Really OT, Stall Converters

I think I understand that a stall converter is basically like a clutch for an automatic, but could someone explain to me how changing one in a diesel truck makes the truck faster?

Allows high engine rpms to spool up the turbo before converter engagement occurs.
It's a critical science to match the torque converter stall speed, torque converter torque capacity. To the turbo A/R ratio plus the compressor wheel size.
Multi-plate lockup high stall torque converters allow a good launch then lockup to prevent slippage. Once the engine is into it's power band and turbo is up speed.

All TC's on cars/truck stall. That stalling is what "disconnects" the engine from the drive line when stopped in gear. The original high rpm converters were made for the race (drag) track and engines running big cams that had to idle say at 1500 RPMs. Small converters also slip more during acceleration and at speeds if they are the older type with no lock up.

On newer applications like a truck putting in a smaller diameter converter allows more slippage from a stop or when accelerating hard enough to disconnect the lockup. That allows the engine to rev faster which could be desirable in a true off road truck and some may even like that in a daily driver. But when you hit the point that a converter locks up they all spin at engine RPM so no advantage to top end. Thing of it is just because the engine winds up faster don't mean that the vehicle is accelating faster if it's slipping too much.

Now knowing that a TC is a liquid coupling between the engine and tranny and has to have a certain amount of slipage to work too much spilage on a daily driver or work truck is bad. That slipage creates heat and heat is one of the biggest killers of auto trannies. Thats why when the came out with the lock up converters and better coolers in most cases they about doubled the life of the tranny.

Rick

(quoted from post at 06:00:09 01/13/12) Allows high engine rpms to spool up the turbo before converter engagement occurs.
It's a critical science to match the torque converter stall speed, torque converter torque capacity. To the turbo A/R ratio plus the compressor wheel size.
Multi-plate lockup high stall torque converters allow a good launch then lockup to prevent slippage. Once the engine is into it's power band and turbo is up speed.

Click to expand...

Learn something every day. I can see why pullers and mud racers would want that. But it's kinda funny, setting here remembering my dad complaining about them "darn kids and thier hot rods".......and I'm thinking about the darn kids and thier hot rod trucks.....ROFL!

I haven't stayed up with the performance things in the past 15 years or so, guess I'm getting old.....

Rick

The stall RPM (or stall speed) of a converter is the rpm at which the converter no longer slips, or slips very little. An OEM converter is set to provide good fuel economy and smooth acceleration, so the rpm is set low. A modified engine needs to get at or near it's torque peak to achieve the best acceleration, so the converter rpm is set as close to this as possible. Advertised rpm of a given torque converter is only an estimate- it depends on the engine's torque band for it's true stall rpm. For my drag cars, I always had a converter custom built to match the characteristics of the engine. Coan was always my favorite builder- they asked every possible parameter, such as car weight, tire size, camshaft specs, rear gear ratio, etc, and the converter always did what they said it would. I once ordered a 5300 rpm converter (8") from them for a large engine in a heavy car, and it stalled at 5300 rpm. I later used it with a small motor (less torque) in a very light car, and it stalled at 4000 rpm. The car was lazy off the line, since this engine's torque peak was about 6500 rpm. The low buck solution at the time was 200hp of nitrous in low gear, which brought the torque curve down, and also raised the stall speed to about 5000 rpm. The car left like a rocket. In a street car, converters are always a compromise. A high stall is a pain in the butt to drive every day, but also a lot of fun, is matched to the engine.

All torque converters have a stall-speed - high or low performance.

A torque converter is either a locking torque converter, or non locking. Locking torque converters are useful to fuel economy because they provide a direct link to the trans (no fluid coupling) when locked. They are locked by the external application of controlled hydraulic pressure on internal clutches (or magnetic systems using electricity) these have nothing to do with the reality of a ""stall" converter.

An engine produces a torque curve as it accellerates (or tries to) from idle. If coupled to a torque converter that is held stationary on its output shaft (stalled)by the transmission, (or in the case of a dynaflow buick, direct to the driveshaft clutch), the output turbine is stationary.
The input (cover and input turbine) is spun by the flex plate. AS engine torque increases, its speed increases. This RPM increase continues until the torque multiplication of the convertwe at stall prevents further rpm increase. (Mutch heat is generated doing this because the entire engine output is being converted to heat.)
A 2000RPM stall speed thus depends on a specific engine HP and torque curve to be met. It will change its measured "stall" speed with different engines based on that factor alone.
A high "stall" converter just lets the engine spin higher into its HP curve than a low speed "stall" converter. It will also allow higher engine speeds due to fluid dynamics thab a low stall converter. This does reduce MPG and produces more wast heat. If it is also a l;ocking converter, when locked it is not a High Stall anything.
There are complex torque converters in those buicks. a simple fluid coupling is not used today for automotive applications. Jim

All converters stock or aftermarket have a stall speed. With a after markert, you pick the stall speed that matches the power curve of your engine combination. Ive only used one in gas powered vehicles, but if i had to guess I would say for the black smoke crowd that it is pretty effective at getting the their oversized turbo spooled up. If this is something your looking at, you should know that the high stall converters build a lot of heat in a short amount of time, and heat is the absolute #1 enemy of a slush box.

Converter stall speed is the RPM at which slippage lessens and more power is transmitted to the transmission.

Truck usually have a "loose" convertor, or one whose stall speed is too high over the peak torque RPM. Replacing it with a "tighter" converter, one whose stall speed is closer to the peak torque RPM, will cause less slippage and more power transfer. The old Cummins trucks like mine, the peak torque is around 1800, but the convertors don't stall til around 2000 RPM, so I have 2000 RPM in my powerband that really doesn't do anything because the converter is slipping too much in that RPM range.

Highly modified sled pullers, like drag racers, want a high-stall converter so that they can get the RPM up into the powerband while still stopped.