Curiosity Question: High Stall Speed

[wthif]

I'm still confused with how the torque converter's stall speed effects drive ability and it has been explained several times to me. I feel I'm starting to get it, but have a couple of questions.

Story first: My job occasionally gets me out on steep mountain fire roads and last week I had to rescue my boss who had two flat tires. So with a jack, jack stands, and a two hundred pound bronze sculpture (couldn't easily get it out) in my truck I headed off to get my boss. There was no problem getting to him and back in 2wd but on the way back I went up a steep grade. The really steep part of the grade wasn't real long, about 100 feet but long enough to notice that it seemed like the engine revs were somewhat higher than usual for my current speed. The grade went up about 40 feet in elevation in that hundred feet. The rest of the road is 20% to 30% grade, all fairly steep and honestly never seen a fire truck try it.
So the question:

How much does the stall speed of a torque converter effect the torque converter's ability to pull a steep grade? Especially from a dead stop?

I do expect my revs to be higher in a situation like this, but what are the chances of being on a hill with a 2,000 pound load and my truck not moving?

At the time I ordered my transmission, I ordered a higher stall speed because I didn't plan on towing or doing much off road driving any more. My stall speed is advertised around 1,700 RPM.

[DA BIG ONE]

Automatic requires the lowest stall speed you can get, if you have deep pockets a custom unit can get your stall speed down even more.

However, the tq of a diesel engine is much more than most gassers at low rpm's; so let's talk tq multiplication of a converter if the tq is increased x 2 and your motor is putting down say 300 ft lbs tq @ say 1,000 rpms then you get 600 lbs tq just as wheels start to spin then it goes back to the 300 mark after you are moving.

[Robyn]

Let me add some here.

The "STALL SPEED"
stall speed is the maximum speed the engine will run (With it running properly)
with the wheels locked and the throttle wide open. (No accessories running)

The stall speed is designed to compliment the engines HP and torque producing ability.
A little V6 cracker can engine that uses a 700R4 will have a far different converter than the 5.7 gasser or the 6.2 diesel.

The converter on the diesel is not only heavier having six bolts to attach it to the flywheel but also all the internals are beefier too.
The bladeing in the turbine section as well as the pump section are far different on a HD unit.
Also the stator is different too.

First off to really explain what is happening in the converter, think of it as two fans sitting look at one another real close together.
One fan is powered and the other is not.
As the powered fan turns the none powered one will also start to turn.

Now back to the stator, the stator is a bladed unit that sits between the Pump (blades that are powered by the engine) and the turbine section (blades that are being driven and are connected to the input shaft of the tranny)
The stator is the magical little goody that causes the multplication of torque and it does so by redirecting the oil that is exiting the turbine blades and routing it back through again to extract more useful power to the turbine.

Now in the stator there is another device, the one way clutch or sprague.
The Stator stays locked against the sprague under high demand situations requiring major torque mutiplication.
As the convertor pump speed and the turbine speed reach equalibrium the sprague allows the stator to turn free and rotate with the consist. (Highway speeds or cruiseing)

The other goody in the 700 converter is the lockup clutch.
The TCC is its refered to is a lined disc that is attached to the input shaft and floats along for the ride.
This disc is in the front of the converter and when conditions have been reached that will allow for total lockup oil pressure is routed through the input shaft to a point behind the clutch assembly and this forces the disc to engage the converter shell effectively locking the converter up as though it were a mechanical clutch.

This is the basics of whats happening.

Now if you get a condition where you mash on the throttle at low speed and the sucker howls, screams, roars, and then starts to move the very real possibilty is that the sprague clutch in the stator has failed allowing the stator to spin backward without giving the needed torque multiplication.

I have had One (1) do this in my career. It was my 86 dually TH400.
Classic textbook example, doggy off the line but ok at road speed.

If you start seeing more of the condition you described it could be things going away.
Check the tranny fluid to see if it smells burnt or has crud in it.
A failing sprague will usually leave crap in the pan but not the burnt smell.
If it smells bad its clutches or bands going away.

Hope this helps some
Robyn

[Robyn]

Just a foot note on the stator.

If the stator sprague fails in a locked condition the rig will start off fine and run good up untill maybe 40-45 then it will get real sluggish as though its got a big load behind.
This will happen because the stator has not spun free and is causing a big drag on the system.
Also will cause a lot of heat to build too.
The excess heat condition can occur with either type of failure due to the excessive amount of churning of the oil.

With a low speed failure (Sprague slipping) the unit is spinning wildly inside and the resulting excess friction causes the oil to heat up rapidly.

At road speed the turbine and pump are rotating at very close to eqaul speeds and very little heat is produced.

With a high speed failure (sprague locked) the sprague acts like a giant sail in the wind (OIL) and cause a huge drag on the oil that increases heat.

Also a failed sprague clutch in the stator can cause the tranny to produce a funny whinning noise during take off.

Its not always very pronounced but is usually there, may seem like a faint buzz or a slushy sound that was not there before.

Ah yessssss the good O'll Gooooy Go

[wthif]

Thanks for all the great info Robyn.

What are the advatages of having a higher stall speed? In custom truck mags. you'll see trucks with high horse power gassers with 2500+ RPM stall speeds.

[SoTxPollock]

If your a drag racer the advantage of higher stall speed is to get the engine up to a higher rpm where its making more horsepower before it locks up and sends you down the track. Most gassers don't recommend more than 2000 rpm stall speed on the street, but some go up around 6000 on all out racers with small engines that really buzz going down the track. I've driven stall speed cars with 3500 on the street, but its not fun unless you nail the throttle to make it hook up, kind of gets anoying light after light.
On diesels the lower the better, unless you are BMDMAX and setting drag race records.