you had the traction to use it if there were no downside. However, it's unlikely that the STR is independent of other important converter operating characterisitics. Efficiency is the big one. Generally speaking, a higher STR equates to less efficiency. Also. it is said that the higher the STR, the quicker it drops off as rpm rises, which may not be desireable. When I raised questions about these issues, what I was looking for was someone who could explain the "why", but they are generally accepted as true.

So, you would want maximum STR if it weren't accompanied by lower efficiency and a faster drop off of the STR toward zero, but this may not be possible. It's too bad there are no standards for testing converters. If there were, we might have a rational basis for selecting one over another!

Rich Krause