There are a number of technological advancements in drag racing to thank for the mind-blowing performance across the vast spectrum of the sport; tires, suspension, fuel injection, and wheel speed management devices, just to name a few. But one of the sometimes-unsung heroes of the puzzle is the transmission. Through considerable research and development and on-track product testing, manufacturers like Coan Engineering and ATI Performance Products have pushed the envelope beyond the wildest dreams of Detroit engineers in the 1960s, producing racing-dedicated transmissions that are a far cry from those once toyed with by your father or grandfather. In the process, they’ve all but rendered the once-mighty manual transmission and clutch obsolete across much of the sport, ushering in a new wave of automatic transmissions capable of handling horsepower measured in the thousands.
The formidable GM Turbo-Hydramatic 400 has been front and center in this new wave of all-out automatic transmissions, heralded for its impressive strength and the efficiency of its planetary gear design — both necessary to withstand the power to push an 8.5-inch tire car into the 4-second range or a Pro Mod over 270 mph. Naturally, the Turbo 400 was our first and only choice with our Evil 8.5 Ford Mustang project, as we sourced a two-speed build from ATI with their Wicked Quick two-speed billet aluminum valve body and a stock output shaft with a custom 1.56 gear ratio.
If there’s two planets that make up the entire gear-train of the transmission, you change one of them and re-use some of the stock components, and you’re able to manipulate ratio. – Jason Coan, Coan Engineering
At the center of this build — and what is ultimately the center of any automatic transmission — is the planetary assembly, manufactured by Jason Coan and his team at Coan Engineering in Indiana. As one of the leaders in the design of planetary gearsets, Coan’s gears are found in a range of performance transmissions — both their own and by competing manufacturers — simply because of the time and know-how they’ve invested into developing the parts to produce a superior piece and the gear options made available through the company’s development process.
The planetary assembly is where the rubber meets the road — figuratively and literally — in an automatic transmission, as every element of the transmission works in tandem to turn the planetaries, thereby transmitting the power from the crankshaft and torque converter to the output shaft and driveshaft and produce the desired gear changes. The planetary is also where the gearing — be it two-speed, three-speed, or more — is determined. And for drag racers, gearing is key: the proper first gear ratio for the power level and tire type allows the car to launch with the desired wheel-speed, and the subsequent ratios keep the engine in its power band (and thus the tires and suspension loaded to the desired degree) going down the racetrack.
In a Turbo 400, the stock first gear checks in at 2.48, with 1.48 as the second cog, but with the power of our supercharged small-block in Evil 8.5 paired with the tiny 235 drag radials running on radial-type prep, a custom 2.73/1.56 gearing was deemed optimal. With that, we sourced a 2.75 ratio, steel-carrier planetary (Part #COA-22702) from Coan Engineering for our ATI-assembled transmission. Jason Coan notes this is the company’s “most economical entry into changing ratios in a 400,” with the use of aftermarket steel and re-furbished OEM components to make the price affordable.
To iron out the discrepancy in the numbers, Coan shares that, while the actual ratio of this gearset is 2.73, it has always been marketed as a 2.75-1.56. Early attempts to change the ratios in the Turbo 400 centered around borrowing components from other OE transmissions, such as the TH200 and TH350. In doing so, these “Frankenstein concoctions,” as he refers to them, produced a ratio very near 2.75, and the naming configuration carried forward when Coan began producing better billet parts.
[This gearset] is very popular in two-speeds, because a 1.56 bridges the gap between a Powerglide — which is commonly a 1.80 — and a 400 factory second gear ratio of 1.48. – Jason Coan, Coan Engineering
The reaction carrier in this build, refurbished by ATI, is an OEM GM piece with OEM helical-cut planetaries. The helical-cut sun gear is OEM, as well.
“If there’s two pinions that make up the entire gear-train of the transmission, you change one of them, re-use some of the stock components, and you’re able to manipulate ratio,” Coan explains.
Every pinion (planet) that Coan produces is straight-cut, making it easier to identify in the assembly what is aftermarket and what is OEM. For this kit, Coan has taken the helical OEM sun gear, split it in half, and set it to the proper length for the pinions and the reaction shaft. The helical cut gear has the same strength as the straight-cut, so this is merely a re-purposing of the factory gear.
The reaction carrier in our transmission, not part of Coan’s kit, is entirely OEM refurbished, including the shell and pinions.
The output carrier uses an OEM steel shell. Coan removes the factory carrier from this, then replaces it with a 7075 aluminum piece that uses straight-cut, 16-tooth pinions manufactured from a proprietary blend of heat-treated steel. In order to make their products modular and easily serviceable, Coan utilizes splines and snap-rings on their assemblies so everything is interchangeable. This particular setup features six pinions, while some of their beefier planetary kits for higher-horsepower and higher-torque cars feature eight, which provide greater load-sharing for the sun gear and ring gear.
“This is a 1.56 second gear, and we’re using the 16-tooth pinion gears, so we found that utilizing six of them versus the four used by the factory, we share the load better and can enhance the strength of the gear-set and extend the life of the gears,” Coan explains.
The gear ratio math, however, doesn’t come down to the tooth count of the pinions, but rather on the sun gears and the ring gear. The pinions, instead, simply fill the space so the two can run together.
“What this kit does is it creates a 2.73 first gear, which is really unusable but people buy it because it makes second gear 1.56. In terms of drag racing, it’s not very popular as a three-speed ratio unless for a reasonably heavy naturally-aspirated car. But it’s very popular in two-speeds, because a 1.56 bridges the gap between a Powerglide — which is commonly a 1.80 — and a 400 factory second gear ratio of 1.48. So it lets a car that’s been configured around using a two-speed — maybe it had a Powerglide in it but a 1.8 is too much ratio or they’ve reached the threshold of parts durability of the Powerglide — it allows them to get into a 400 with a very manageable first gear of 1.56,” Coan explains.
Coan notes that when using the 400 as a two-speed, the reaction carrier is only used to transfer power in Reverse. “That being said, OE gears are adequate,” he adds. “The low roller clutch may also be omitted from the transmission in two-speed configuration as long as the unit has a billet direct clutch drum. Omitting the low roller allows the reaction carrier to be lightened or replaced with a 2-speed specific aluminum carrier.”
Coan offers both of these options.
While this package offers more strength than the factory components, in terms of drag racing, until you get into an extreme level of horsepower, upgraded kits like this are more about gear ratios than they are durability, according to Coan. But for the off-road race trucks and other extreme applications that use Turbo 400s and these kits, the durability of the assembly is certainly paramount. With the material selection and the design of the 400, Coan notes there aren’t many clear-cut weak links in the planetaries, and their goal is always to steer customers in the right direction to pair the proper components with the application.
“This is a very economical gearset, but we’re working within the space constraints of the stock output carrier shell. The ring gear can only be so big and fit within that shell and still attach to the output shaft, so you’re limited in terms of the tooth counts and therefore ratio availability, and by the outside diameter of the ring gear, which somewhat limits strength by way of wall thickness,” Coan says.
For that, Coan offers a whole range of planetary kits and components to cover the entire spectrum of applications and power levels.
The next step up from this kit, according to Coan, would include a billet-steel output carrier shell. If you want to reduce rotating weight, a billet steel shell larger in diameter with larger potential tooth counts and wall thicknesses is also an option. One-piece reation shafts, in which the shaft and sun gears are machined as one for strength, is also part of Coan’s product lineup, as are billet reaction carriers with straight-cut gears. Compound, close-ratio planetaries, which are complete redesigns of the Turbo 400 planetary manufactured exclusively by Coan for big-power applications, include the arrangement of the aforementioned eight planets in the output carrier. All of this, Coan notes, is a proverbial step above the previous with regard to durability and gear ratio options.
The benefit here, again, is two-fold: we’ve achieved the desired gear ratio for our engine, vehicle and vehicle weight, and tire combination, while at the same time adding some strength and longevity in all the right places. Truth be told, GM hit a home-run with the design of the Turbo 400, but bright minds like those at Coan Engineering are making a great transmission just that much better, at a price many heads-up racers can afford.