Torque converters are way up there on the list of Dark and Mysterious Things. 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.
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. You'll learn about 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
Simply put, 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.
The insides of a converter consist of 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 device 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 most misunderstood aspect of torque converters is stall speed. Many people think if a converter is rated at 2,500 rpm, their 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 an 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 (or true stall) is the maximum engine rpm achieved from a complete stop with the transmission in gear, the brakes apllie,d and the engine at full throttle. The rpm reached just before the vehicle begins to move forward 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.
This brings us to flash stall. It is the maximum engine rpm reached when you do a full-throttle launch with the transmission in low gear and no brakes applied. Flash stall is always lower than foot (true) stall because there is less load on the converter. Changing the load on the converter can change the flash stall rating. Additional engine torque, a higher(numerically lower) rear axle gear, or adding vehicle weight will increase flash stall. Less torque, a lower (numerically higher) gear, and less weight will decrease flash stall.
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 think about the type of engine you have or are building. For the street, 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, don't expect it to be much fun on the street because of the high stall converter and big rear axle gear required to lock up the converter.
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, 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.
Nitrous oxide and superchargers also affect converter selection. An engine with a power adder 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.
The Final Ratio
Rear axle gear ratio and tire diameter are very important to proper converter selection. You need to have a final cruise rpm (rpm generated based on tire diameter and rear axle gear ratio) that allows the converter to function at full lockup at cruising speeds. If you don't, the converter will constantly slip.
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 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. 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. This will help you position the new converter properly.
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. 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 directly with the companies that build the converters. Hopefully, this guide will help you ask the right questions-and understand the answers.
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