torque converter question

[albertarose]

Hi Every one !

My question is about Torque converters ..my friend says stock stall speed is best for street ..I have a big block fairly stock with a turbo 400 .. he also claims the turbo 350 is better than the turbo 400 for dependability and strength.. he also claims that torque converters wear out ..any one have any info I can use here ? what do you think about manual shift kits are they worth it? I have never been able to get my turbo 400 to kick down ..I have tryed using a factory swith set up but it did not activate it ..so i have always driven it without kick down working but it still goes pretty good any opinions to share ? thanks in advance...Leon

[kwmech]

If your engine is reasonable stock, then yes, a stock convertor would be ok. Turbo 400 is the better trans for your big block. If the big block is fairly built
and you want a little more zip, then you can use a stock turbo 350 convertor which will bring the stall up to about 15-1600 range. Then again a lot of things are variable and somewhat unlimited depending upon your checkbook

[albertarose]

thanks for the info sounds good and I think the 350 pattern will bolt up to my flywheel(its drilled for both if I remember correctly so you think stock is the best bet for street ..yes my engine is fairly stock ..high 9s comp edelbroc performer mild rv crane cam oval port truck hrads ..hydr valves ..thanks again that was good info to have ..Leon

[mnunn454]

Leon,

I have a BBC, TH400 with stock converter. Like you I'm having problems getting mine to kick down with or without the switch in place. I figure I'll fix that when and if I ever go inside it.

Stock TH400 converter should be fine unless you want a lot more performance off the line. Since converter stall = slippage, and since there's no such thing as a free lunch, you'll get better economy out of the stock converter.

If performance is your aim, then personally, I wouldn't go with the marginal increase of a stock TH350 conveter. I'd go with a 2200 to 2500 stall TH400converter. JMO.

[albertarose]

Thanks that sounds fair enough ..with a big block you hardly need a kick down anyway ! but I have always wanted to see mine work ..it could be the module on the tranny itself not working.. ? A question about the 2200 converter ..how will the truck drive then ..harder shifts ? also does it hurt fuel economy on the highway too or just while shifting (stupid question I know) my truck has highway gears and does the speed limit at around 2200 rpm does that mean it will want to shift at that speed with the 2200 stall verter? Im kind of confused here..
One thing I have learned is that if you put it in first and drive it up to about 5500 the turbo 500 shifts itself and man does it launch !..mine chirps both tires hard and slaps your head back,.. but I know it cant be any good for the tranny,and driveline although mine is still working and I have done it a few dozen times ..(it is pretty fun ) this turbo 400 will puke toast one of these days ..my buddy said GM designed them to shift themselves from first like that for drunks with loud stereos!! LOL ..anyway thanks for any more feedback about TCs I am learning here from your experiance and i do appriciate it!

Leon

[mnunn454]

There's a whole science behind converters, so the best thing to do is call and talk to a few experts like B&M and TCI, etc.

2200 isn't the RPM that it shifts, it's the RPM that it stops slipping.

If your truck normally turns 2200 RPM going down the highway, you don't want a converter with an equal or higher stall than that because it'll be slipping most of the time.

Sounds like you should stay with your stock converter.

[albertarose]

yeah mabee your right !

[NF65Fleetside]

Hey guys, Im new to all this but recently rebuilt my motor in th 65 c-10. 350 shortblock with 283 heads, Edelbrock perf. int. mani., 4barrel carb, block hugger headers. TH350 with 2800 stall converter....is the stall too much for daily driving/a bit of fwy?...when test driving it, the fawker spun the wheels at any type of throttle pressure! fun...BUT took it on the fwy for a few and it just seems to rev like crazy! not sure if this combo is good for the highway or not.

[mnunn454]

NF65, what kind of rear end ratio do you have? How did it behave before the stall converter was added?

If it's a numerically low one (like a 3.07:1 or similar, then yes, you're probably slipping the converter and generating extra heat (that's bad for the tranny). If you have a numerically high, say 3.73, 4.10 or in that range, then you're probably experiencing is the normal engine revs, since the converter is effectively "locked up" at anything above 2500 RPM and therefore not slipping or generating extra heat.

If I were mostly performance oriented (which I am) ideally I'd want the converter locking up somewhere between (a) a little above where my cam starts coming alive and (b) about 10% lower RPMs than I normally drive on the highway. If that's somewhere around 2500 RPMs then you're all set. Just get bigger or stickier tires.

If driveability is more important than takeoff ability, then I'd swap back to a stock or nearly stock converter and enjoy the cruise.

Hope this helps.

Welcome also!

[1968SWBBigBlock]

What is stall speed and how do I determine what stall I have now?

Stall speed is a term used to describe the rpm at which the torque converter transfers the power from the engine to the transmission. There are different ways to test stall speed. "Foot brake stall" is when you press the brake pedal and then press the gas pedal. When the car doesn't go anymore or the tires start to spin that is "brake stall". (DO NOT TRY THIS! THIS MEASUREMENT IS MEANINGLESS AND POTENTIALLY DANGEROUS TO YOU AND THE TORQUE CONVERTER!) "Flash stall" is when you, from a dead stop, press the gas pedal to wide open throttle (wot). Watch your tach needle, you will see the needle jump to a certain rpm, that is "flash stall". A drag racing-style trans brake will give you closer to the true stall speed of a torque converter.

What is torque multiplication?

Torque multiplication is a term used to describe the amount of torque the torque converter will multiply. A torque converter is basically a fluid coupling between your engine and transmission. There is no direct mechanical link until the torque converter is in lockup mode. The pump assembly of the torque converter is directly linked to the crankshaft of the engine, however the turbine assembly of the torque converter is not. It is connected to the transmission input shaft. When the oil is pumped to move the turbine it is actually pushing the turbine at a higher rate. Each torque converter pump and design pushes the fluid differently, therefore giving you different rates of turbine speed . All of this only means that different torque converters will give you different torque multiplication rates simply by design. Our goal is to design the best torque converter using the highest multiplication rates for your application.

What does torque converter efficiency mean?

Once the torque converter does it's job multiplying torque for take off, the function of the torque converter is to be a link (sometimes called a fluid coupling)from the engine to the transmission. Keeping in mind that the pump assembly and the turbine assembly spin at different speeds, every torque converter has a different rate of slippage between the two. The amount of slippage is what determines efficiency. This is why auto manufacturers created lockup torque converters. Rather than having a torque converter that is let's say 92% efficient, they mechanically link the engine to the transmission, giving it a 1:1 ratio or creating 100% efficiency. This lowers the rpm of the engine, therefore increasing fuel efficiency.

Why do I need a higher stall speed converter?

Theoretically, for maximum acceleration the stall speed of the torque converter should match the peak torque rpm of the engine. A good explanation for the way it works is this: when you go outside jogging you start to breathe in and out faster and harder. Well the same thing goes for a performance engine. The engine is breathing in and out harder and faster, at a higher rpm. If a high performance engine makes power at a higher rpm, then a higher stall speed torque converter is what you need to put more power to the ground quicker
http://www.protorque.com/techi/ti_faq.htm

[1968SWBBigBlock]

High technology trickles down from cutting-edge industries, eventually making its way into the performance aftermarket, and helps to demystify the “black arts” of car crafting. Even stock EFI computers are no longer hallowed ground; there’s enough information on the Internet to reprogram ’em any way you like. Nevertheless, some black arts have never been totally understood by the masses.

For most people, ourselves included, selecting the right torque converter remains a mystery. Unless you’re bolstered with years of experience in the field, it’s near impossible to predict precisely how an individual torque converter will behave in your own car. Faced with a page-long listing of converters, what can you do to narrow down the right one for your street machine? When and why would you need upgraded Torrington bearings and furnace-brazed fins? Will an 11-inch 3,000-stall converter work the same as a 10-inch converter rated at the same stall speed? Throwing darts is a bad decision-making process; our advice is simple: call the manufacturer. In fact, we called five torque converter manufacturers, grilled their tech guys on this very subject, and asked each of them to recommend a converter for a typical car. Before making that call yourself, this article should help narrow down your converter choices so you don’t have to spend hours on the phone like we did.

Why A Converter?

There are two basic reasons why automatic transmissions need torque converters. First, they allow the engine to decouple from the rest of the drivetrain at slow speeds, eliminating the need for a clutch. Second, the slip that’s built into the converter not only lets the engine rev immediately into its powerband, it also allows this slipping action to multiply the engine’s torque. This explains the abnormally sky-high torque curves often generated by an auto trans car on a chassis dyno. So even though many of our favorite slush boxes only have two or three forward gears, the converter adds some effective gear multiplication to get the car out of the hole faster.

So what’s all the talk about stall speed? Stall speed boils down to the rpm at which the converter effectively locks the motor to the driveline and multiplies torque at a one-to-one ratio. Since the guts of a converter constitute a fluid coupling, the two halves of a conventional converter are never directly locked together like a clutch disc to a flywheel—unless, of course, it’s a modern lockup version with an integral friction clutch (see sidebar). A good converter should produce less than 10 percent slippage after the engine rpm has exceeded the stall speed, otherwise the constant, heat-building slippage will lead to wasted engine output and an early death for the trans.

The Hows And Whys of Stall

Converters are rated to stall within a certain rpm range. That’s not because the converter companies don’t know their product; it’s just that your individual engine characteristics and vehicle specs influence stall speed and make a precise number nearly impossible to predict. Even the term “stall speed” itself isn’t perfectly defined. Here are the three distinctions you’re likely to encounter.

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 wide open throttle (WOT).

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.

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.

When a converter company quotes you a stall speed, verify whether it’s flash stall or true stall. Most converter companies quoted us flash stall figures. Many factors determine where the converter will flash stall once it’s 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.

Of course, the power and torque curves of your motor will have a huge effect on stall speed. Generally speaking, engines that produce more low-end torque will bump the stall speed to a higher rpm. Conversely, the same converter will stall to a lower rpm behind a less torquey, higher-winding engine. Converter companies often designate the former as “big-block” and the latter “small-block.” When you buy a typical converter that’s rated at 2,000-2,500-rpm stall, that rating is meant to span a variety of motors with different power curves. Scott Miller of TCI points out his company’s 12-inch Saturday Night Special converters typically stall at 1,600-1,800 rpm behind a 325-375 lb-ft “small-block” and up to 2,000 rpm behind a 400-450 lb-ft “big-block.” Sure, they’ll stall even higher behind a torquier motor, but they’re intended for mild, conservatively cammed motors.


Miller explains that any converter’s size limits the amount of torque it can safely handle. Although larger-diameter converters have bigger parts, that’s not always a good thing. Larger fins mean the fluid can exert more bending forces and result in failure. In stock form, bigger converters also allow more torque multiplication and lower potential stall speed, but the internals can handle only so much torque before parts start breaking. Reducing the diameter of the converter reduces its ability to multiply torque, puts less stress on the fins, and raises the stall speed, so small converters are generally better suited to peaky high-performance engines with higher-winding powerbands.

Other internal tweaks, such as fin angle and stator design, can have enormous effects on the converter’s fluid coupling, which changes stall speed and torque multiplication. So it’s possible to build a “tight” 2,600-stall converter or a “loose” 4,000-stall converter in identical 10-inch housings, just by varying the internal design. Most internal mods are proprietary, but building a converter to achieve the right flash stall while maintaining around-town driveability takes a combination of proper stator design and fin angle in the correctly sized case. While it may be possible to build a 7,000-rpm 11-inch converter, it’d be horrendously inefficient—starting with an 8-inch converter housing would be a smarter choice.

Get Smart

No matter what your buddy tells you, or what advice you’re given by the speed shop counter guy, the converter companies recommend you use a converter suited to your specific application. Even TCI and B&M, which sell almost exclusively through off-the-shelf dealer networks, suggest you contact them directly for a recommendation before purchasing. “Choosing the right converter is like a visit to the doctor,” says Jim Hughes of Hughes Performance. “The more information you can give him, the better he’s able to correctly diagnose the problem.”

Be honest with yourself and decide how you want to use the car. Most of our own cars are 80 percent street driven, and 20 percent track abused. Given this information, the converter tech guy would suggest a converter that retains good street manners. If you tell the tech guy your street machine is “100 percent race car,” but you’re really intending to drive the thing on the highway, you’re only setting yourself up for disappointment.


After you’ve declared your intent for the car, you also need to tell the tech guy the weight of your vehicle, and the following information about your powertrain combo:

• Displacement

• Compression ratio

• Cam profile (duration at 0.050-inch lift, lobe separation angle)

• Carburetor or injector size

• Transmission model year and gear ratios

• Rearend gear ratio

Off The Shelf, Or Off The Wall?

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 ’71 Nova is an easy application to match up, there’s probably no converter ready to install behind a twin-turbo 400ci ’68 Caprice.

JW and Hughes sell fewer off- the-shelf converters because their clientele is comprised of a greater percentage of racers. John Winters, President of JW, told us that although his off-the-shelf converters work well for certain vehicles, he believes many gearheads shy away from custom units to save money. Art Carr builds every one of its converters to order, and technical sales guy Steve Lancaster doesn’t usually recommend anything less than a mid-level Heavy Duty Super Torque converter. Steve’s philosophy is, “You get what you pay for. Why back a $6,000 engine with a cheap converter?”

We’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 our specific cars have always been dynamite. We’re not sure the stars of last month’s “Street Car Shootout” would have run the times they did were it not for their custom 9-inch converters we spec’d from Art Carr, although the $900 list price hurt us a little.


Car Craft Tech Article