Quest for the Nines: Upgrading an S550 Transmission and Converter

Quest for the Nines: Upgrading an S550 Transmission and Converter

Pony Wars is a battle of the Big Three’s muscle cars going head-to-head in a variety of challenges to test the limits and determine the best option for ultimate fun-having capabilities. Last season, a 2019 Mustang GT took on a Chevrolet Camaro and a Dodge Challenger on the autocross, braking pad, drag strip, and dyno to see who would reign supreme. You can catch up on the play-by-play here, but we’ll give you an in-depth look into the Mustang’s performance.

The Mustang GT is equipped with a ProCharger P-1X, Stage II intercooler, and Race bypass valve, putting it in the power range of a smidge over 900 horsepower and 637 lb-ft of torque on an otherwise stock engine. The Mustang held up well through the competition, but the team found its weak spot – the stock 10R80 transmission.

While the Mustang had flown through autocross and the braking challenge with nary a technical issue, the drag strip threw a wrench in the gears.

The Steeda drag-suspension-equipped Mustang was outfitted with Weld RT-S S71 wheels wrapped in Toyo Proxes TQ drag radials, and started off its time on the quarter-mile doing pretty well.

Though the weather was hot (we’re talking nearly 100 degrees) at Muscle Cars at the Strip at Las Vegas Motor Speedway, the first round went off without a hitch, granting the Mustang an 11.03-second pass at 131.83 mph following a soft launch. The team opted to skip round two to adjust the Mustang’s tune, but came back with a vengeance for round three, hammering out a 10.83-second pass at 131 mph. It was obvious on this pass that something wasn’t right, as the 5-6 gear change proved impossible. After blowing the tires off in round four on a 140-degree track surface, a 10.64 at 134+ mph was achieved in round five, although the transmission woes continued. It was decided that the amount of power the Mustang was cranking out was just too much for the stock transmission. These issues of course did not improve, causing the Mustang to take the loss on the drag strip.

The current method of trial and error in our industry is fading, and being able to see a simulated converter run is insightful as well as amazing,” Chris Sehorn, Circle D Specialities

Once put on the dyno, the Mustang team was delighted to find out just how much power it was making. It put down more than 900 horsepower at the rear wheels, and the team decided to try its luck on the strip once again (this time, not in competition mode). The Mustang managed a 10.35 at 136 mph – a huge improvement. For reference, it was on this pass that the Mustang achieved a 6.962 at 106.64 in the eighth-mile. This will be important later! But even still, the transmission wouldn’t shift properly, and Fifth gear was left unvisited. The team pulled the 10-speed automatic transmission and realized the clutches of the 10R80 were badly smoked.

Mission: Fresh Transmission

It was decided that in order to handle over 900 wheel horsepower, the Mustang would be equipped with Circle D Specialties’ new billet torque converter, several 10R80 upgrades, and 3.31 gears. Previously, the supercharged S550 was utilizing all-stock components with 3.55 gears, and they didn’t handle the pressure all that well. Brett Lasala of MSRacing was the transmission builder of choice (you may remember his green, twin-turbo, 6R80-equipped S197 we featured a while back).

We asked Brett a whole slew of questions about the transmission build and he broke it down for us. Basically, the only upgrade that the 10R80 needed was new clutches, which was the transmission’s weakest link. The new clutches will provide better “holding power” than its stock counterparts, and this upgrade won’t break the bank either. For the McLeod clutches and installation, it’ll run you about $1,700 at LaSala’s shop.

“We haven’t seen any common hard part failures in the 10R80 yet, but now that there’s a better holding clutch kit, I’m sure we will,” he explained.

LaSala noted that the stock clutches do well up to about 600 rear wheel horsepower. “Above that, the clutches slip, and you’ll experience flared and missed shifts.” Upgrading to the McLeod clutches should allow us to put down horsepower numbers in the 1,000-1,200-wheel horsepower range without issue.

While there aren’t currently different options available, LaSala explained that his team is working on modifications that will allow for increased clutch count for the problematic drums.

The McLeod clutches (PN: 88098) were designed in collaboration with Raybestos and are advertised as able to withstand high-stress applications and high temperatures, as well as repeated cycling, resulting in an improvement in transmission performance and durability — both things that our Mustang could benefit from. The heat-treated steel clutch plates are said to outperform OE materials by up to 20%, and have a finish that supplies a greater apply area and friction clutch life. Noise and shudder are lowered, and the OE thickness is maintained for maximum heat sink.

LaSala got the fresh clutch kit installed, and with the transmission back in order, it’s almost time to move on to the installation. But first, let’s talk about the Circle D Specialties torque converter we chose for this application.

Torque Converter Talk

A torque converter is made up of five components: a housing, pump, turbine, stator, and transmission fluid. When a car is at idle, the engine is turning slowly, delivering just a small amount of torque through the converter. When you hit the gas, the engine spins harder, pumping more fluid into the converter, and transferring more torque to the wheels. The converter’s housing is bolted to the flywheel, and the pump of the converter is made up of fins that turn at the same speed as the engine.

That sounds simple enough, but consider that the new 10-speed transmission stuffs four extra gears into nearly the same overall size thanks to its revised, “squashed” torque converter design. In addition, extremely precise timing of clutch engagement is needed to make each shift in milliseconds.

The Circle D billet torque converter was chosen for this Mustang because it is based off the OE converter but paired with Circle D’s CNC-machined billet stator and impeller modifications. Circle D offers a variety of options to accommodate a number of engine, power adder, and vehicle applications. Plus, you can make yourself $250 back if you turn in your original converter for a core credit!

Circle D says this particular converter features a fifth axis CNC-machined billet stator, hand TIG welding throughout, and impeller fin recalibration. It also features custom CDS internal clearances and Torrington-style bearings.

Using the correct torque converter for your application, even with a bone-stock engine, can result in improved performance (specifically when it comes to those pesky 60-foot times).

The Stator

Perhaps considered the most “interesting” part of a torque converter, the stator is the finned piece of the operation in the center of the torque converter. The way it’s designed directly affects efficiency and stall speeds, and its job is to recirculate transmission fluid from turbine to pump, resulting in torque multiplication.

“Stator technology has improved from using OE stators to full five-axis CNC machined billet stators. Utilizing the multitude of OE options on our 245mm, 258mm, and 265mm platforms, we can target different torque curves and power levels to achieve exceptional performance,” Chris Sehorn, owner of Circle D explains. “We have learned a lot over the last ten years by trial and error, and we can dial-in a setup. Our latest in stator design comes from our engineering department using Computational Fluid Dynamics (CFD).”

Engineering Jargon

We spoke with Ronnie Combs, Senior Design Engineer at Circle D, and he explained Circle D’s Computational Fluid Dynamics software, utilized during the design process to ensure that the converter is built for the torque curve of the Mustang.

“Computational Fluid Dynamics (CFD) is an engineering tool that makes use of computers to analyze the behavior of fluids interacting with physical systems. This is accomplished by solving the Navier-Stokes equations, which is a set of partial differential equations that represent the mathematical statements of three conservation laws of physics. The conservation of mass states that mass cannot be created or destroyed. For incompressible flow, this means the mass flow rate in must equal the mass flow rate out. Based on Newton’s second law of motion, the conservation of momentum states that the rate of change of momentum is the sum of the forces. The conservation of energy is described by the first law of thermodynamics which states that the rate of change of energy equals the sum of the rate of heat transfer and work done.

“From the information gained in the simulation solution, Leonhard Euler’s equations for turbomachinery are used to produce the dimensionless and semi-dimensionless performance parameters: K-factor and torque ratio. These parameters have advanced our ability to design the torque converter by allowing us to be concerned with more than the stall speed and the slip when crossing the traps.”

Simply put, CFD software helps develop products for new transmissions, while also allowing Circle D to study older units and make improvements. Circle D invested in the tool in 2018 and has been utilizing it tirelessly ever since. It’s not new technology persay (companies like Ford have been using it for years). In the old days, a one-off part would need to be built before testing it and making changes to adjust and make it perfect. This software completely removes the extra steps.

“In theory, CFD allows us to control the torque multiplication a little more. We can hit areas hard, and it will show us what the number is. The current method of trial and error in our industry is fading, and being able to see a simulated converter run is insightful as well as amazing,” Chris says. “We can take a look with the software and apply certain theories to see where the pressures are too low or too high. We can even see where the fluid is as the converter starts to couple. It will even calculate torque multiplication for you, and it applies to all two- to 10-speed transmissions.”

The Installation Process

Of course, the transmission had previously been removed and sent out to LaSala for a refresh, so our Mustang sat on the two-post lift in the meantime. Once it made it back into our hands, it was time to put it back where it belonged, along with the new torque converter. This part was pretty easy, but it helped to have a couple of sets of hands (and that aforementioned lift).

We were sure that we had the correct torque converter for our application, thanks to the help from our friends at Circle-D. But you should always be sure you’ve received the correct part prior to installation by making sure that the pilot fits into the rear of the crankshaft without excess clearance. You’ll also want to be sure that the converter’s bolt holes or studs line up with the bolt pattern of the flywheel.

Before installing the converter onto the transmission, we poured about a half of a quart of automatic transmission fluid (ATF) into the converter. We also used some of the ATF to coat the transmission seal, front pump bushing, and the converter neck. The converter was installed onto the transmission with extra care not to damage the front seal or bushing, before the pilot of the converter was held to center it as much as possible. The front mounting pads were rotated in a clockwise direction, allowing the splines and hub flats to engage with the pump. We knew installation was going as planned, thanks to two “drops” of the converter before it was all the way in.

Light lithium grease was used to grease the torque converter pilot and crankshaft pilot so that the converter could easily slide into the crank with no binding.

With the torque converter in place, the transmission was ready to be installed back into the Mustang. The transmission was guided onto the engine dowel pins before the mounting bolts were installed, continually checking converter clearance. With this step done, we pushed the converter into the transmission as far as we could and measured the distance between the end of the stud and the flywheel. The converter was then pulled forward a fraction of an inch, and flat washers were utilized to remove any additional space between our new converter and the flywheel. Finally, the converter nuts were tightened before measuring again.

The transmission was reinstalled and bolted to the block, being sure to continually check the converter clearance. A drill bit or feeler gauge can be used to check clearance. We were looking for 1/8-inch clearance or pull back, plus or minus .025-inch. If during your installation you find there to be excessive clearance, supplied SAE washers can be added. If there is no clearance, STOP. Circle-D tells us this is probably because the converter is not seated.

The transmission installation was completed before the recommended 13 quarts of ATF were added gradually. The team also installed 3.31 gears into the Mustang’s Torsen differential, replacing the stock 3.73 gears, for better utilization of each gear throughout the quarter-mile. Finally, taller Mickey Thompson ET Street R tires (305/45R17) were chosen in an effort to soften the hit in conjunction with the lower gear, helping to keep the tire planted upon launch.

Heading Back to the Track

With the transmission refreshed and torque converter and gears installed, it was time to head back to the track to see what the Mustang had up its sleeve. Unfortunately, the team was unable to run a quarter-mile track, so the results we’re able to share are that of the eighth-mile variety. As we mentioned earlier, the Mustang’s best pass previously had an eighth-mile elapsed time of 6.962 at 106.64 mph.

On the first run out, the Mustang trapped a 7.342 at nearly 73 miles per hour. That pass was followed up by its best of the day — a 6.480 at 113.26 mph with a 60-foot time of 1.592. Track conditions on passes three and four proved difficult for the Pony, causing tire spin and wheel hop issues and rendering the time slips just about useless.

After a few adjustments, the Mustang managed a 6.274 at 116.65, followed by a 6.878 at 73.08 and finally, the last pass of the session, a 7.048 at 73.04. While these runs are kind of all over the place, it’s important to note that this was the first time the Mustang returned to the track following the removal of its stock torque converter.

The return to the track definitely showed a clear improvement! The best pass should’ve translated to somewhere around a 9.96-second pass at 143 mph in the quarter-mile (using the Wallace Racing calculator). We are counting this as a success, but we’d like to hear what you think. Please comment below and let us know if you prefer quarter-mile or eighth-mile testing.

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About the author

Stephanie Davies-Bardekoff

Stephanie Davies-Bardekoff got her start in automotive media while attending Rutgers. She worked for Roush Performance for a while, before eventually landing here at Power Automedia. Her Coyote-swapped 1992 Fox-body drag car is her prized possession.
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