torque converter 101

[Wicked Wolf Auto]

Please school me on torque converters what exactly is their function and how do I determine wich one I need?

 

[rjfguitar]

A torque coonverter is the hydraulic connection between the engine and rest of the transmission. There are fins that look like a turbo fan on an air plane. There are two sets of fins inside the converter, the outside fins are connected to the shell of the converter which is bolted to the flexplate and engine. The engine spins the shell and outer fins. (this is whree my knowledge gets shakey) The second smaller fins are inside the stator and the transmissions input shaft is slid into the stator. The fluid is spun by the outer fins and I think is then pushed through the stator spinning it which then turns the input shaft. I think that is the basics of it, I may be off here and there though.

 

[colbystephens]

i think you're pretty much right on - though this is where my knowledge stops too. i just want to clarify, assuming i understand it correctly, the motor doesn't directly spin the tranny input shaft, but rather the fluid spinning the stator, thus the motor can remain running w/ out having a clutch in as in a manual trans.

 

[Wicked Wolf Auto]

so how do I determine the stall speed I need? and what is stall speed?

 

[rjfguitar]

so how do I determine the stall speed I need? and what is stall speed?

I think it has to do with the angle of the fins. The different angles make it where the oil will start to spin the stator, I think. Meaning. The fins can be shaped to require lots of oil flow before they start to spin the input shaft, raising the stall because it takes more RPM's to push that much oil through it.

You don't really need to know how it works EXACTLY to figure out what you need.

What are you running for an engine? If it's stock, you want stock stall. An RV cam, intake, headers(your standard SBC upgrades) and you would probably want around 500rpms above stock. The more aggressive the engine, the more stall you want over stock. I have seen them go as high as 3,500RPM's over stock for racing.

 

[Mastiff]

Would it be true that an off-road machine would generally want a low stall torque converter so you can crawl at low RPM? A high stall converter would require you to rev it up to get going? I'm not sure what the downside is to a low stall converter as long as the engine doesn't die at idle with the brake held.

 

[colbystephens]

Would it be true that an off-road machine would generally want a low stall torque converter so you can crawl at low RPM? A high stall converter would require you to rev it up to get going? I'm not sure what the downside is to a low stall converter as long as the engine doesn't die at idle with the brake held.

inspite of the fact that i don't really know what i'm talking about, i'm going to take a stab at this one. it would seem to me that you would not want a lower-than-stock stall b/c you want to stay in the lowest gear for increased torque in most offroad situations. course, it would seem to me you wouldn't want a high stall such that you don't have to get your rpm's super high (again assuming stock motor) to finally get into the next gear. someone correct me.

 

[muscle]

In my understanding and the way my tranny guy described it..

its like you take two room fans and put them facing eachother. when you turn one on, the forced air will cause the other fan to spin. Same theory in a TC except the air is hydraulic fluid.

I believe the stall for a th350 is 1200 rpm and 1100 rpm for a th400.

 

[LVJimmy]

Stall speed has a lot to due with the fins but mostly with the convertor diameter. The rated stall speed is just a comparison between convertors it doesn't mean the converter will stall at that RPM. A 2500 stall might stall at 1800 behind a 454 built for low end torque likewise it could stall at 3500 rpm behing a high reving 4 cyl. It used to be that you could get a cheap drag racing stall convertor by pulling one out of a Chevy Vega. Your best bet is to talk to the convertor builder NOT THE TRANNY BUILDER

 

[ntsqd]

My understanding: think of a donut. Now take a wide slice out of the middle, from the center to the center of the 'ring'. What's left of the donut is the inside of the tc. That part cut out is the Stator. The fluid travels in a circular swirling path around the 'donut' while going around with the crank's rotation.

The angle of the fins also has to do with the torque multiplication that the tc is capable of.

Stall RPM depends on the torque the engine can produce. If you can manually put the trans in direct (not "Drive") while in high range and your brakes are good enough you can floor both the brakes and the throttle. The max RPM will be the tc's stall speed. The problem is that few have manual valve bodies so the trans goes to first & you roast the tires.

 

[goldwing2000]

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

 

[rjfguitar]

Would it be true that an off-road machine would generally want a low stall torque converter so you can crawl at low RPM? A high stall converter would require you to rev it up to get going? I'm not sure what the downside is to a low stall converter as long as the engine doesn't die at idle with the brake held.

Yes, generally offroaders try to stary with a conservative stall for just the reason you mentioned, depending on the engine and application.

You match your stall speed to where the engine makes power. The drawback of a low stall converter is that if you have a rev happy 377SBC that makes peak power very high in it's RPM band because it has a large cam and heads, the converter will want to engage to soon and not let it reach it's powerband and make it a bogged out dog.

It goes the other way too, I just had the 47RE auto in my Dodge Ram diesel done. The stock Dodge converter is set a little too high, it wants to over rev the engine and let it go ABOVE it's powerband. I used a converter that actually has a stall 500rpm's below stock. I did this because of the massive amounts of torque the Cummins makes around 1,600RPM's. The quicker the stator grabs the better with the Cummins.

Hopefully that will help a little.

Here is another thing I just thought of....ever looked through a Camshaft catolog or even in Summit Racing. Ever noticed how they say "this cam works good from idle to 5,000RPM's." This cam works well from 2,500RPM's to 6,000RPM's" blah blah. The bigger your cam, the higher your power band will probably be, and with a higher power band you need to bring the stall on your converter up to match that power band.

 

[wicked jester]

The bit about the 2 fans faceing each other is about the easiest "visual" for how they work. The stall speed is based upon both size and fin angle. The same converter will stall at different speeds based upon the torque the engine is makeing. ie. bbc vs sbc The more torque that is being made the higer the stall speed to a certain point. Your engine combo will be the determing factor on what converter you will need. The higher the rpm that you start to make power at the higher the stall speed needed. For a stock or slightly modded engine you can use a stock converter or one with a few hundred more rpm and be happy. Just keep in mind that the higher the stall speed the more heat that gets produced, then you need to add an aux. trans cooler. Just dont go getting carried away looking for a high stall converter thinknig it will be better, all that will do is make it miserable to drive.