My '72 Supreme has a 2800 stall converter, and even after doing some research, I still don't fully understand how a stall converter works. I'm thinking that the "stall speed" is when the converter fully engages to allow optimum power transfer to the drivetrain, but is that right? And what are the benefits?

 

 

 

If you have a racing cam with all the power on the top, a high stall will let you get the rpm up there before launch. They're not fun for street use or everyday use.

 

Addressing your 1st statement - first. After which, stall speed should make more sense to you. So, first things first.

How Does a Torque Converter Work?

 

People also don't realize that stall speed isn't a fixed thing. It's really a resistance to engine torque. If you have an engine that makes more torque, the stall speed of a given converter will be higher than with the same converter behind a lower torque engine.

 

Exactly as you have pointed out, the gentleman in the video Matt identified (above) speaks specifically to your statement:

 

Stall speed is what rpm the engine will get to with the brakes applied and wheels not turning with your foot matted on the gas pedal.
The converter rated stall speed is a ballpark number for on the shelf converters when behind a v8 with 350ft lb of torque.
The more torque your engine makes the higher the stall speed will be from it's rating, If your engine is lacking in torque at the rated stall speed, it may stall the engine rpm rise lower than the rating.
Stall speed is NOT the engine rpm that the transmission will start moving the car/truck.
For a performance car you want the stall speed in the meat of the torque curve, so when the transmission shifts it is still in the meat(power band) of the torque curve. Example if your engine makes 350 ft lp of torque from 3200 rpm to 5200 rpm. you want the stall to keep the engine speed between That Curve on a street car when matting the gas to the floor, under cruise driving it will act differently. A track car, you want it to shift at peak hp and drop the rpm back down to peak torque and rpm back to peak hp, on a street car that rpm power band is wider than a race only engine that will have a narrower power band.
Hope this helps.
Remember the converters rated stall is a ballpark based on a engine making 350 ft lb . The stall in your set up might end up higher or lower than the rating depending on engine power and where in the rpm it makes it.
A engine that makes 500hp at 6800 rpm, but almost nothing at 2500 rpm, will be a dog with a 2500 rpm stall converter, because the engine is starting to be loaded before it is in it's power curve.
In the 80's I had a car with a 1800-2000 stall converter but the engine because of the way to big intake ports(heads) and cam, didn't make power till 3600-3800 rpm. So the car would move out ok from idle when cruising and when getting on it, it go ok at stall speed to that 3600 rpm then go like a raped ape from 3600-7000 rpm.
A product of very mismatched parts.

 

"Stall speed is what rpm the engine will get to with the brakes applied and wheels not turning with your foot matted on the gas pedal."

So does this mean that once the torque converter reaches stall speed, the wheels will start turning?

 

The engine starts to stall out, hence the term. You would need to overpower the rear brakes to start turning the wheels.

 

So let's see if I can get this...
Sitting still, you apply the brakes in gear and press on the gas. The rpms climb until stall speed is reached. At that point, the engine starts to bug (or stall). Then that's when peak torque is being applied (if you have everything set up correctly).

Is that right? And if so, then ideally you would want to launch at stall speed, correct?

 

My take is you let off the brake before the engine begins to stall. The weight (mass) of the vehicle will keep it stationary for a moment, and the converter will flash to its stall RPM, then the full engine torque will be transmitted to the driveline.

Or you can use a line lock to apply the front brakes only and let the rears break loose at their traction limit.

 

Lots of misunderstanding here. I’ll try to clear up the confusion. I know far more about transmissions than converters, there is a lot of science involved.

First misconception: brake stall speed, and flash stall speed. Some people think if you stand on the brakes and torque up the engine, when the tires begins to spin hats the stall speed. Things like vehicle weight, brake capacity, tore grip, all affect when the tire breaks traction. Have a car with skinny hard tires on asphalt? The stall speed will be lower than the exact same car with sticky tires on concrete.

Let’s say you have a stout 455 in a 68 4 door Olds 98 with a 3.08 gear. On a sticky track, with sticky tires, the converter might flash to say 3000rpm. Do absolutely nothing to the engine, swap in a set of 4.10 gears, that same engine/converter/car might have a stall speed of 2700. Why? The mechanical advantage of the gears LOWERED the load the converter sees. Take the same drivetrain and swap it into a late 70s/early 80s Olds Starfire lightweight car, the converter might only go to 2500, once again due to the lower load.

Another misconception: high stall converter slip excessively during normal driving. Nope!! Since a converter is load sensitive, low throttle/low power=equals low stall speed. Case in point: the 10 inch Coan converter flashes to 4200 at the track, but is tight enough to daily drive. While it definitely isn’t as “tight” as a factory converter, it’s mild enough that I can and do drive it around town, on the interstates, without issue. A good quality converter won’t feel mushy at part throttle.

The Buick Grand National D5 factory converter will flash to 2800, no way would GM have done that if it was going to cause customer complaints or warranty costs.


There are a ton of things that influence stall speed in the converter itself, the angle of the find and blades of the pump and turbine, the angles on the stater, the diameter of the converter, the list goes on and on. And due to the cost of the tooling and manufacturing, almost nobody “makes” a converter from scratch. The performance converters you see in the Summit or JEGS catalog all started life in a production vehicle. Most of the 10 inch converters are built from smaller fwd cars. Since those converters were never designed to handle the torque of a big engine most of the manufacturer if cost is in upgrades. Converter companies machine and adapt sprags (watch the converter video to understand the function of the sprag) from larger converters to fit the smaller converters. They furnace braze the fins to ensure they don’t work loose from the pumps or turbines. They mix and match different pumps/turbines with different fin angles to design into the converter the desired stall speed. They machine or use different stators for the same reason. Then they add ballon plates or modify the covers to add strength, abd to adapt bolt patterns to fit engines the converter was never designed to fit behind.

I worked at Coan engineering for 3 years. Dave Coan and Steve Griner built the very first Powerglide transbrake in the early 70s, and really started the performance converter and transmission performance industry. Dave Coan isn’t the easiest person to deal with or to work for, but he is a genius when I comes to transmission stuff. I learned a lot working there.

 

While most of what you said is correct, I'll comment on this one. I don't think it rates a blind "Nope!!" as you cited a performance converter, and said you drove it around without issue. The switch pitch converter would not have existed if high stall converters did not slip at low speeds. I think you are taking today's tech and how its advances have expanded to allow wider areas of performance to mask the original point of high stall vs low stall in production applications when these cars were new. It's certainly not a misconception, even if it is mitigated by modern tech, which I am glad for, incidentally.

 

Or, place the front bumper up against a concrete wall - no need for brakes or a line lock.

 

I believe the video Matt put up by Hughes is like the most complete basic Introduction 101 regarding torque converter stall speeds (foot-brake, flash & static) I've ever viewed. The gentleman was spot-on, IMO.

 

No, in the context of "stall" is the engine rpm stops getting higher, the engine will run fine, just the converter will hold the engine from turning faster (higher rpm) unless you over power your rear brakes and the wheels are allowed to turn /spin.
If while trying to load the engine against the brakes like you posted above and the engine starts to run like crap, you have tune up issues, either in fueling or spark or both.
The word stall, doesn't mean the engine cuts off, or shuts off, it means the engine rpm rise is STALLED.
Sitting still, you apply the brakes in gear and press on the gas. The rpms climb until stall speed is reached. At that point, the engine rpm stops rising (or stalls) .
Where you have it stalled to at launch depends on gearing, as you want it to leave in the torque curve near or at peak, but when the transmission shifts into the next gear, you want the engine speed to stay in the meat of the torque curve, these depends on your gearing in the transmission, rear end, tire size, and vehicle speed at gear change. and the converter stall speed you installed.
This is why racers have custom converters built, to stall at the rpm they want with their package.
Example I have a th400 with a "off the shelf," (you order it from summit and they have it in stock hense the on the shelf) That is rated at 3500 rpm.
With a small block chevy 327 c.i.d. making about 300 hp and 310 ft lb (going by online calculators of drag race et and mph at the line) It stall at 3200 rpm, the same car, same converter, same transmission, gearing and tires, but a Fire breathing big block chevy 468 c.i.d. 13 to 1 compression, it stall at 3900 rpm. remember this converter is rated at 3500 rpm stall.

 

There is still a big following of the switch pitch converter today. There are some companies that build those converters.

The switch pitch was phased out due to the expense to manufacture, and because some people complained bout the “whiring” noise the transmission made while in the high stall mode. There was nothing wrong with the whiring noise, it just sounded different, people weren’t familiar with it. If something costs more to make, and creates warranty and complaints, that is one of the first things to be eliminated.

Converter design has come a long way since the switch pitch. People have figured out how to make the spread between low snd high stall far greater than it was originally designed.

Kinda irrelevant to this discussion, the biggest drawback to the switch pitch is it’s inherent weakness. The guys on the Buick forums seem to think the limit of the stator is around 550-600 hp,