rskrause

There have been a number of posts over time related to stall speed, and I wanted to clarify a couple of points with respect to this seemingly simple concept. For the purposes of this discussion, I am using "stall speed" to refer to "true" stall speed. What you would see with the tranny input shaft locked, as in a transbrake equipped unit. However, the same principles apply to brake stall, flash stall, etc.

The main point I am trying to clarify is that stall speed is not just a function of the converter. It is also a function of engine torque. This can be relatively easily described by defining the "K" factor. K is simply the constant in the equation K = rpm/sqrt{torque}. The equation describes the observed behavior of the converter behind a specific engine. What this allows us to do is determine what the stall speed of a given converter will be if we put it behind a different engine.

For example, if a motor has 400 ft-lbs. of torque and stalls a particular converter at 3,000rpm, K = 3,000/sqrt{400} = 150. Since we know K = 150, we can predict the new stall speed if torque is increased to say 500 ft-lbs by rearranging the equation to rpm = K*sqrt{torque}. In this case, the new stall would be rpm = 150*sqrt{500} = 3350. This formula isn't perfect. It won't work if the engines have wildly different torque curve, for example. And it won't tell if a particular converter will hold togther under markedly increased torque. But it gives a decent ballpark estimate and serves to illustrate a basic aspect of torque converter funtion.

One peripherally related question that may have occurred to you is "why are high stall converters more expensive". In general, it's because all else being equal, a higher stall converter will be physically smaller. Because it has to absorb the same energy with smaller components, the components must be stronger, which is to say more expensive!

The forces in a converter are quite high, and they generate a lot of heat. When a converter is stalled, all of the hp the motor produces is being turned into heat. Since a converter is always "slipping", even when cruising, a lot of heat is generated. And since high stall converters "slip" more under acceleration and at lower speeds, they generate a lot more heat than tighter converters. That's why you must have a trans cooler if you run a high stall converter, especially if you are racing and stalling the converter. Another way to keep heat under control is to use a larger capacity pan. The fabricated pans look cool, but are quite expensive. And compared to cast aluminum, they do not reinforce the transmission case, which can contribute to longevity under extreme use. So I recommend a deep, cast aluminum pan for performance users. Heat control is not only important for longevity. If you are a bracket racer, keeping tranny temps relatively constant will contribute to consistency.

Rich

Cmr0z28

Great post Rich!

rskrause

Thanks for the positive feedback!

Rich

Capn Pete

Makes sense to me . I never knew about the "K" formula ... very interesting .

LWillmann

Great post, very informative!

faded93bird

Dammitttttt!!!!!

Thats just great, you found a real life use for Algebra. Now what are all the students supposed to tell their teachers?

Maybe you should teach these guys what Total power under the curve means next (I have a program to do it the easy way). But that is calculus and we can use that everyday

Good post
Faded

00cls1camaross

alright i just read and i understand that the stall speed is relative to the torque being applied. so when we say the car has a torque converter that has a stall at 3000 what are we saying? is it something to do with the lockup or unlocking of the converter?

rskrause

No, as I posted in the other thread, when the converter is locked up, it is as if it isn't there. The speed of the crankshaft and tranny input shaft will be the same. No slip at all. If you have a lockup converter, once it is locked up the stall seed becomes irrelevant.

Rich

DarkHorse

Rich - Nice post and thanks for the information.

A question - what footbrake stall do you normally see from, for instance, a 3000 stall 2.5 str - roughly when would you expect the car to start rolling thru the brakes?

The reason I ask is because I can't seem to get my car to footbrake any higher than say 2000-2100rpm's before it starts to roll thru the front brakes

rskrause

That's what you would expect. The TC transmits torque even at idle, that's why the car will creep forward if the brakes are not applied. The more torque going in, the more going out even before the (true) stall speed is reached. At some point, the torque going out of the converter into the tranny (and multipled by the first gear ratio, the rear end ratio, and the converter multiplication) will exceed the holding power of the rear brakes and the rear wheels will spin and.or the car push the locked fronts forward.

How much torque are we talking about? Well, if the engine puts out 350ft/lbs at 2,500rpm, the overall driveline ratio is 10:1 in 1st gear, and the converter is multiplying torque by a factor of 2, there will be ~7,200 ft.lbs to the wheels. If you have a high stall converter and a transbrake, with a hi-torque motor and short gears, the total can easily be triple that or more. Obviously, this is a hell of a lot more torque than the rear brakes can absorb and it's one of the reasons that drag race automatics have a transbrake.

Rich

tfs95z28

So if i am reading your post correctly rskrause, a transbrake should eliminate this "roll through" that Dark Horse is talking about? Because i have the same problem.

I was either going to do an upgraded brake system to the LS1 setup since their calipers are two piston or a line lock. If the line lock is all i will need, then i will definitely do that instead because it is a heck of a lot less expensive.

TQdrivenws6

A line lock and a trans brake are two very different things.

A line lock is just a solenoid that (when activated) closes to hold pressure to your front brakes. This allows you to release the brake pedal to free up the rear brakes from excessive wear during a burnout. A line lock can also be used to prevent movement on the starting line, or to preload the driveline in a manual transmission car by slipping the clutch.

A trans brake uses the internal components of the transmission to eliminate the output shaft from turning. How this works is that an electrical solenoid locks the trans in forward and reverse at the same time on the starting line, the gas is floored when a button controlling the solenoid is released, so is reverse- and the car will launch forward instantaneously.

rskrause

FWIW, transbrakes do release quickly, but far from instantaneously. For brake equipped cars running a pro tree (0.4 sec between yellow and green), most use special pro tree brakes and tranny mods to speed up transbrake release. A full tree brake designed for a bracket racer (0.5 sec between the last yellow and green) will not release quickly enough to cut a good light on a pro tree. The rpo tree brake iincludes valve body and case modifications to allow the pressure on the reverse piston to drop more quickly when the brake is released.

Rich

tfs95z28

thanks TQdrivenWS6. But another question, if all a line lock is is a solenoid that closes the front brakes, isnt it only going to hold as good as your front brakes can handle? I mean can it hold any better than if you are just standing on the brake pedal yourself? And if so, wont you be spinning the rear wheels the whole time while getting ready to launch since it only locks up the fronts?

Im new to this, and really wanna solve this roll through at the tree problem i have been having.

TQdrivenws6

All it does is hold your front brakes on as hard as you can from the brake pedal. When you push the brake pedal in and activate the line lock, it prevents any brake fluid from returning from the front brakes, thus locking the front brakes on. So it is very handy for doing a burnout. If you give it so much gas that the rear tires start spinning, you probably are trying to launch too hard for your setup.

As far as a roll through issue, I know most bolt-on to cam only guys have the best luck flashing the converter, a.k.a. leaving off idle. It may not seem logical, but taking advantage of flashing the converter can really get you out of the hole fast.