Stall Speed 101

[tsleg]

Can somebody explain how a torque convertor works and the differences/benefits of the different stall speeds? I have a 55 Chevy sedan that I'm putting a 96 LT1 and 4L60E in. Motor has a LT4 HC, 1.6RR, ported heads, and Sanderson cast headers (not the best performance, but they fit and look cool). Also has 3:55 gears. Car is a driver with maybe a few trips down the strip to see what she can do.

Any advice?

[rskrause]

Very simplified, a TC has three major internal components. The turbine, stator, and impeller. They reside inside the case, which is filled with ATF. The crankshaft is connected to the case through the flexplate and the impeller is mounted to the case. When the crank turns the impeller rotates and generates fluid flow. The fluid flow is directed first against the turbine, which is coupled to the input shaft of the transmission. The force of the fluid flow causes the turbine to rotate. The fluid flows from the turbine to the stator (in the middle of the converter), which freely rotates in one direction due to a one way clutch. The stator plays a role in directing the fluid so that it efficiently interacts with the impeller and the turbine.

A characterisic of a TC is that it multiples torque, by a factor as much as 2-2.5 times. Maximum torque multiplication occurs at rest, as the vehicle just starts moving. As speed increases, the torque multiplication decreases. Once the impeller and turbine speeds approach each other, torque multiplication decreases to essentially zero. A non-lockup TC absorbs ~2-7% of total engine power due to the "slippage" that exists between the impeller and turbine. A lockup TC has clutches that link the impeller and turbine, thus allowing near 100% efficiency at high speeds.

Stall refers to the maximum speed a the motor can achieve against the converter when the tubine is locked and prevented from rotating. The rpm achieved (stall speed) will be a function of the engine torque and the converter design. In general, the higher the stall, the less efficient the converter is at high speed. So why would you want a high stall converter? To allow the engine to get into the meat of the power band quicker. A converter optimized for drag racing will have a stall speed much higher than a street converter. Allowing the engine to get into the powerband quickly more than compensates for the disadvantage of lower efficiency. On a street car, the penalty in gas mileage and heat generated by a high stall converter favors a lower stall. Also, a high stall can be annoying to drive on the street because it degrades throttle response. When you stab the gas, there is a delay while the engine revs before the car starts to accelerate. To illustrate how this works, there can be up to 0.5sec or more improvement in 1/4 miles times by switching to a high stall converter. The improvement all comes in the first part of the run, and interestingly, trap speeds may decrease due to the ineffciency of the higher stall converter at high speeds.

So for a street/strip car, converter stall is a compromise. For example, with my combo a 4,500-5,000rpm stall would likely give the best 1/4 mile times, but I have a converter which stalls at 3,000-3,500 to allow tolerable street driving. Stock conveters typically stall in the 1,500-2,000rpm range. Of course, what is tolerable to one person may not be to another and vice versa.

A race quality converter can be quite expensive. All of the parts including the case need to be strengthened to absorb the high loads. When a transbrake is used, for example, the converter has to absorb the full power of the engine with the turbine locked. This generates enormous heat and pressure. Obviously, you need a good tranny cooler if you use a high stall converter. Good race quality converters for a hi-po application start at the $900-1,000 range. True race converters do not usually have lockup clutches. Most sanctioning bodies don't allow them, so the manufacturers haven't perfected components able to handle the loads of racing.

To get a converter that stalls at the right speed for your aplication, you need to speak to the converter manufacturer. Labelling a converter as having a specific stall without reference to the motor it is behind is misleading and naive. That stock converter, which stalls at ~1,500rpm behind a stock motor, might stall at 4,000rpm behind a blown big block, if it didn't blow up first! For a stock LT1 with Hot cam, you probably want a stall in the 2,500-3,000rpm range for street/strip use. But do not buy an off the shelf converter thinking it will give you the advertised stall. Speak to the manufacturer first to be sure you are getting what you need.

Hope this helps.

Rich Krause

[tsleg]

Wow, great response. Just the info I was looking for. Thanks Rich!

[rskrause]

Originally posted by tsleg
Wow, great response. Just the info I was looking for. Thanks Rich!

You are welcome! I just re-read the post and thought it was so good that I would TTT it. I am a legend in my own mind

Rich Krause

[Laymaster]

wow that was an awesome read!

[94SLUG]

Originally posted by rskrause
You are welcome! I just re-read the post and thought it was so good that I would TTT it. I am a legend in my own mind

Rich Krause

You are a god

[97Z-M6]

Originally posted by 94SLUG
You are a god

hes worth keeping around, but god....come on now.

[rskrause]

Originally posted by 97Z-M6
hes worth keeping around, but god....come on now.

Well, I know for sure that I am not God (capital "G") but can't I be a god (small "g"). Reminds of how much I am looking forward to that new Jim Carrey movie "Bruce Almighty". That scene they show on the ads where his wife is saying "honey, do my breasts look bigger to you?" cracks me up. Just what I would do if I "had the power"

Rich Krause

[FyreLance]

this is an old post but thought I'd ttt it as I came across it in a search. Excellent post, Rich.

[rskrause]

Originally Posted by FyreLance

this is an old post but thought I'd ttt it as I came across it in a search. Excellent post, Rich.

Thanks!

Rich

[jsk96z28]

Thanks Rich, you just talked me out of a convertor upgrade, I think. Just one quick question, how does a higher stall effect traction? As it is I have to baby it off the line before nailing it down to limit tire spin and I have lca's, panhard bar and a spohn torque arm upgrades. This monster just has a ton of torque and its bone stock inside.

[rskrause]

Unless you are using sticky tires, a high stall converter in a car that already has enough torque to easily spin the tires is not going to benefit from a stall much higher than stock. It will just make it harder to control the car off the line. An LT1, with all that low end torque would be such an example.

A very mild increase in stall speed (say 500rpm) might improve your chances in impromptu traffic light drag races IF you are a skilled driver. By wheel braking up to ~1,500rpm but at part throttle you can improve your launch on street tires, and the stock converter is too tight to allow this. But to succeed at this takes some practice and with the differing surfaces encountered on the street is somewhat hit or miss. After the car starts moving you need to feed it just enough throttle but not too mcuh so as to spin the tires. It takes quite a bit of skill to do this. Once the car is rolling beyond 10-15mph there will be no benefit from a looser converter on a stock motor on the street. Anyone who says otherwise is WRONG, but you can believe what you wish. And most street "races" are from a roll anyway. An LS1 stands to benefit more with the higher rpm range it likes. But again, much looser than stock on the street with street tires is not a good idea and the benefit is not automatic by any means. My own car, with the last converter I had in it (4,200 true transbrake stall) was impossible to launch quickly on the street with the street tires. I would regularly have cars like Mini Coopers get a hole shot on me! Funny in a way. At the track with slick, launching off the brake with N2O coming on just after the tires started to roll was a blast but frankly it was a PITA on the street. Racing is a whole 'nother thing.

Rich

[jsk96z28]

Thanks Rich, I appreciate the response. Pretty much closed the door for me in thinking of increasing stall, I appreciate it, it would be a waste of money and I'd be worse off than stock. I don't run at the track that often, which seems to be the place I'd have some sort of increase over stock. I've pretty much got it down on how to pull away on the street, thanks to the G-Tech, it sure gives me the feedback I needed to see what worked and what didn't. Thanks again.

[rskrause]

3,000 is ok for an LS1, 2,500 for an LT1 but no more for wither an less would be my preference unless there was a lot of track use involved. But then again, tastes differ.

Rich

[frmula1]

excellant... man your good.


last question for now.


i heard in ls1 tech from one of the vendors, that the tci converter comes with larger bolts to connect it to the flexplate (obviously)... it was also said that 1 of the pilot holes in the flexplate has to be bored out with a drill so the larger bolt can fit. i did not have to do this with my lt1 stall, install and its making me extremely nervous. i don't consider myself having the know-how, comfort, or tools to drill on my flexplate. i was wondering if using 2 of the tci bolts and 1 stock bolt would work? or if the stock bolts would be too small to fit into the threads of the converter


would ya know?

thanks

Jared