Frank H. Cicerale
March 1, 2008
We swapped out the stock torque converter in this S197 Mustang for one of TCI's Super Streetfighter converters. The results speak for themselves. You can almost slip a couple of credit cards under the left front.

In today's performance world, adding power to your pushrod or modular engine has never been easier. With the advent and increased usage of power adders such as nitrous oxide, turbochargers, and both centrifugal and positive-displacement superchargers, 400-or-more rear-wheel horse-power is certainly attainable.

With the increase in horsepower output from the engine comes the need to effectively transfer the power from the crankshaft to the rear tires. While beefing up the rearend and adding a set of gears to take full advantage of the available power is on just about everyone's "to-do" list, it's just as important to be able to transfer the power efficiently through the driveline. In the case of a manual-transmission-equipped car, picking the right combination of clutch, flywheel, and transmission (and transmission gearing) is the key to leaving the starting line hard without smoking the clutch and/or wasting power. For cars that are equipped with an automatic, though, the job of making sure the power moves from the engine to the transmission, and ultimately to the rear tires, is a task handled by the torque converter.

We cruised to JDM Engineering for the converter swap. Things were kicked off when we removed the exhaust system, then unbolted the driveshaft loop from the transmission.

In this article, we will show you what an efficient torque converter is worth on the track. Pick a converter that's too tight, and the engine will struggle to get into the powerband; ultimately, the e.t. will suffer. Select one that's too loose, and the power will slip away, leaving you with poor on-track performance.

When it comes to driveline components, the torque converter is arguably the most complex part you'll choose. It is a fluid coupler, which also acts as a torque multiplier during initial acceleration. What this means is that, using the engine's torque, the converter and all of its internals multiply that torque to get the car to move out of the gate. Still, the converter can slip so the engine doesn't stall while idling in gear. Furthermore, today's converters have provisions for full lockup, and there's virtually no slippage once the vehicle is up to speed. This enhances the life expectancy of the transmission and also increases fuel mileage.

Before we delve into the innards of upgrading to a performance converter and the protocol for choosing the right one, let's take a look at what makes a converter tick.

A torque converter consists of five (or sometimes six) parts: the cover, the turbine, the stator, the sprag, and the impeller pump (and sometimes a clutch/lockup mechanism). The cover is the basic part of the converter, and in actuality, serves to contain the parts. Used for bolting up the unit, its job is to attach the converter to the flywheel. The turbine rides within the cover and is attached to the drivetrain with a spline to the input shaft of the transmission. As the transmission pump forces fluid through the converter, one turbine sends the fluid to the turbine facing it. This movement of fluid across the turbine blades is what drives the converter and the trans. Think of having two electric desk fans facing each other. Turn on one fan and it drives the blades on the other. Additionally, the amount of blades and their angle greatly affect things such as stall speed and converter flash.

Two other important parts of the torque converter are the stator and the sprag. The stator has been likened to the brain of the converter, and its job is to change the flow of transmission fluid between the turbine and the pump. It's also what makes the converter as a whole a torque con-verter, or multiplier, and not simply a fluid-coupling device. The operation of the stator is directly linked to the operation of the sprag. The sprag is a one-way mechanical clutch that fits inside of the stator. The stator is related to the sprag in that the sprag must work correctly for the stator to do its job. To do this, the stator must hold the sprag in the proper position when the converter is in stall mode. Stall mode is when the turbine speed is slow in relation to the impeller-pump speed. In addition, the stator must allow the sprag to spin with the rest of the converter after the turbine speed nears the speed of the impeller pump.

The transmission lines were undone from the transmission. To keep transmission fluid from leaking all over the place, we plugged both the line ends as well as the inlets on the trans case itself. Note: When working with the 5R55S, try to keep fluid loss to a minimum. There is no dipstick to check the trans fluid level on the 5R55S, which also means there is a special provision for adding fluid to the trans. This is done by removing the hex-head bolt on the trans pan, replacing it with a Zerk fitting, and injecting trans fluid much in the same manner you would grease a bushing.

The sprag is important in that it is the part that multiplies the torque coming from the engine to get the vehicle moving. If the sprag breaks, the car won't move-it's as simple as that. Torque multiplication is done both in stall mode as well as when the vehicle is accelerating. This torque multiplication is what gets an automatic car to move from a standing start. Once the converter has reached a 1:1 ratio, or full lockup, the torque multiplication decreases rapidly. This is because the car is already moving and doesn't need that extra grunt to get going. Additionally, the turbine and pump are at the same speed, thus resulting in the power from the engine being transferred down the line in a direct manner.

The last part of the converter is the impeller pump. This is on the half of the converter facing the transmission. Inside the pump are a series of aforementioned fins, which drive the transmission fluid inside of the converter into the turbine. The size of the converter, the size of the pump, and the number and shape of the pump's fins are what gives each torque converter its unique characteristics. Converters generally range from full racing units in the 7-, 8-, 9-inch range, up to 10-, 11-, and 12-inch converters from the OEM. Larger converters have more internal area so the fluid stays cooler and there is less slip or stall. Smaller converters generally will have a higher stall speed, and that's why they're used in racing. Also, with less fluid moving through them, heat can build quickly.

Now that we have the basics out of the way, let's turn to the differences between the stock torque converter and the new TCI Super Street-fighter converter that we installed. The TCI piece is made specifically for the 5R55S five-speed automatic transmission found in the new S197 Mustangs. According to Tony Akins of TCI, there are a myriad of differences between the stock converter and the TCI one.

The driveshaft bolts were removed from the front and back yokes. Once the bolts were out, the driveshaft was yanked.

"For starters, the stock converter is 12 inches in diameter, while the Super Streetfighter is 10 inches," Akins says. "Internally, our converter has a billet stator and furnace-brazed fins that are tack-welded for durability. The stock converter has a cast-aluminum sprag, while ours is a billet piece. Also, our lockup material consists of different material than the stock converter. All of our converters utilize much of the same equipment we use in our 2,000hp Powerglide converters."

All of the internal modifications add up to a converter with characteristics that are much different than the stock piece. "The sprag that we use actually reduces slippage anywhere from 7 to 10 percent," Akins says. "All of this translates into a converter that has less rotating weight, as well as more stall speed. A stock 5R55S converter will have a stall speed of about 1,600 rpm. Our converter stalls in the neighborhood of 3,500 rpm. All of the modifications combine to lower elapsed time while raising the vehicle's top-end speed."

Before we went further, we disconnected the battery, as is customary with most work JDM does. It's even more important in our case since we had to undo the starter bolts and move the starter out of the way to remove the trans. We removed the starter after disconnecting the trans lines.

This high stall may be a concern for everyday street driving, but we didn't find this to be the case with the TCI unit. A converter's stall speed is the rpm that a given converter has to spin in order for it to overcome a given amount of load and drive the turbine.

Low stall speeds are great when you have gobs of low-rpm torque, as in a Lightning or with a positive-displacement blower. Obviously, the flip side is that the more stall speed, the higher the rpm needs to be before the turbine (and the car for that matter) starts to make headway.

This is the difference between a "tight" (low stall) converter and a "loose" (high stall) one. While you might think that a lower stall converter would get a car to move quicker, that's not necessarily the case. Ideally, for drag racing, you want a converter that stalls a few hundred rpm below maximum torque. If your 4.6 Three-Valve is making peak torque at 3,000 rpm, you want to be able to stall the car on the starting line near or somewhere below that, ideally in the neighborhood of 2,500-2,800 rpm. If you have a converter that stalls to 1,600 rpm, it will take time for the engine to reach its optimum powerband. Therefore, 60-foot times will be slow, and elapsed time as well as trap speed will be hurt as well. This is where torque-converter choice is both crucial and complex.

Here are the converters side by side. The TCI converter (left) measures 10 inches, while the stock converter is a 12-inch piece. The difference lies in the internals, and the TCI converter features some stout pieces that increase durability as well as performance.

"The name of the game is to get the stall speed as high as possible," Akins says. "The stock converter will obviously offer better driveability, but it lacks in the performance department. When it comes to converter selection, first and foremost, the application in which the car is going to be used is key. If the car is a race-only vehicle, then an 8-inch converter would be perfect. For someone who wants to race their car on weekends and get good numbers, yet routinely drive it on the street, a compromise has to be made as something, be it ultimate performance or driveability, will suffer.

"In the performance arena, the perfect compromise is to get away with the most stall you can in street trim. Basically, we think 3,800 rpm is about the maximum amount of stall speed you would want on the street. The goal is to provide maximum power transfer with the least amount of slippage. Slippage creates heat, and heat is what kills a converter and a transmission."

The starter was bolted back into place. We checked the starter in relation to the flywheel and found that no shims were needed other than what were already utilized.

Once the purpose and application of the car are established, Akins and the rest of TCI's tech advisors look at a bunch of other factors before steering a customer in the right direction. "Once we know what the car is going to be used for, we then look at the rear gear ratio, the cam duration specs, the weight of the car, and whether or not the car is equipped with a power adder," Akins says. "Once we have all of that information, we give the customer an honest evaluation of what would best fit their needs based on our tech advisor's experience and knowledge."

TCI To The Test
We decided to test a TCI Super Streetfighter on a stroked naturally aspirated S197. The Mustang GT in question is owned by JDM Engineering's Jim D'Amore III and is equipped with a 298ci stroker engine sporting ported heads, stock camshafts, a custom tune, 4.56 gears, and no power adder. In this trim, the car has made 357 hp at the rear wheels. More importantly, though, with the stock converter, the Mustang has run a best of 11.879 at 115.30 mph while recording a 1.710 60-foot time. Oh, and one final note on our waiting patient: The car is driven on the street every day.

Knowing this little mod motor could benefit from a looser converter, we spent a day at JDM installing the TCI converter. We say a day because a bit of work had to be done to the factory converter plate to make things fit perfectly.

With the trans in place, we hooked up the trans harness and linkage.

The factory crankshaft, as well as the flywheel/ flexplate, is a six-bolt piece, and the stroker crank and current flywheel/flexplate in this car was an aftermarket eight-bolt unit, thus the converter did not mount directly. Had the vehicle had the stock setup, there wouldn't have been an issue. The converter plate needed to be modified to give sufficient room for the two extra bolts, as well as to allow the plate to sit flush on the flywheel. This took a little massaging.

Once the converter was on, we took a cruise on the streets around the Freehold, New Jersey- based shop to see what kind of driveability issues there might be. To put it bluntly, there were none. "The car drives awesome and is totally streetable," D'Amore reports. "There's a slightly looser [throttle] pedal feel when the car is going uphill, and when we put in the converter, it took some of the hit out of the shifts because it is looser."

We then hooked the transmission lines back up, tightening them carefully so we wouldn't round off the bolts.

On the track, the results speak for them-selves. The 60-foot time was reduced to a 1.582, and the elapsed time was lowered to an 11.649. Speed fell off to 113 mph, but D'Amore con-tributes this to there not being a change made to the tune within the transmission. "We did no tuning whatsoever for the converter," he says. "The car could have probably run quicker if we had played with the tune of the transmission."

The better e.t. and 60-foot times came from the fact that D'Amore could launch the car at a much higher rpm, and the engine went right into its peak power range. "With the stock converter, I was struggling to get the car to stall at 2,000 rpm on the starting line," he says. "With the TCI converter, I was able to bring the starting line rpm up to anywhere from 2,600 to 2,800 rpm, and it would flash to well over 3,000 once I got going-there was no bogging; it just took off."

It even lifted the front tires just a bit on launch.

Like they said in the movie Days of Thunder, "Loose is fast." In this case, it certainly was.

Reinstalling the driveshaft was a two-person job for both removal and reinstallation.

It's All In The Timing
The TCI Super Streetfighter converter for the 5R55S transmission (PN 456002) is billed to produce lower elapsed times by as much as a half a second while losing nothing in terms of street driveability. While we didn't see a half-second improvement in e.t., we'd like to think that a difference of 0.230 second is pretty sizeable. Check out the 60-foot and quarter-mile numbers below.

  Stock Converter TCI Converter Difference
60-Foot 1.710 1.582 -0.128
E.T. 11.879 11.649 -0.230
MPH 115.30 113.15 -02.15