Over the last several months, we’ve been upgrading parts and pieces on our Project Silver Bullet machine with an eye on killer dragstrip performance without killing the streetability. The focus of our story in this go-round is the brand-new triple-disc torque converter from the team at Circle D Specialties in Houston, Texas – a beautiful piece of automotive jewelry if we’ve ever seen one.
To provide a bit of background on our hot rod, in the first few installments of this project we’ve added a Phase 2 supercharger system from ROUSH Performance, opened up the exhaust with a set of full-length headers and X-pipe from Kooks Custom Headers to go with Roush’s axle-back exhaust, and finished off the underside with a complete Maximum Motorsports Road and Track Grip Box. The second round of modifications included new high-flow fuel injectors from Deatschworks, then a rearend upgrade with an Eaton Truetrac differential complete with Motive 3.73 gears to maximize our performance both on the track and on the street. In the last installment, we swapped out some of our suspension parts for better launches at the dragstrip, installing a set of Strange Engineering‘s S197 single-adjustable front and double-adjustable rear shocks to go with a set of Cobra Jet coil springs from American Muscle, which improved our short times despite a belt-slippage problem, and set us up for this round of the build.
The How And Why
We’ve discussed torque converters before in previous articles, but a quick rundown is in order.
Principles Of Torque Converter Operation
The torque converter is a fluid coupling, designed to take the torque produced by the engine and turn it into usable power that can be pushed to the vehicle’s drive wheels through the transmission, driveshaft, and rearend assembly. There are four main parts to a torque converter – the turbine, pump, stator, and front cover. The pump is welded to the front cover, and transmission fluid circulates internally to turn all of these pieces to produce an efficient fluid coupling. Check out the video above (it’s a little long but a great watch) to gain a greater understanding of what’s happening inside the converter.
Our new Circle D converter offers an advantage not found in older-style torque converters, and that’s the lockup clutch inside. By using a piston and multiple friction surfaces within the assembly in conjunction with solenoid-fired fluid pressure, the converter locks up – hence the name. The solenoid is controlled by the tune and this can be manipulated to suit individual preferences. Our 6R80 transmission, as spec’d by Ford in our 2013 Mustang, had a lockup converter from the factory, but the dual-disc OE lockup clutch gives up holding power compared to our new triple-disc piece.
“For optimum performance, the factory converter is never going to be the right piece. It’s all set up for fuel economy and drivability, not performance. From an NA setup to centrifugal-supercharged setup to a turbocharged setup, those will all really benefit from a performance torque converter. The positive-displacement supercharger like you have in this project will benefit also – even though it doesn’t have an extreme gain because the torque curve is so low, there is still plenty of room for improvement. You’re trying to match the torque curve better to the engine for the gain in performance.” says Circle D’s Chris Sehorn.
Focused On Performance
There are literally dozens of torque converter manufacturers in the marketplace, but very few have tackled the late-model lockup clutch converter design quite like Circle D has. “Since we make all of our own parts, we have figured out things that have worked, and we’re not limited to waiting on another converter competitor to make parts. We were first to market with the 6R80 torque converters by about a year, and that’s because we make our own parts,” says Sehorn.
The company makes two different diameter torque converters for the 6R80 market at this time – a 245mm version and a 258mm version. Naturally aspirated, centrifugally supercharged, and lower boost pressure (8-10 psi) turbocharged cars will benefit most from the smaller diameter unit, while higher-horsepower and positive-displacement supercharged cars will benefit most from the larger 258mm unit. The smaller the diameter of the centrifugal pump (torque converter), the higher the stall speed of the converter.
Changes And Upgrades
With that thought in mind, we consulted with Chris on what the best piece would be for our application, given the 500-plus horsepower we are trying to get to the ground most efficiently. One of the hallmarks of a Circle D Specialties torque converter is its durability, and our current power level is the breaking point for a factory torque converter, as the clutch material inside the converter will only hold so much power before that material says “Sayonara”.
“We see the factory converter start to give up around 500 horsepower – the lockup clutch starts slipping. We manufacture the billet lockup pistons, the lockup clutch and the billet front cover for these converters. They are designed to handle around 1,200 horsepower. The big-power stuff, you get the benefit of a better-matched stall to the combination, but also a much, much stronger torque converter,” says Sehorn.
The multi-disc torque converter, as built by Circle D, has a number of unique features internally that help it to perform both on the street and at the track. One item Sehorn is specifically proud of is the the deep-groove ball-bearing that’s installed in the front cover. It’s there to center all of the parts and pieces internally. According to Sehorn, this is an extremely durable design that is unique within the industry that helps to contain the axial and radial loads within the converter. These features help to extend the life of the converter.
“We’re extremely proud of our triple-disc design. We make everything we can in-house. It’s a USA-forged front cover that starts out as a 60-pound block of steel that we CNC-machine into its final shape. The high-carbon clutches are also custom-made just for us, these hold up extremely well in the later-model transmissions. The clutches are also heat-treated to eliminate any wear issues over time. The apply piston is full billet that’s USA-forged and machined in house as well, and we use a custom turbine hub in these converters,” says Sehorn.
The converter also has industry standard items built-in like Torrington bearings for the turbine hub to spin upon, anti-balloon plates, and furnace brazed fins, along with TIG-welded parts where necessary.
Going Up, Up, Up
In a positive-displacement supercharged car like ours, getting the engine up into the powerband doesn’t take nearly as much effort as it might for an owner of a naturally aspirated car or centrifugal supercharged car, where the engine needs to get up into the RPM band in order to make power. The converter doesn’t need a stall speed quite so high, and ultimate power-handling is much more important in our case.
We see the factory converter start to give up around 500 horsepower – the lockup clutch starts slipping. – Chris Sehorn, Circle D Specialties
“We can alter the stall speed to match the torque curve, and in a positive displacement application like yours we have to be careful not to overshoot the torque curve by going too loose. Whereas with a centrifugal car, or an NA car, we want to go 4,000 to 4,500 rpm on the flash, so that when you pop out of the hole you’re just popping right into the powerband. A turbo car needs to be loose down low, so you can footbrake it and build some boost. Turbo cars are the hardest ones to do because there’s no power, and then twice the amount of power. Basically, the more power you put to the converter, the higher it’s going to stall,” Sehorn explains.
On the track, he says that users can see as much as a three-quarter-second improvement in elapsed times with a naturally-aspirated or centrifugally-supercharged car with the use of a new torque converter optimized for performance, while a positive-displacement supercharger owner will see a couple of tenths at least, depending upon what was in the car prior to installation.
Another advantage comes in terms of drivability – not on the street, but at the track. “From a competitive standpoint, if you can loosen up the converter up it’s a lot easier to stage the car and be consistent with the tree without having to fight the brakes so much, and that’s what we were after in this application,” Sehorn says.
Positive-displacement supercharged applications like Project Silver Bullet – as a function of design – produce all of their boost nearly off-idle. This can present issues when staging a car in a drag racing application as the car wants to push through the brakes due to the boost coming in as soon as the car loads up on the foot-brake. This problem is not prevalent in trans-brake racing applications, but we don’t ever plan on doing that with Project Silver Bullet.
On The Track Improvements
At the track, we saw a substantial improvement from our ROUSH-supercharged machine in some areas, while others clued us in to some issues to note. Previously, our best 60-foot time was a 1.54, and on both passes this day we were able to click off a 1.51 on less than ideal track conditions – and found out in the process of running the car through the quarter-mile that we need to put some more time in on the dyno and on the street to straighten out our transmission tuneup for the new torque converter.
We saw a substantial improvement on the bottom side of the elapsed time with a three hundredths decrease in the short-track, but didn’t see the improvement through the eighth due the fact that since this is a completely different converter than stock and realized we needed to adjust our transmission shift points accordingly.
Every time we go out to the track, we learn something new. This time out, we found out that our new converter does exactly what we expected it to, which is get us out of the hole more quickly and efficiently. Next time back to the track, we’ll have the transmission tuneup ironed out and expect to see a substantial elapsed time decrease in the process.