stall convertor question?

trav_19 said:

I have always had sticks but my dad has a auto chevelle ss and its got an auto in it my question is to you guys he just put in a ......2800 stall i believe maybe 3000 not sure but my understanding of them are they are supposta act like they are in neutral till it reaches the rpm then they engauge....am i right?????? just tryin to help my dad out hes put tons of money in this and i think something wrong with his stall... it starts moving if you lift off the brake ??? could it be installed wrong???? whats the deal???

thanks for any help guys


Trav

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I know someone can explain it better but I can tell you I have a 3400 stall....if I let off of the brake the car rolls....If I give it " easy peddle" it pulls on".......now....if I "floor" it I'm gone.........the stall is meant to allow the rpms to get to where they need to be for hard launch's.....but you don't want to hard launch it every where you go.......
Someone help me out with the "splanation"

Essentially, it doesn't grip 100% until it reaches that RPM.
If you just lift your foot off of the pedal, it has the time to start getting it spinning, whereas when you mash it, it hardly has time to get you moving until it reaches that rpm. So, it really could have like a percentage of load for a certain rpm window. BAsically like fluid catching on paddels or fins.

Or another super crude way of thinking about it is that it's like slipping the clutch on a stick (like if you start in second gear). It spools up quicker because it's slipping until it gets up in the powerband and you fully engage the clutch. We used to do it with the small sports bikes with no low end grunt.

trav_19 said:

...? i dont get it...when i drove it if you accelerate slow it takes off like normal if you at a light and lift off the brake your going if you smash the pedal your roasting the tires???? what am i not getting your sayin easy throttle should be less than normal acceleration?

Click to expand...

Maybe if you rephrased the question in "normal" English we can understand what you are asking.
RC

trav_19 said:

...? i dont get it...when i drove it if you accelerate slow it takes off like normal if you at a light and lift off the brake your going if you smash the pedal your roasting the tires???? what am i not getting your sayin easy throttle should be less than normal acceleration?

Click to expand...

Stab at this I'll take one...

For example, say your power band starts at 3,000rpm and ends at 4,500rpm.

In an ideal performance world your car would never leave the powerband.

The stall allows the car to "slip" quickly to the start of the powerband and then it engages right as the engine hits its sweet spot.

HISSIN's example of slipping the clutch is very good.

To see an example of how critical being in the powerband is, in a stick Mustang, put it in 3rd at 2000RPM and got WOT... it will be really slow. Do the same at 2500 and 3000... it will pull progressively harder, sooner. You are getting into the powerband and can really feel it.

The stall does the same for auto guys... it lets the engine quickly build to it's powerband then fully engages when it gets there.

Wes

trav_19,

I spent years restoring old 60s and 70s muscle cars for a hobby. I’ll try to take a shot at explaining this.

The engines in those cars were really quite unsophisticated compared to today’s engines that use variable valve timing and computer controlled ignition. As such, it was very important to match the torque converter stall speed (in an auto tranny car) with the operating range of the cam shaft installed in the engine.

The “bigger” the cam shaft (meaning more overlap and more lift) the more top end horsepower you could generate with the engine. However, the bottom end suffered as a result (meaning poor idle quality and less low end torque). (Since there was no variable valve timing back in those years, you just had to pick either top end power or low end torque when you designed the engine.)

A mild streetable cam shaft in those years had an operating range of about 1,500 RPM to 5,000 RPM. A “big” cam shaft in a typical 60s muscle car might have an operating range of say 3,000 RPM to 6,5000 RPM. That means that the engine won’t really start producing good power until you pass 3,000 RPM. I a big heavy muscle car like a Chevelle, you need the engine to get up to that speed before the torque converter starts to act on the turbine to rotate the output shaft and put power to the wheels. (You might get away with a lower stall speed if the car was relatively light like a Plymouth Barracuda, for example.) A torque converter is really nothing more than a fluid pump! Understanding the internals of a torque converter would really help you get your mind around this whole issue.

Here’s a great article with pictures that explains how a torque converter works.

http://auto.howstuffworks.com/torque-converter.htm

All of this said, I think that the problem with your Dad’s car creeping at idle (unless the brake is applied) is not related to the stall speed of the torque converter. This was a typical problem in cars with “big” cam shafts in them in those years.

As I mentioned a big cam shaft causes poor idle quality. That caused many carburetor tuners in those days (me included) to bump up the idle speed screw on the carb to keep the engine from stalling at idle. Raising the idle speed to perhaps as much as 1,000 RPM or even higher, was not uncommon. I’d say your Dad’s got the idle speed set a bit too high and that’s why the car creeps. Since it’s really not a big problem, many choose to just let it like that. Others carb tuners back then resorted to drilling bypass holes in the carb butterfly valves to improve airflow a bit at idle. This was a typical tuner trick to deal with big cams.

The main question I have for your Dad is what are the specs of the cam he has installed in the engine, and does the stall speed rating of his torque converter match the operating range of the cam.

Hope some of that helps.

Pictures are worth a thousand words.... movies well, they are a bunch of pictures.

Here's an old vid I made to show the difference:

http://www.100mphclub.com/movies/edgestall.mov 6.1MB - Quicktime

I had to change the springs and shocks in the car to get traction after that. Thus my current times compared to those shown in the vid.

Paul...as usual, nice job with the Vid!
RC

Thanks.... to me a stall is harder to explain than it is to watch/experience.