Nice posts Jim, Rich, Bret, OldSStroker, SStrokerAce, Zero_to_69, and everyone else.

It was a very nice read and thought provoking


What really causes problems as I think we all found out is that drivetrain plays a huge part in this and it's not easy to isolate just the items you want to test (hp/tq).

The reason you shift at a higher rpm and not at peak torque is like what Jim was saying---it's because of the spacing of the gear ratios. You want maximum tq at the axles and you take whatever gear will give you that And when 2nd gear isn't giving you as much axle torque as you would have in 3rd you rock her into the next gear

 

Hey, you have to give mention to "B4C". He kicked me in the *** with his initial post.

You guys are correct, you can really feel the heat in the replies from the last
two pages.

I guess I've learned another important factor about drivelines:

Even well beyond the torque converter stall/flash point, multiplication can still
occur.

 

I think you have that backward. The higher the gear, the higher the load on the torque converter, and therefore the more multiplication takes place. The load isn't coming from the engine, but rather from the mass of the car multiplied by tire diameter and the total drive ratio. so likewise, a heavier car will also load the torque converter more.

Torque converters do lots of fun things to the power curve at the drive wheels. This is easily seen on any chassis dyno. I vividly recall watching a friend of mine dyno his turbo V6 Firebird with a very loose converter. The power curve was literally a flat line from the start of the run to redline! The torque curve started with something 800lb-ft and fell steadily throughout the run to perhaps 300. That was a great demonstration of torque multiplication. Obviously, the typical street-use converter won't be nearly that extreme, but the effect is still there.

 

Yes, my bad.

Let's change that to:

So...if my dyno was taken in 1st gear, the crank output would have had a
higher force on the transmission input which would have allowed more multiplication
throughout the sample.

Does that make more sense now?

 

Well, no, you still have the multiplication thing backward and you're kind of confusing load and force. The only thing between your crankshaft and the transmission input shaft is the flexplate and torque converter. Any gear changes happen "downstream" in the drivetrain. You can't change engine's output by changing gears (well, unless your car is a turbo, but let's not go there right now). By upshifting, you are changing the total mass load on the torque converter by multiplying the mass of the car through the gears. It's this greater load on the output side of the converter versus the torque output of the engine (which hasn't changed) that causes the greater difference in speed between the converter's turbine and impeller - i.e., it slips more and multiplies the engine's torque more.

So it would be accurate to say: "So...if my dyno was taken in 1st gear, the transmission input would have presented a smaller load to the converter's turbine, which would have allowed less multiplication throughout the sample."

 

Tino,

In your reading you noticed that a torque converter only multiplies torque when there is a difference between the input (engine) rpm and the TC output (which is the trans input shaft) rpm. Max multplication occurs at Zero (!) vehicle speed and converter input (engine) at stall speed. Obviously this can only occur prior to launch. Let's say the true stall speed is 2400 rpm. At stall and vehicle still at rest, the STR is max, say 2.0. The delta rpm (input-output) is (2400-0 = 2400).

Now there is almost no other situation with this converter when you get a 2400 rpm difference, or delta, between the converter input (aka 'pump') rpm and the converter output (aka 'turbine' or trans input) rpm. Even if you did, because the turbine isn't at zero rpm in some higher gear on the road or on the dyno at a speed above Zero, you most likely will never see the STR at any time with the vehicle moving.

This might not exactly relate to "The higher the gear, the higher the load on the torque converter, and therefore the more multiplication takes place." but different folks see things in different ways. I'm not sure I can reconcile both explanations. Perhaps you can.

If you could start off in 3rd or 4th gear (2nd gear starts are available on some automatics), you'd have lots of initial multiplicaton, and lots of slippage of course, until the vehicle speed (trans input) got closer to the engine speed. The Buick Dynaflow was pretty much a torque converter only trans in drive (but with a more complex converter) back in the 50s and 60s.

FWIW, I think your 'Yank' link, while self-serving, is the better of the two.

FWIW (Ver II): 'Most everyone is familiar with the Chevy Powerglide, but anyone remember the "TurboGlide" Chevy trans? What made it different?

Bonus FWIW: Does "Switch Pitch" ring a bell, transmission-wise?

 

Understood about the connection train...but here's how I thought of it.

What if we remove the car and transmission from the equation.

Let's lock the turbine a 4000 lb. wheel.

The engine is connected to the impeller via the flex plate.

So, we're 1:1 essentially, minus any fluid coupling loss.

If the engine is rev'd, it may reach 2200 RPM and then stall because the input
torque is 100 lbs./ft.

If we installed a gear box and allowed the engine to spin up a touch, it may
reach 2600 RPM before stalling. We are effectively dividing the output load
using the gear box.

I always thought of the torque converter STR as being a two-way street.
The input and the load determine the multiplication.


I can see what you mean by less load.

^ That confuses me?

The engine is winding up easier against 1st gear, the converter 'flashes'
easier. Doesn't that allow more multiplication? The initial difference
between input and output shaft rotation?

IE: the more difference in shaft RPM, the more STR is realized?

 

Aren't overlapping posts fun? It's like sitting around having a few beers and bench racin'.

Hmmm... good thought.

 

Yes Jon,

You're too quick for me!

BTW: I think I owe you a couple of cases by now?

What do you think of my previous reply?

 

Introduced in 1957/58.

The 'Flight Pitch Stator' is able to vary the position of it's vanes
(via hydraulic pressure) in an infinite number of angles. Throttle position directly governs this angle. The three turbines are connected to 3 coaxial
shafts (one inside the other) to 2 planetary gear sets within the transmission.

Engine torque is transmitted in an uninterrupted flow from turbine to turbine and through the varying stator vanes.

 

Tino, I don't see how you figure that changing gears to a lower gear allows the engine to flash the converter more. Let's go back to this because this says almost all:

So that stalled condition is like the ultimate, sustained "flash." No matter what launch techniques you use you'll never see higher delta rpm than 2400 with this converter and this motor. Think of this car with the brakes set (or a trans brake set) as an infinite load, which is causing the stall. Now consider the total opposite condition, where converter's output shaft isn't connected to anything - no load at all, zero. In this case, your delta rpm is 0, because the output shaft will spin at the same rpm as the input at all times (the converter has its own inertail mass, of course, so we have to ignore that). With no delta, there is no torque multiplication at all. So there are the two ends of the continuum of stall condtions: 1) infinite load, max delta rpm, max torque multiplication; and 2) zero load, zero delta rpm, zero torque multiplication.

So in between those two conditions are what takes place with any actual car with mass when it's rolling during WOT acceleration. There is definitely some load and some torque multiplication, not zero load but also not infinite. If you have a 5sp car, then the load in 5th gear is a lot closer to infinite than the load in 1st. In 1st gear, the output side of the converter will have an easier time accelerating the vehicle through the engine's rpm range, so the delta rpm will be fairly low as the output side "keeps up" more easily with the input side. In 5th, the converter is straining against roughly five times the load compared to 1st gear, and the delta rpm is higher as the output of the converter lags further behind (F=MA and all). And as OldSStroker said, the higher the delta the more the torque multiplication.

It worked for me!

 

Above Jon also said that starting off in a higher gear with an auto would
provide 'lots of initial multiplication.'

If I were to start a launch in 2nd gear, with the RPM foot-brake to 2400 RPM,
the initial multiplier spike would be bigger because the torque converter is
trying to move a higher load (the car - transmission selected gear ratio).

So you're both stating that, at launch, since the gears are locked by a trans-
brake, the torque converter doesn't "Know" what gear my transmission
has currrently selected.

The foot-brake RPM will not change because the motor output will not change
at this point.

Once I release the trans-brake, the torque converter will flash higher because
of the shock it receives from the higher load.

On the other hand, repeating this launch in 1st gear may not result in a higher
flash RPM as the transmission gearing helps to move the car away from a dead
stop with more mechanical advantage sooner.

How's that?

 

Tino,

It's the difference between input and output rpm of the converter, not the transmission that causes the multiplication. Converter output is trans input. The delta rpm occurs only across the converter.

My observation has shown more converter delta or slippage (and the resulting torque multiplicaton) in the lower gears while accelerating from a standstill, either at WOT or part throttle. My automatic C5 has a fairly tight (17-1800 rpm stall). Even at WOT (converter unlocked) in 3rd at hp peak or torque peak, there is only a few hundred rpm delta. The engine is working hard there (about 140 @ hp peak), but very little delta. Same thing at torque or power peak in second climbing a grade. Hmmmm..

When observable results differ from one's explanation of what should happen,

perhaps the explanation needs to be modified.

Mom knows why, but we need to figure it out. Remember that our explanation of why something occurs, correct or incorrect doesn't affect how it occurs. Mom Nature is a practicing physicist and one though broad. No one can change her "mind".

 

Jon!

You're still too quick for me!

I thought about this a little more and thankfully posted just before you did.

HAve a read, and please grade my answer.

 

I have to say I've not had the same empirical experience as you have in automatic cars. However, that in itself doesn't invalidate what you've reported, and I agree that either the explanation much change or we'd have to come up with mitigating reason for your obvservations in your C5 (i.e., maybe there is another reason other than converter lockup for the low delta). Persuant to that, I have an auto VW Passat that, when held manually in 2nd gear, appears to shift or lockup the converter. All of them do this - mine is not an isolated case. However, the maps show that lockup isn't programmed to occur in 2nd gear. I've read conjecture that the fluid pressure is increased at this point and is somehow affecting stall speed and delta. I really have no idea, though.