For straight line guys, spending hundreds of dollars on something that doesn't give you any extra horsepower doesn't exactly seem like a good deal. Even a federal bean counter could figure that one out. Unless that money buys extra hook off the line, your suspicions of lameness are usually true. However, the one exception to the rule is a high-stall torque converter. Sticking one of these fluid-filled donuts between the engine and trans can often shave a half-second off your quarter-mile e.t.'s. It's all about maximizing power off the line, and nothing accomplishes this better than an optimized torque converter. By allowing a motor to freely rev to the fat part of its powerband almost instantaneously, instead of lugging along at low rpm where torque is at a premium, the result is blistering 60-foot times and phenomenal gains in acceleration. To put it succinctly, all automatic-equipped street machines and drag cars need an aftermarket torque converter.
In essence, a torque converter is nothing more than a fluid coupling that transfers power from the engine to the transmission. Unlike the clutch in a manual transmission, the input side (impeller) of the converter isn't physically connected to the output side (turbine). Instead, converters transfer power from one side to the other by swirling transmission fluid around inside their housings. It's this peculiar dynamic that offers endless tuning possibilities when the task at hand is torque multiplication and powerband optimization. Consequently, torque converter design is a very complex science where simple changes to the length, size, and shape of its internal blades and fins dramatically alter its performance characteristics. Likewise, there are a multitude of ways to build a converter to achieve the same specific performance goal. To get a handle on it all, we contacted two of the sharpest minds in the business, Stanley Poff of TCI and Joe Rivera of Pro Torque. Here's what they had to say.
Stanley Poff: "When extracting more horsepower from an engine with components such as a performance intake manifold, fuel delivery system, camshaft, and valvetrain, it's imperative to upgrade from a factory converter to a performance aftermarket unit. In most cases, horsepower has been increased and a more durable torque converter is needed. Likewise, changing the torque converter to a higher stall speed increases torque multiplication and allows the engine to reach its powerband quicker. Raising the powerband of the engine requires flashing the converter's stall speed closer to the peak torque of the engine, and locking it as soon afterwards as possible. For example, if you have an engine that is making peak torque around 2,500 rpm and you use a tight stock converter which averages around 1,500-1,700 rpm of stall, there will be a lag time where the engine will simply not perform as well as it could until it reaches 2,500 rpm. By installing a torque converter that flash stalls to 2,500 rpm, the engine is allowed to reach its optimum hp range quicker and will therefore perform much better."
Joe Rivera: "A performance torque converter is usually the best bang for buck mod for a hot rod. Our customers with 2010 Camaros are picking up four- to six-tenths in the quarter-mile with one of our high-stall converters. It's hard to match that with a set of headers, or a cold-air induction system. It's not that a performance converter makes more power, but what it does is allow your car to transfer power better. By raising the stall speed, or slippage, a performance converter raises engine rpm at launch, which increases power at launch. For example, if an engine makes 100 lb-ft of torque at 1,500 rpm and 175 lb-ft at 2,500, which would you rather have? With a higher-stall converter, your engine isn't making any more power, but it's allowing the motor to get to a place where it's happier more quickly. OE converters follow a one-size-fits-all philosophy. They want grandma to be able step on the gas without the tires spinning. Performance enthusiasts don't want that. They want more power to reach the wheels quicker. As a result, new-car manufacturers stick with conservative stall speeds to prevent you from going WOT in First gear and frying the tires. On the other hand, aftermarket manufacturers want you to fry the tires. When tuning a converter, the goal is to match it to the operating range of the engine. If you have a converter that's closely matched to the engine's dynamics that gets it into the fat part of the powerband more quickly, it will transfer power to wheels more quickly, which translates to faster acceleration."
Converter Components Stanley Poff: "To the untrained eye, a torque converter may look like a big circle with a bunch of fins inside it, but the various components it contains all serve important functions. GM converters can be broken down into two basic groups: non-lockup converters used in the TH350 and TH400, and lockup converters used in overdrive transmissions like the 700-R4, 200-4R, 4L60, 4L65, 4L80, and 4L85. The main components in a conventional non-lockup converter are the mounting cover, turbine, stator, and pump impeller. The mounting cover is what bolts to the flexplate and acts as a housing for all the internal parts. The pump impeller pumps and circulates fluid through the torque converter. As engine rpm increases, the rate of fluid flow through the converter increases until it reaches lockup. The turbine then catches the fluid from the pump impeller and circulates it to the stator and finally back to the pump, in a circular motion. By directing fluid from the turbine back to the pump impeller, the stator is responsible for the stall speed and torque multiplication of the converter. In addition to those four components, lockup converters add a lockup piston to the mix, which enables the converter to lock up at cruising speed. Once the engine reaches a certain temperature and rpm, a solenoid located on the transmission activates the lockup function."
Joe Rivera: "Data acquisition is the key to understanding and improving performance in all areas of racing. The same is true in the development and tuning of performance torque converters. The many years of racing and street car development we have been a part of has allowed us access to some of the best testing equipment available today, from high-end race data acquisition systems to rear-wheel dyno equipment. Even so, it's the race car that is the ultimate test for what we're doing.
"The benefits of data acquisition technology in the development of race converters gets trickled down throughout our entire product line. From the fastest automatic-equipped race cars to the tree-trunk pulling torque of modified diesels, and right down to your street car. For instance, the converters we developed for the fifth-gen Camaro is really a byproduct of all the work we are doing with high-horsepower Outlaw turbo cars. Our billet stators feature a proprietary blade design that offers more stall speed and unbelievable part-throttle characteristics. The process used to create these new billet stators is really revolutionary. We CAD/CAM design the individual blades, and then create a stator based off the physical space requirements. Next, we four-axis machine the parts out of one solid chunk of steel or aluminum. During testing, we rely heavily on our Racepak data acquisition system to find out exactly how to apply the power to the tires.
"When you're making converter changes, you need to understand that every time you change something in the converter, you're making a dynamic change. Kind of like playing a game of Jenga, if you move one piece of the puzzle everything moves along with it. In turbo cars, we worked very hard to find a converter design that both spools the turbos quickly and efficiently transfers power to the wheels. In the past that was very difficult, but with data acquisition and CAD/CAM technology we have been able to create completely different stator designs very quickly to get the best of both worlds. With the Racepak data logging system, we can measure wheel speed, driveshaft speed, g's at launch, and the average g's throughout the run to see how changes to the converter improves performance at the track. Truthfully, we'd be in the dark without data acquisition systems that chart our progress at the track."
Stanley Poff: "The stall speed of a torque converter plays an important role in overall vehicle performance. Stall speed is determined by the amount of flash rpm the converter is allowed to reach before the vehicle starts to move. Different torque converter cores, stator combinations, and pump fin angles can allow for different stall ranges in a torque converter. It is possible to reach the same stall-speed in a torque converter with different stators and pump/fin combinations, and one of the combinations may yield a better torque multiplication ratio than the others. The optimum stall speed and torque multiplication of a converter is achieved with R&D time on a dynamometer. TCI has an in-house dynamometer that uses a 900hp engine. This dyno was built specifically to test torque converter stall speeds without using a transmission. TCI can map different torque converters with different stall speeds and achieve a unit that will deliver the best stall speed and torque multiplication ratio.
"There are dozens of variables involved in selecting the correct stall speed for an application. First, we ask the customer a series of questions to determine what the vehicle will be used for. Is the car a grocery getter, a street machine, or a weekend warrior? Will it be a heavy cruiser or a lightweight all-out race application? Next, we examine the engine combo by asking for things like the duration of the camshaft and the type of induction system they're running to determine what type of horsepower we are dealing with. We're actually trying to dyno the engine in our heads with the specs we gather, so the customer being truthful when relaying those specs ensures a closer fit in stall speed and an optimum torque converter. Obviously, a converter will flash stall more in a heavy car than it will in a light car. Likewise, a converter will normally stall higher in a big-block application than it will in a small-block application, unless a power-adder is used on the small-block. A higher-rpm engine normally has to use a much higher stall speed than a low-rpm torque engine."
Joe Rivera: "Most people are familiar with what a converter's stall speed is, but a converter's multiplication ratio is an important performance factor as well. Back in day, all you'd hear about is stall speed, but you can have a converter with a lower stall speed and a higher torque multiplication ratio that accelerates a car faster than a converter with a higher stall speed and lower multiplication ratio. Every torque converter has the ability to multiply torque. This is a product of the converter taking energy from the trans fluid and redirecting it, which multiplies torque. As the ratio of turbine speed to pump speed increases, torque multiplication decreases. The confusing part is that multiplication ratio is independent of stall speed, and it's based mostly upon the design of the stator and pump. The average converter has a multiplication ratio between 1.9- and 2.5:1. The goal is to match the stall speed with an engine's power curve, and maximize the multiplication ratio as much as possible."
Stanley Poff: "Furnace brazing is a term that's thrown around a lot by converter manufacturers, and is a very common method of strengthening the internals. During the furnace brazing process, the turbine and the pump impeller are loaded with a powder brazing and run through an oven that causes that powder to melt and conform along the entire length of the fins. These fins are not very stationary from the factory since they're mass-produced pieces, so they're tightened down at TCI before the process begins. The powdered brazing material melts in the oven, and conforms to the complex contours of the fins. After it cools, the brazing materials bonds to the fins, creating one solid and uniform component. The final product is substantially stronger than an OEM torque converter, which are rarely furnace brazed."
Stanley Poff: "Even though the pump impeller and turbine inside a torque converter don't physically touch each other, the extreme pressures inside a converter can distort its internals. Factory torque converters are mass-produced, and aren't designed to work or last under higher-horsepower applications. They usually incorporate thrust washers instead of needle bearings, inferior stamped steel mounting fronts, and non-brazed internals. To endure the demands of high-hp engines, a TCI performance torque converter has a fully furnace brazed pump impeller and turbine, a machined stator, and sometimes even handbuilt steel stators with bearings that ride on both the surface of the turbine and pump impeller. In some instances we install anti-ballooning plates to the pump impeller and the mounting front to reinforce them for strength. In extreme-hp applications, we use forged or billet front covers."
Joe Rivera: "When it comes to torque converters, everyone hears about features like billet covers and furnace brazing, but do they really understand what that means? There's a lot of time and energy that goes into building a durable product that performs properly. What really good shops do is pay attention to the finer details. One of the things we do differently is box weld the anti-ballooning plates to ensure there is no flex or movement in the pump. We also use anti-ballooning plates on the turbines to make sure they don't move around either. Other tricks include billet stators that are machined from a single hunk of billet steel, and top-of-the-line billet front covers to make sure that they are as light and as strong as they can be. No one talks much about bearings, but they're a critical component in building a durable converter. We use oversized bearings that can withstand the load of up to 4,000 horsepower.
"At Pro Torque, we rate our converters up to a certain power level. Our current catalog includes converters rated at 500-, 650-, 850-, and 1,000-plus horsepower. At the 500hp mark, features such as anti-ballooning plates, furnace brazing, Torrington bearings, reinforced blades, and billet covers and clutches are used to fortify the converter. As power levels increase, the basic components stay the same, but they're just a little beefier. The converters we make for 3,800hp twin-turbo cars actually have similar features as our converters rated at 650 hp. They might have billet steel stators, but the basic components are the same. This is yet another example of how race technology from our top-of-the-line converters trickles down to our street product line."
Joe Rivera: "Through exhaustive R&D, we've learned that modifying the stator is one of the most effective tools in tuning the characteristics of a torque converter. We can take the same pump and turbine, and completely alter the characteristics of the converter just by changing the stator. This is because how a converter transfers power is determined by how the stator redirects the fluid. Thanks to our billet stator program, we've been able to make huge strides in understanding how different stator designs affect performance. The process starts with a solid chunk of steel, which gets machined down into a stator in our four-axis CNC mill. Last year alone, we made 175 different billet stators designs, all with various blade angles, blade pitches, blade lengths, blade counts, blade shapes, and window sizes. All of these factors affect how oil gets redirected inside the converter. By creating our own stator design while leaving the pump and turbine untouched, we can take a 10.5-inch converter and change stall speed between 3,000 and 5,500 rpm.
"There are always exceptions, but generally speaking, adding the number of blades raises the stall speed while changing the pitch of the blades can lower stall speed. Blade design also affects the multiplication ratio, so stator design can get very complex. Let's say you have five different blade counts with three different blade pitches that you want to test. That's 15 different combinations just based on one blade design. Add blade length and window design to the equation, and the number of possibilities goes up exponentially. As you can see, it takes a lot of time and R&D to design the right stator for an engine combination."
Stanley Poff: "Torque converters are inherently complex due to their multitude of moving parts, and we take extreme measures to ensure quality control. Many of the components are made to specifications with tolerances held to 0.002- to 0.005-inch. Items that are heat-treated have to be within a certain range or hardness in order to have adequate durability. Furthermore, endplay in the torque converter is set so the torque converter has the correct preload on the bearings inside. Each torque converter is air-checked in a water tank under 120 psi of pressure to ensure there are no leaks, and every torque converter TCI produces is balanced to within 5-10 grams on a torque converter balancer. TCI also has quality control engineers that spot-check all of its products including transmissions, torque converters, and shift kits."
Loose & Tight
Joe Rivera: "Traditionally, part-throttle slippage has been one of the drawbacks of a performance converter in a street application. It makes the converter feel mushy at part throttle and compromises driveability. Thanks to our race technology trickling down to our street converters, that compromise is a thing of the past. Outlaw turbo drag cars are the most difficult application to build a converter for because you need them loose on the bottom end of the track and tight on the top end. This forced us to get creative and test several new stator designs. Through lots of experimentation, we can now loosen up a converter on bottom without reducing efficiency up top. To accomplish this, we take a stock converter and put one of our stators in it. That one change alone raises stall speed to 3,000 rpm. Our billet stator yields excellent driveability with little to no part-throttle slip, which eliminates that mushy feeling typical of a high-stall converter."