torque convert question

[DLang]

I understand the basic theory about how a torque converter and automatic transmission work, but what I don't know, and don't know where to look is how to calculate their effect on how torque is transfered from the engine to the driveshaft. I've put togeater a perforamce calcualtor to assist me with thought experiments of different engine/transmission/transfer case/gear/tire combinations (it's at lang.hm/scout/nph-performance6.cgi if you want to take a look). it's fairly good for manual transmissions, and probably not too bad for the newer automatics with locking converters where you can assume no slippage in the drivetrain, but for older automatics (like the TH400 in my truck) I know that there are other factors that come into play, but I don't know how to account for them in the graphs.

I know that

1. there is some slippage between the engine and the transmission, the transmission will never turn quite as fast as the engine

2. if there is a substantial difference between the engine speed and the transmission speed the torque from the engine will be multiplied in the transmission (but I have no idea what this curve would look like, I seem to remember reading somewhere that this approached a 2x multiplier under ideal conditions)

3. the closer the engine and transmission speeds are the less efficiant the torque converter is.

4. I've seen the term 'stall speed' used, and I think it refers to a speed below which the converter transfers almost no power.

5. I know that there are pumping requirements that eat some power (but how much? not just in peak HP, but at any arbatrary speed)

can folks here help me (or point me in the right direction?)

David Lang

[NH2112]

Well I'm not sure of the best way to define "stall speed" but what you do to check it is put the tranny in D (not OD), push brakes to the floor as hard as possible (you have to keep the drive wheels from spinning), and floor the accelerator. Your engine RPM should reach stall speed, but not exceed it. If you don't reach stall speed you have an engine problem, if you exceed it there's a TC or tranny problem.

[Robyn]

Stall speed is the speed that a good running engine cant pull beyond with the brakes locked and full throttle aplied.
This can vary wildly with toque convertors that have been rebuilt.
There are so many types ie low stall, high stall, RV ect.
The higher the stall the "looser" the convertor also the more heat it produces at low speed.
At road speed (60 MPH) on a stock 400 your engine will only be about 150 200 RPM over output speed.
Your 400 is about the best choice for a non lockup box regardless of transfer case or rear gears, tires and so on.
Not really sure here what you are trying to accomplish???????/
But I hope these tid bits help.

[DLang]

what I'm mostly trying to accomplish is to figure out the formulas to amke my graphing accurately reflect the effects of a torque converter. right now it treats an automatic transmission as if it was a manual, and I know that that's wrong, I just don't know how to do any better.

it was mentioned in another thread (possibly on a different board) that auto transmissions eat a fair amount of power just to run the transmission (figures were given that range from 15hp to 50hp depending on the transmission), but saying that a transmission eats '15hp' means drasticly different things at 1800 rpm then it does at 4000 rpm, so I doubt that the effect is that consistant in the hp cost (it could be, I am just doubting it)

David Lang

[gmctd]

Do a search on the 'net, or google for automotive torque converter theory - narrows the search for curves specific to infernal combustion engines designed for traffic service.

Iirc, there's some stuff specific from GM, and may address the two main tc's for the T400, the one for the 350 and the D4 for the 6.2-5 and the 454.

Also check for info on the D6\D7 (iirc) for the Cummins\Dodge, with extremely low stall, almost TCC-style hookup, but absolutely no engine braking - drive\driven elements and stator are inter-changeable with the GM D4 version.

There's some info on the ATA site, butcha gotta pay fer that stuff - it's for professionals in the business.

[DLang]

Ok, I found a page that seems to be reasonable at http://www.leeric.lsu.edu/bgbb/7/ecep/auto/f/f.htm

it indicates that for cruising you get about 90% of your rpm through to the drivetrain, and loose ~10% of your power to the pumping overheads in the transmission.

I'm going to assume that that's not the same 10% and change my model to take this into account.

I also see that when the difference between engine speed and drivetrain speed approaches the stall speed you get a torque multiplier of 2-2.5:1 (although you really don't want to do a lot of driving in this mode, it will heat the transmission up a LOT). I'm debating exactly how to account for this.

what are common stock stall speeds?

I also found a facinating article about a purely mechanical torque converter that was designed in the 1920's it would be interesting to see what could be done with this today (the designer produced cars that got 100 mpg in 1924, with a 500cc 5.6hp engine. but nobody cared about milage then) this wasn't just for tiny cars either, romania used the design to power railroad trolly cars (40 ton cars powered by a 10hp motor instead of the ~140hp motors commonly used on cars that size)

http://fluid.power.net/fpn/const/const005.html

[Robyn]

1500 would be a good ball park "stock" stall.
The "power loss" through the box is the reason we have seen the advent of the "Lockup" convertor.
With the unit in lockup the power drain drops to the equivalent of a stick shift.
Generally though the power loss is so small its not worth worrying about.
The torque multiplication factor far outweighs any savings.
If it were not for the torque convertor you would literally run to death many of the pickup engines used to pull fairly heavy loads.
You would be limited to doing the job with lower starting gears and if mileage was a concern, overdrive on top.
I remember back in the 60's my father had a new ford truck he built a camper for. The Ford had a 292 with a 4 speed. The rig went ok but on the road and pulling a grade 4th was too tall and 3rd was too low.
Most all Class 8 stuff now runs at minimum a 10 speed.
My western Star has an 18 speed with the 500hp Cat up front.
This gives a good broad range of ratios to make live workable.
I had a 1982 Blazer with a 6.2 and a 700R4 tranny the rig got 24 MPG highway.
The 700 went away and I replaced it with a 350 which is a 3 speed and no lockup. The mileage on the road never changed much.
The only loss was the deep low gear of the 700 versus the much higher one in the 350.
The great thing about the lockups is that the heat created in the converter drops drastically when in lockup. Most all of the heat in an auto comes from the converter.
During hard pulls "Nearing stall" there is copious quantities of heat built up by the convertor that must be dissapated.
Heat takes HP to create and this heat is blown off to atmosphere via the coolers. Therein lies the power loss spoken of.
In an ideal world that heat would be reused to produce power, but unfortunately we dont live there.
Back in the days of the first autos they did not have torque converters, they had a fluid coupling which got them around the need for a clutch but provided nothing in the way of torque multiplication.
Them old suckers would sit at the stop light and roar, scream and howl and finally move off down the road and then they would shift into a "Locked" mode for the remainder of the gear ranges.
They were harsh shifting and in many cases not real durable, but they were the start of great things to come.
Just a little history here :0)
Robyn

[CleviteKid]

Back in the days of the first autos they did not have torque converters, they had a fluid coupling which got them around the need for a clutch but provided nothing in the way of torque multiplication.
Them old suckers would sit at the stop light and roar, scream and howl and finally move off down the road and then they would shift into a "Locked" mode for the remainder of the gear ranges.

Robyn

. . . which is why Buick's version, the Dynaflow, was so often referred to as the Dynaslow.

[gmctd]

..and Dyna-slush............

[Robyn]

Yes yes the old Buick's
Brings to mind a hair raising event when I was about 14. I was riding with my Uncle in his old Buick, we were heading down a long grade with a train crossing at the bottom. The brakes faded out on the buick so O'll Uncle Clarence shoved the Buick in reverse and floored it. OMG I had S!@T in my pants on that one.
He got the bloody thing stopped about 3 feet from the tracks. Now dont try this with your 4L80 cause it aintagonna hack it!!
The old Buick took us on home as though nothing was amiss..
Another old box was the Roto Hydramatics. What a POS. had to learn to rebuild those in College auto shop. OMG a nightmare they are.
the 4 speed hydro's were an ok box.
Robyn

[DA BIG ONE]

The higher the tq ratio of a converter the better for say drag racing. The tq will multiply until the tires break great for that hookup then drop off quickly.......

Custom converters can be built to fine tune the tq X, and stall speed but can run into $3,000+ cost w/kevlar clutch, or multi clutch setup.

Consider tq output of a diesel and things can go bad quickly if the tranny can't take the tq multiplication load.

For a given stall speed od say 2,000 rpm's the actual stall would be in relation to the engine tq output, higher than stock then the stall rpm's will increase, engine tq output below stock and the converter will stall sooner.

[damork]

I still have an old 1950 Model B2B Dodge pickup with the Fluid Drive option, which was basically a standard 3 speed transmission behind a torque converter and a clutch. Was my first vehicle and although not a lot of power, it would put many 4WD units to shame in muddy or snowy conditions. It had 6.00-16" tires, tall and skinny.