Practicalities of auto gearboxes

[TuffRR]

Can someone explain to me how torque converters and ratios work with auto gearboxes. I understand that normally they run away down hill but can they be modded to stop this, is that what the torque converter does? What about de-stalling, what do people mean when they talk about this?
It has also been mentioned that torque converters offer a 2:1 reduction? Why/How?

What is a good auto box to put in for off-roading?

Thanks.

[jessie928]

Yes, you can de-stall the torque converter in he tranny, Just drops the stall speed from a preset 1500rpm or so to whatever you want it to be. This is dependant on what your car can idle at. bout 800-1000 rpm might be ok.
the stall speed is the point where the torque converter build up enough fluid pressure to turn the gearboxefficiently. fir instance a 4000rpm stall speed will allow the gearbox to slip upto 4000rpm after which you get an almighty kick of power right before the peak of your engines torque curve.

If you drop your stall speed, make sure your engine has enough torque at that rpm to pull the weight of the car, otherwise it wil lbe a pig and wont pull the skin of a rice pudding.


A good gearbox.. Chev TH350 lighter strong as 3 speed, TH400 Heaviyer stong as 3speed
Th700r4 heaviyer, offeres 4th and is not quite as strong as the others but when modified and kitted out a little is enough to cop a severe pounding from a warm BB Chev.
I would go the TH700R4 simply because it offers better economy and drivability.

There is also the TH700 out of the V6 Commodors which are popular in the states behind mild V8's as you can get adapter kits to mount them up.
this may be ok in your rangie and is ALOT cheaper to buy and rebuild than the V8 version.
a buggered V8 700 will dent your wallet about 700bucks depending on availability, a good V6 700 you can get for at 400$ anyday of the week.

Jes

[TuffRR]

the stall speed is the point where the torque converter build up enough fluid pressure to turn the gearboxefficiently. fir instance a 4000rpm stall speed will allow the gearbox to slip upto 4000rpm
Jes

Does that mean that if the revs are above the stall speed, the vehicle will not run away going down hills and would have the same engine braking as an equivilantly geared manual box?

[jessie928]

Not quite,
there is still slippage associated to a Torque converter because its not actually a mechanical link, its a fluid link, like the clutch fan setup. so its never going to be the same as a Manual cluch setup.

A locking torque converter ( such as teh one in the TH700r4) "may" beable to be modified so that it locks in 1st at slow speeds, which would more or less give direct drive through the tranny but i dont know if anyone has done this before, or if its at all possible.??.

the ultimate setup would be, a disk brake in place of the drum brake on the back of the transfer case with a brembo 6 piston caliper, and a small vacume booster as on the GQclucth for enginebraking

Jes

[awill4x4]

I've got a turbo 700R4 in the workshop at the moment with about 3 or 4 large chunks broken on the bellhousing. A friend of mine bought it from someone who thought it was buggered because of the broken casting for $125.
My next job is to Tig weld it all back together
I just hope it's a "nice" casting.
Regards Andrew.

[Punchy]

From the "small" experience i have with convertors i can tell you this.
If you are wanting consistant torque convertor performance and suitable engine braking from it, I can sugest one type of convertor.

NB!!!!!!!:
Its best only suited for competition vehilces only.!!!!!!!!
And you require a very large- effective fan cooled auto trans oil cooler to go with it

They are a special build and not to many places do them...

They are called a Diode torque convertor.

The are very consistant ..and produce larger than normal amounts of heat in the system, thus the requirement of the extra cooling

The thery behind it is this.
There are three main parts to a auto torque convertor (now called ATC for short)
1 the turbine
2 the impellor
3 the stator

In short -the stator which normally is fitted with an internal bearing (sprag or roller) and thrust washers is "locked" and is not permitted to rotate..

(i will not go into the physics and mechanical BS of it all and ill keep it sweet.)

By not allowing the stator to rotate , the pressure inside of the convertor is controlled, and also increases as engine rpm rises. HIGH SUSTAINED RPM IS NOT RECOMMENDED.. This is a throw back from a drag racing application. the Controlled pressure does not allow over run or slip back from the transmssion ( like a locked up clutch).

Engagement point (or stall) can still be pre determined by the shaping and configuration of the turbine, impellor and stator... the higher the engagment point the more heat involved due to fluid transfer.

This is an option, but it can be an expesive one to build, and failure is a reality. They generally dont last forever...but thats talking from a drag racing point of veiw...4x4ing does not include 5-7000 rpm launches ! heheh does it now..
Providing the unit is kept under controll for the heat i should last..

Finally the th700r trans lock up convertor isnt strong enough to cope as a mechanical brake.. they are just too weak in construction and design..hell they burn out in a car doing constant town driving in 4th....

[jessie928]

Your being sarcastic now arnt you
your right about loading up overdrive.

I am interested in this Converter type, first time i hearing of it. Morre Info PLEASE ?

The trans cooling should be no problem with a big cooler as you say.

Interested,

Jes

[LOCKY]

I run a Chev 305 Turbo 400 in a 85 Rangie, with 4.7 Diffs and 36 Swampers. Std Convertor & LT230 Transfer. Fantastic retardation downhill, need to accelerate down most things including ledges as the gearing is so low.

Very Happy

[Punchy]

Let me know what the application is
engine
gearbox
horsepower (real figure not the i will have mayby this much one)
Torque ( re above)


I would assume that if you are wanting engine braking you want it locked up from about 1200 rpm.

After i have the above i will make some inquires or you could try ringing "melva converters" in brisbane..
there is another guy i know that could build it to suit your job...but i refusee to associate or do bussiness with him anymore... He cost me thousands once..i shant be doing that again.. :(

[Punchy]

(is that the TPI 305 locky ?

I'm gearing up to install a 307 chev into my 1989 75 series tray..and am of two minds whether to put in the th400, the th700r or just to leave the 5 speed manual in place.

how is it with the auto 400 in there ?
any problems with the box ?

[RUFF]

After i have the above i will make some inquires or you could try ringing "melva converters" in brisbane..
there is another guy i know that could build it to suit your job...but i refusee to associate or do bussiness with him anymore... He cost me thousands once..i shant be doing that again.. :(

Phil from Melva Is a Top guy and a 4wd nut as well its just he drives a suzuki If you call him tell him that Tony from Outerlimits gave you his number and he should do you a good deal.

Melva's number is 07-38089376 Ask for Phil Harris

Punchy who is the other guy you have had probs with. Pm me if you can im just curious who it is as i have dealt with most converter builders in Bris and its good to know who is F**king who around.[/url]

[Punchy]

have PM'ed ruff

[RUFF]

Yeah no worries i knew who you had to be talking about

[LOCKY]

TPI removed and Quady & Performer installed by previous owner of motor, damn!

Turbo 400 is great but could do with an overdrive but I own it and don't want to change adaptors etc. Strong and reliable, and did I mention cheap.

[Punchy]

i've looked at the back of the th400 and th700 cases..and the look very similar in mounting paterns..i just haven't checked to see if the out put shafts are different...

IF things were similar would it be just a simple case of a different spud shaft for an upgrade to the th700 ?

[LOCKY]

Think it is more involved than that including length.