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torque converter and stall converter info

18668 Views 0 Replies 1 Participant Last post by  So Money
Original article from Summit Racing's website.
Torque converters are way up there on the list of Dark and Mysterious Things along with sorcery, corporate accounting practices, and figuring out dial-ins. Because there are so many factors involved in choosing the right converter, many people end up getting the wrong one for their application, resulting in poor vehicle performance and broken transmissions.

But it doesn’t have to be this way. You too can learn how to choose the right torque converter, and this little primer will help you get started. We’re going to highlight the basics on things like stall speed, matching a converter to cam size and rear axle ratio, special considerations for nitrous and blowers, and more.

What a Converter Does
Without going into a bookful of theory, a torque converter is a hydraulic coupler between the engine and the transmission. It changes mechanical torque (engine torque) into hydraulic pressure before sending it back to the transmission. The converter also multiplies the torque at low speed or during periods of high engine load.

Inside a converter are an impeller, a stator, and a turbine, all surrounded by transmission fluid. The impeller rotates at engine crank speed, acting as a fluid pump. The turbine is the output member hooked to the transmission input shaft. The stator sits between the two, acting as a torque multiplier when impeller speed exceeds turbine speed. When the converter reaches its stall or lockup speed, the stator stops multiplying torque and the converter essentially acts as a fluid coupling. When the vehicle is coasting (no load), the converter directs torque back towards the engine, acting as a brake.

What is Stall Speed, Anyway?
The one thing people misunderstand the most about torque converters is stall speed. Many think that if they have a converter rated at 2,500 rpm, the car will rev up to that rpm and then take off. That’s not how it works. Stall speed is a function of engine rpm—the more torque your engine makes, the higher the rpm the converter will stall, or lock up at, and transfer that torque to the transmission.

There are two types of stall speed—foot brake stall and flash stall. Foot brake stall (also called true stall) is the maximum rpm that can be achieved with the transmission in gear, the brakes locked, and the engine at full throttle. The rpm reached before the vehicle moves is the true stall speed of the converter.

The problem with foot brake stall is that you will end up overpowering the brakes and suspension before you reach the converter’s stall speed. The only way to really measure true stall is by using a trans-brake. This will keep the vehicle from moving, allowing the converter to absorb 100 percent of the engine’s torque.

Race classes that do not allow trans-brakes are often called foot-brake classes. In this type of racing, the rpm obtained when the brakes are applied and the vehicle is not moving is considered to be foot brake stall. When the brakes are released, the engine goes to full throttle and “flashes” the converter.

That leads us to flash stall. It is the maximum engine rpm reached when you launch a vehicle at full throttle from a dead stop with no brakes applied. Flash stall is always lower than true stall because there is less load on the converter. It is dependent on many factors including vehicle weight, rear-axle gear ratios, and tire height. Anything that decreases the load on a converter (numerically higher gears, less engine torque, low vehicle weight) will cause the flash stall speed to be lower. Conversely, increasing torque, going to a numerically lower gear, or building a heavy car will increase flash stall speed.

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.

What Kind of Engine Are You Building?
Before you even crack open a torque converter catalog, you need to be realistic about the type of engine you are building. You need to match low and midrange engine torque to the converter’s stall speed. For example, if you are building a street small block that makes most of its torque around 2,500 to 3,000 rpm, don’t get a converter that stalls at 4,000. Not only will the car be hard to drive, the converter will constantly slip and will eventually be destroyed due to overheating. If you build a big block that makes its torque at 4,500 rpm or so, don’t expect it to be much fun on the street because of the high stall converter and big rear axle gear required to get the converter to lock up.

Camshaft selection is also critical to torque converter selection. On the street, many people will choose a cam that will put an engine’s rpm range 1,500 to 2,000 rpm higher than stock. Not only does that reduce bottom end torque, but a higher stall converter will be required to match the new torque peak. Many people will get the recommended converter, but neglect to upgrade the rear axle gear to compliment the higher stall speed (more on gear ratios and tire sizes in a minute).

Say you built a small block V8 with a 235 degree (at .050)/.488 inch lift cam and added a converter rated at 3,000 to 3,500 rpm. To make the combination work properly with a minimum of converter slippage, you will need a 4.10 or higher rear axle gear with 26 to 27 inch tall tires. Illustration One shows you approximate stall speeds based on engine type, cam duration, and rear axle gear ratio.

Nitrous or supercharging also affect converter selection. An engine with one of these power adders produces more torque than it would if it was normally aspirated. That means a nitrous or blown engine needs a converter with a lower stall speed range. Otherwise, the converter will stall too high, causing it to slip and eventually self-destruct due to the extra heat generated.

The Final Ratio
Rear axle gear ratio and tire diameter are very important to proper converter selection. You need to have enough of a final cruise rpm (rpm generated based on tire diameter and rear axle gear ratio) to allow the converter to function at full lockup at cruising speeds. If you don’t, the converter will constantly slip.

Illustration Two shows you the rpm generated at 60 miles per hour with various gear ratios and tire diameters. This will help you determine where your converter should stall. You can see how close your vehicle’s actual cruise rpm is to the chart by reading the tach at a steady 60 mph, find your gear ratio and tire diameter in the chart, then compare your rpm reading to the chart’s suggested rpm.

Size Does Matter
Torque converter size can also be confusing. Converters can range from 11 and 12 inches in diameter all the way down to 7 inches. Basically, the smaller the converter, the less fluid has to be pumped through it. Less fluid means less drag on the converter internals, which allows it to stall at higher speeds. That’s why you see 8, 9, and 10 inch converters listed for racing applications. In general, you want to avoid small converters on a typical street car due to the much higher stall speeds (usually 3,000 rpm and up).

If you are adding a lot of nitrous (over 200 horsepower), running high blower pressure (over 12 psi), or use a trans-brake, you will need a converter built to handle the extra stress. The extra torque generated can cause a converter to “balloon”, or expand in diameter. Look for a converter with a high quality stator assembly and an anti-ballooning plate to keep it from expanding.

The Fitting Room
A common complaint about aftermarket torque converters is fitment. Often, a new converter will not fit the transmission’s input shaft because it is built to much closer tolerances than OEM converters, so the hub-to-input shaft fit is tighter. Just because the new converter will not slip onto the input shaft doesn’t mean the converter is defective—just use a little extra effort.

A good way to check if a new converter will fit properly is to compare it to the stock converter you are taking out. Illustration Three shows the three critical dimensions: overall length (from engine mounting face to end of hub), hub slot depth, and hub slot inside diameter. Before you remove the old converter, check the dimension from the bellhousing to the front with the stock converter in place. This will help you position the new converter properly.

Other Considerations
A higher stall converter will place extra stress where it mates to the engine, so make sure you use quality converter bolts (like those made by ARP) and an SFI approved flexplate. SFI approved flexplates for Chevys are usually double drilled for small and large bolt patterns, eliminating the need to guess which bolt pattern you have.

Aftermarket torque converters are neutral balanced, designed for internally balanced engines. Most externally balanced engines have the balance weight on the flexplate, so this is no big deal. But on externally balanced Chryslers—340 and 360 small block and 440 big block—the factory put the balance weight on the torque converter. If you have one of these engines, make sure to get the appropriate flexplate counterweighted to match the engine balance. Most SFI approved Chrysler flexplates have this counterweight.

Don’t consider this to be the end-all and be-all on torque converters. The best way to get the perfect converter for your application is to talk with the tech guys at Summit or even better, directly with the companies that build the converters. Hopefully, this guide will help you ask your converter builder the right questions—and understand his answers.
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