||08-12-2009 06:01 PM
This is from a PM I sent to Dragoneye a little over a year ago modified a tad
All torque converters have a stall speed. however 99.9% of stock torque converters will never be spoken of as a "stall converter". there is no difference in the way a stock converter works and a "stall converter" honestly, a "stall converter" is an oxymoron. all torque converters stall, thats how they work.
Lets start at the beginning of torque converters. The torque converter is nothing more than a fluid coupler that connects the engine crankshaft to the drive shaft using hydraulic fluid (transmission fluid). There is no direct mechanical connection between the crankshaft and the drive shaft in an automatic transmission. This fluid coupling method allows the transmission to shift gears automatically and without breaking any mechanical connection. It does this by monitoring transmission fluid pressures and/or engine vacuum (depending on the transmission). The rpm that the converter stalls at can be varied by any number of methods. Chief among them is the diameter of the converter or the pitch of the stator vanes. A converter that stalls at a higher rpm than the typical "stock" unit could be called a high stall converter. Stall speed varies with engine torque, so a 4000 RPM converter behind a 502 might provide a 1700 RPM stall behind a 283 (which is one thing people dont take into account when they purchase a stall, a "universal" 3000rpm stall will not stall at that speed for every motor).
things to look at:
True Stall: The rpm the engine cannot exceed when the driveline is locked. The most accurate way to determine true stall is by locking First gear and Reverse with a transbrake and observing engine rpm at WOT. (however, most of us dont have a reverse manual valve body and a transbrake to practice this, so we have to rely on the manufacturer's recomendation).
Flash Stall: The rpm the engine “flashes” to when launched from rest at WOT. A converter will often briefly flash to a higher rpm than its true stall speed. This is due to the fact that when you go to WOT, the transmission builds pressure faster than when you gradually "stall up". and basically "overstalls" the converter before you launch.
Brake Stall: The rpm the engine cannot exceed with the brakes locked and the driveshaft not spinning. Brake stall isn’t usually an accurate measuring tool since the engine often overpowers the wheels before the true stall speed is reached.
now, those are all things about how converters work...now why would you want one that stalls higher than stock?
Advantages of a higher stall converter include: More torque multiplication which means better acceleration. An engine that is set up with a high rpm cam produces poor low speed torque and usually idles at a higher than normal speed. A higher stall converter slips more at idle speeds and eliminates idle creep along with allowing the engine to wind up into its power band when launching. A high stall converter is usually smaller and lighter, producing less flywheel effect and faster rpm gain. In short, your engine will idle easier and rev quicker.
Disadvantages include heat. High stall converters are less efficient at transmitting power and the wasted energy turns up as heat. You can expect your fuel mileage to go down especially in town.
Another factor that gets confusing is converter slip. Slip is basically a measure of converter efficiency. Due to the difference in rotating speeds between the impeller and the turbine, there is usually a five to 10 percent efficiency loss at cruising speeds for non-lockup converters. Because a converter gradually slips, or creeps up, to full stall/lockup rpm, the higher the stall speed, the more slippage you get. On a street-driven vehicle, that can lead to poor idle and low end performance, worse gas mileage, and most importantly, greater heat buildup-the number one killer of converters and transmissions. If you do run a high stall converter, a good transmission cooler is a must.
Unless you have years of experience in the field, it’s nearly impossible to predict precisely how an individual torque converter will behave in your own car. Things like upgraded Torrington bearings and furnace-brazed fins come to play, an 11-inch 3,000 converter vs a 10-inch converter and which is better for your application.
and cus im lazy, here's a link with more info on how converters work, inside and out
If you run an oddball combination of parts, don’t be surprised if a tech guy recommends a custom-built converter. Although companies like TCI and B&M believe an off-the-shelf converter will suit 90 percent of their customers, remember that even these ready-to-ship converters have been designed for common yet specific engine/vehicle applications. But while a mild 350 in a ’78 Camaro is an easy application to match up, there’s probably no converter ready to install behind a twin-turbo 400ci ’68 Caprice.
most manufacturers dont usually recommend anything less than a mid-level Heavy Duty Super Torque converter. the best philosophy is, “You get what you pay for. Why back a $16,000 engine with a cheap converter?”
i've used off-the-shelf converters in a myriad of cars; sometimes they work wonderfully, and sometimes they’re less than great. Custom converters built for specific cars have always been dynamite. although the high list price hurts a little, its worth paying to know that what you are putting into the car is going to last thru the abuse.
When a converter company quotes you a stall speed, verify whether it’s flash stall or true stall. Most converter companies quote flash stall figures. Many factors determine where the converter will flash stall once its installed in your car. Heavy cars with tall (numerically low) gears and large-diameter tires offer more resistance to forward motion, so the converter will stall at a higher rpm than it would in a light car with steep gears and short tires. The easier the motor can accelerate the vehicle, the lower the converter will need to stall to get the car moving.
These are primarily used with nitrous setups as the more horsepower you have, the more cover pump pressure there is. With enough pressure your converter will balloon. Pump pressure drives the impeller hub into your transmission pump causing the pump to fail. It also creates bearing endplay creating premature converter failure. When the converter balloons, it loses the dynamic flow through your converter, resulting in loss of efficiency and horsepower. Anti Ballooning flanged hubs keep the converter from expanding outward. The converter will actually become longer from front to rear when it balloons.
some more stuff
Disclaimer: This is from a "hobbyist" point of view, and although I do my own work and have personal experience I am by no means a professional mechanic. I tried to keep everything as factual as possible, but like any piece of writing some opinion my always invoulunterily be snuck in. To be blunt: I am not responsible if you choose the totally wrong stall and your car is a turd!
I am going to start from the top and explain everything as best as I can.
What is a Torque Converter, or "Stall"?
The proper name is a "Torque Converter", but I will refer to it as simply a "stall" for simplicity and popularities sake. The stall converter is the piece inside an automatic transmission that not only transfers, but multiplies the output of the engine and sends it out the transmission. It is basically a fluid coupler, which can engage and disengage the connection between your engine and transmission. Stalls are rated by their "Stall Rating", which can be a bit confusing and cannot always be compared. This is because while some companies rate their stalls by their "flash stall"(I'll get into that later...), others are rated by their significantly lower "brake stall"(more on that later too!). To add more confusion to the mix, how a stall acts in a car will depend a lot on the cars torque peak, power curves, and even how sticky your tires are. A stall rated as a 2800 for a stocker can stall much higher on a higly modified car!
Cam surge??? One reason to get a stall...
Cam surge is the jumping variation in idle that most cam'd cars have. With a tight converter, naturally the car will buck forward everytime the idle twitches, since the connection between the engine and driveshaft is fully engaged at lower rpms. This is not as serious an issue for a manual car, since with the clutch held down the connection between the engine and drivetrain is completely seperated. You can rev the car at will and it will not move. With an automatic though, the connection is still there in varying degrees as long as you are in gear. With a larger(more loose) converter the idle will still jump due to the cam, but the car will not buck and move as much because of the stall "eating" the low rpm changes by allowing the connection between the engine and drivetrain to slip at lower rpms. The rpms twitch, but it will not be transfered to the drive wheels and push against your brakes http://www.z28.com/forum/images/smilies/smile.gif
Brake Stalling! More RPMs off the line!
Also, when you hold the brake and then apply gas to get the rpms up from a standstill, a stalled car will brake stall much higher then a stocker. You can bring it up to higher rpms before your tires start to brake loose and spin. It has been a while, but I believe my stock stall would start to spin the tires at around 1.5k rpms if I held the brake. With a bigger stall, if I hold the brake firmly and slowly give it gas I can bring the rpms up to 2800-3k rpms. Usually you can bring the rpms up to a bit lower then the stall rating, but brake stall is highly dependent on engine torque and tires used. This is very helpful on the street, where full on "flash" launches will just result in tire spin. You can brake stall to where your tires still have traction, then let off the brake while gradually leaning into the throttle to prevent wheelspin.
...but we want SPEED! A stalls true potential...
The real benefit though is the amazing launching and shifting it gives an automatic car. When properly matched to the cam and heads, when you punch the gas you will instantly be in your cars power band - and equally importantly stay there. The drop in rpm's during every upshift at WOT will dissapear, keeping you right where you want to be for the whole run - no rowing through the gears and climbing/falling rpms like a manual. Partial throttle drivability will barely be affected with a good quality stall, and gas mileage should not change much assuming you do not have a heavy foot. A full .5 off the 1/4 is common with a good quality 3k+ stall.
How will all that all effect ME? A stall in action...
As a real life example, I've ran a 3200 level two "Performance" stall from fuddle racing. Unlike many people think of larger stalls that you cannot move until the stall rating, it still pulls against the brakes and I can still idle crawl at a lumpy 600rpm idle and do not need much throttle to get going on a flat road. Uphill you can def feel the difference though, and if it is a steep hill the car can actually roll backwards in gear if you let go of the brake - just like holding the clutch in on a manual, since the connection between the engine and drivetrain is not strong enough to overcome the weight of the car pulling backwards. If I accelerate normally the car will stay at low rpms rising pretty typically and dropping rpms between gears like a stock car. At 40mph most stalls will lockup(no more slipping), and it will act totally normal and have no difference in gas consumption during lockup driving. If I punch the gas from a stand still though(theoretically of course - I have zero traction on the street ) the rpms instantly flash to ~3400rpms and climb very quickly from there. Flash stall is usually a bit higher then rated stall, and always higher then foot brake stall - but again the stall rating you get from a particular stall will vary on your power level and torque curve(This is why a stall should be custom ordered for your setup and horsepower levels, since more hp and tq will raise a stalls rating in your car.) I do not have to worry about chugging past 1k-3k rpms before reaching peak acceleration, since I start right up in my power band. Then, if I am still WOT, when the car upshifts the rpm's will barely drop any - keeping me in my powerband still but in a higher gear. Your rpms will not drop down as much and make you start the rpm climb all over again.
So just grab one and go? Not all Stalls are created equal...
Notice how much I use "good quality" to describe a stall. A cheapo TCI, B&M, or budget restalled stocker will do nothing but slip like crazy, feel extremely loose on the street, make a ton of heat, and kill your ET's and your rwhp. Nobody I have ever seen that has actually used one has had a complaint that I know of, even people who are coming from other big brand stalls. I have driven cars with both TCI's and Edge stalls before, and the TCI felt extremely loose for its rating but the edge seemed at least on par with the fuddle, but is around 50 bucks more. Yank is widely regarded as the "top dog", but costs quite a bit. My shop had recommended Yank, Vigi, or Fuddle stalls when I was shopping around for one(in that order). They did not say Edge, but they are also great track proven stalls. Stick with those four for most street/strip stalls and you will not be dissapointed! There are then the ultra high performance race orientated stalls like neil chance and such, but that is getting into the high $$$ not very street friendly stuff - springless WOT lockup is INTENSE!
What's the difference???
Stalls have a lot of features that cannot be overlooked when selecting. A few of them are:
Like I said, some manufacturers rate this differently then others. It id best to get in touch with them and ask if it is flash or brake stall, and at what hp/tq level it is rated at.
Best bet? Get a dyno run in! Most shops only charge 65-65 bucks for multiple runs with A/F, and it really is needed to choose the proper stall. Again, its 300-600 below peak torque for a DD or car that sees a lot of street duty, and up to 200rpm below peak torque for more "racey" applications.
To put it in short words, smaller is better to a point. This is helpful in different ways. A smaller stall of the same rating will feel "tighter" on the street, and be mroe fun to put around town in. There is less fluid needed to engage it(its a fluid coupler!), which means less drag. Also, they are a lot lighter - which means not only weight off the car but less rotating mass hanging off your flywheel. This means quicker, more crisp rpms! Be careful though - in general, the smaller the stall, the higher the stall rating will be. There is no point in overshooting your peak power rpms, and the proper stall for a street car should be anywhere from 400-600 rpm below peak torque, based on driver preferance and what you consider "streetable". In a more serious racing application you can go up to 200rpm below peak torque to really stay in your power band.
This information can be harder to find. The size and material of the clutch in a stall is very important! Some stalls may give their "slipping" by just having a cheap clutch that cannot hold up to much power. This not only adds a ton of drivetrain loss, but makes a ton of heat and kills your times.
Neccesity for nitrous users. Nitrous hits HARD, and can wreak havoc on your drivetrain and transmission. You need a stall that will not expand under the stress!
Stall Torque Ratio. A very misleading way stalls are rated, this is supposedly a rating of the multiplication ability of the stall. Different companies use different methods to find this number, but most range from 1.9-~2.2. It is often "guesstimated", and no street converter will EVER have an STR much over 2.2...some cheapos are advertised as 2.5 or even 2.7, but that amount of STR is reserved for all but the stoutest springless racing converters. Pure BS, careful here! This is why brand choice is so critical in choosing a stall.
Most modern stall offer this, and IMO is a necessity for a street driven car. At a certain speed, the computer commands the transmission to lock the converter. This eliminates slip, and greatly improves gas mileage. Unlocked stalls will dyno lower due to more slipping, but can run quicker times in the right setup by allowing higher average rpms during a run.
...is that all? What else is needed?
You cannot just throw in a high stall and call it a day! The stall works its magic by intentionally slipping! This adds a lot of heat to the transmission fluid, and can cook one after too much use - especially stop/go traffic and unlocked cruising speeds. A transmission cooler is mandatory, and NO the factory crap is not enough! Luckily, it is a very easy install thanks to the factory lines being easily accessable, and you can snag a huge GVW RV rated universal transmission cooler for a couple bucks at most auto part stores. Get the biggest one you can get your hands on - mine was like 30 bucks and rated for something crazy like 25,000 GVW. A good transmission cooler is essential to extend the life of your transmission! The internet is your friend - there are a ton of guides on installing one.
Of course there is a lot more to stalls - they will literally make one of the biggest differences in an autos ET when changed. That is the basics though, and should help someone in understanding their benefits and why they are needed in a cammed car.
Any questions/spelling errors/etc dont hesitate to PM me!
and some more reading...