Choosing Torque Converters For Street And Strip Applications
Because there are so many factors involved in choosing torque converters, many people end up getting the wrong one for their application, resulting in poor performance. Some of those factors to consider are stall speed, matching a converter to cam size and rear axle ratio, etc. A torque converter is a hydraulic coupler between the engine and the transmission. It changes 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 torque 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, the converter directs torque back towards the engine. There are two types of stall, speed-foot brake stall and flash stall. Foot brake stall is the maximum engine rpm achieved from a complete stop with the transmission in gear, the brakes applied and the engine at full throttle. The rpm reached just before the vehicle begins to move forward is the true stall speed of the torque 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 transmission brake. This will keep the car from moving, allowing the converter to absorb 100 percent of the engine's torque. Foot-brake stall is the rpm obtained when the brakes are applied and the vehicle is not moving. When the brakes are released, the engine goes to full throttle and “flashes" the converter, and that's called flash Stall. Flash stall is the maximum engine rpm reached when you do a full-throttle launch with the transmission in low gear with no brakes applied. Flash stall is always lower than foot 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 rear axle gear, or adding vehicle weight will increase flash stall. Less torque, a lower racing 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, which is the number one killer of converters and trannys. If you do run a high stall converter, a good transmission cooler is a must. For the street, you need to match low and mid-range engine torque to the converter's stall speed. So, if your engine makes most of its torque around 2500 to 3000 rpm, don't get a converter that stalls at 4 Grand. The car will not only be hard to handle, but the converter will constantly slip and will eventually be destroyed due to overheating. If you build a big block that makes its torque at 4500 rpm, it won't be much fun on the street because of the high stall converter and big rear axle gear required to lock up the converter. For instance, if you build a high performance small block, and put in a cam with a 235 degree duration and a .488 inch lift and then added a converter rated at 3,000 to 3,500 rpm, to make this combination work properly with a minimum of converter slippage, you will need a 4.10 or higher gear ratio with 26 to 27 inch tall tires. Gear ratio and tire diameter are very important to proper converter selection. You need to have a final cruise rpm that allows the converter to function at full lockup at cruising speeds. If you don't, the converter will constantly slip. Nitrous or blown engines produce more torque, so they need 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. If you are adding a lot of nitrous, running high blower pressure, 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. A higher stall converter will place extra stress where it mates to the engine, so make sure you use quality converter bolts and an SFI approved flexplate. 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 Chrysler 340 and 360 small blocks and 440 big blocks the factory put the balance weight on the torque converter. If you have one of these engines, make sure to get the appropriate flexplate counter-weighted to match the engine balance. Most SFI approved Chrysler flexplates have this counterweight. The best way to get the perfect converter for your application is to talk with the tech guys at your local speed shop or directly with the companies that build the converters. Performance Parts Directory
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