HOV

Many here have said that the stall speed for the stock torque converter is 2200 RPM. I don't doubt what the good denizens of this board say, but I don't understand it.

If 2200 RPM is the stall speed, then the truck would be completely unable to put power to the wheels at anything less than 2200 engine RPM. I know this isn't true, because my truck starts moving well before that.

Anyone have an explanation? I don't know jack about torque converters.

new2ctd

a fair read here: http://www.bankspower.com/Tech_understandstallspeed.cfm

HOV

Thanks. I read that a while ago, but it doesn't answer my question. My truck starts moving well before 2000 RPM.

RowJ

This might help.....
Let’s start with a TRUE FULL STALL SPEED. If the transmission were in drive, the brakes were held down (so the vehicle will not move) and the throttle was held "wide open" the torque converter will "stall" the engine at a certain rpm. When "stalled" the engine will not be able to spin any faster unless the vehicle is allowed to move. This is a TRUE STALL SPEED. Do NOT test for True Stall Speed... it can damage shafts and overheat your Tq Converter!

The next stall speed is generally called BREAK AWAY STALL SPEED. If a truck is stopped on a steep hill and held in position using light throttle we are almost at the "break away" stall speed.
The Break Away Stall Speed is the RPM required to start the truck moving.

The last stall speed is generally referred to as the FLASH STALL SPEED. The flash stall speed takes effect under hard acceleration. If, from a standing start, you were to floor the throttle, the engine would start to accelerate quickly and then pause at an rpm as it starts to pull the truck. If the engine went from idle to 1500 rpm in 1.5 seconds when floored and then took another 2 or 3 seconds to get from 1500 to 1700 rpm, this would mean the "flash stall" speed was at 1500 rpm.

RJ

new2ctd

It moves but it is not close to its full power potential. Our powerband is in the 1700 range so with a 2200 rpm stall convertor we have gone over our peak torque curve to get the full potential out of the convertor. so you're never getting your full power with the stock convertor.
Lower stall speed put the peak convertor power in the same place as the peak engine power.
so stall does have quite a bit to do with moving the truck but not just moving it from a dead stop.

***
As engine speed increases, the speed of the impeller and the turbine become nearly the same (reaching their point of minimum slippage). This is called coupling speed or stall speed and is where the converter is generally more efficient. Because the turbine is spinning faster than the fluid can exit its radial chambers, the net angular momentum of the exiting fluid is in the same direction as the turbine's rotation, rather than opposite it. As the impeller approaches this speed, the torque multiplication provided by the stator decreases. At that critical speed (the converter's stall speed) the fluid strikes the back of the stator blades, causing the stator to freewheel so that it will not interfere with the return flow of fluid.

HOV

So which stall speed on our trucks is the 2200 RPM one?

HOV

Okay, with the previous answer about 3 stall speeds and what you provided, I'm seeing that stall speed isn't really an absolute cutoff of go/no go. But tell me this: if the TC is in fluid coupling, it's creating a TQ multiplication effect (that's what your quote says too), so we probably DO make peak TQ at 1700 RPM anyway.

But I also have to question the 1700 peak TQ number. Dyno charts posted here for stock trucks show that there's a small peak around 2400 RPM, and yes there is a steep ramp up to max torque at 1700 RPM, but the curve stays generally flat (save for the 2400 RPM peak) after that.

Regardless of stall speed, we would still hit max TQ just fine for towing at speed when the TC is locked, which is generally when you need it most.

new2ctd

I agree with you here because I'm still pulling strong upwards of 3000rpm on mine! It sure doesn't feel like its at the end of the rope.

But I think their saying that our effective torque peak is near 1700. the TC's perfect sweet spot is the 2200 range. so it is said that we will not get the best that we can in that scenario.

HOV

Correct me if I'm wrong:

Once the TC is in lockup, all questions of stall speed are moot. So anytime the truck is in tow-haul and 3rd gear with lockup, or even second gear if selcted on the lever and at speed, we can spin 1700, 1900, or whatever RPMs would be appropriate and the engine would still be mated to the transmission with no fluid coupling.

The only time the stall speed would be an issue is when starting with a load from a dead stop. The engine will rev a little more than with a lower stall speed converter, but that's about it. I bet a lot of people with manuals slip the clutch to around 2200 rpm anyway when pulling a heavy load from a start. Anything less would be a risk to bogging the engine anyway. Once the load is moving and the truck is in higher gears, the TC will lock and the heavens align.

Am I way off base here?

KVD

Little off topic.

What is the stall speed of most of these performance TCs?? I've heard 1500, is that right??

Also I see people with DTT TCs that are 89% or 91%, what is that?? Also some people say they have a lose TC or it's too tight, could someone explain the difference and why I would or wouldn't want a tight TC.

RowJ

Don't know for sure?

DTT no longer uses that differentiation... but it had to do with line pressures, I believe. Higher pressures (91%) gave firmer shifts for drag racing. 89% was also called "cattle hauler" set up. Had nothing to do with Tq Converters. Many, including myself, drag race with the smoother shifting 89% set up.

Loose TC means a higher stall speed. Easier on drivetrain.... for example, reduces need for billet shafts around 500 hp.

RJ

new2ctd

HOV

In theory yes once in lock-up it's 1:1 but that's not really capable. The Auto being fluid coupled has a constant fluid leak. So I believe it reduces the 1:1 a little.
On the manual, there is no fluid leaking to reduce the 1:1

KVD

Rowj, sou would 1500 be a tight stall??

I don't pull or race with my truck just drive it daily so I'm kinda wonderin what I need on terms of a stall speed.

redpoint5

From what I'm reading it seems people should be aware of the difference between torque and power. Power is a function of torque and engine speed. Think of torque as a man holding a box of stuff that needs to be moved. The larger the box he can carry, the more torque he has. The faster he carries it to the destination, the more RPM he has. Now, combine them and you have larger boxes being carried faster equating to more work being done; this is called horsepower.

The peak torque comes at a different RPM than the peak Horsepower. As RPM continues to build past the peak torque, the engine becomes much less efficient. Power still continues to build but not as fast anymore because the torque is dropping. Imagine the man running around with the box faster than ever, but with a much smaller box.

Horsepower = Torque * RPM /5252

Don't ask me where the constant 5252 comes from, it was mathematically derived and I haven't taken the time to learn where the number came from.

So, high stall is good when you want to go as fast as possible like in a race. Not good when you want to get good fuel mileage and drive normally.

Just so people are clear, stall speed is indeed moot once the converter locks up. There is no slipping once that occurs and it is 100% efficient. Not sure on this but I believe the other efficiencies relate to the ratio between the speed of the input shaft to the output of the torque converter. For example, if the input shaft spins at 1000 RPM while the output spins at 900rpm, the converter would be 90% efficient.

XLR8R

That would be an awful lot of clutch slippage!

Since the CTD produces hundreds of ft-lbs of torque off idle, there's plenty of "grunt" to get the load moving... of course, the newer 3rd gens have the "zero-throttle" launch feature - works in the bottom half of the shift pattern in my experience.