When it comes to a rugged, automatic transmission, the General Motors Turbo 400 is known as a brute. It has been the most-used transmission in heavy cars for drag racing for many years. The gearbox’s unparalleled strength is combined with an ability to survive in a myriad of configurations and unforgiving situations. But as we all know, the Turbo 400 has not been produced by GM since 1987.
Electronic overdrives have taken over, and that means rebuildable cores are getting much harder — even impossible — to find. That is a thing of the past because ATI Performance Products considered it prudent to create an aftermarket transmission case that is stronger than the original and SFI-approved.
The SuperCase is actually available for the Powerglide, Turbo 400, and new this year, Turbo 350 transmission. But for this article, we’ll be focusing on building a bullet-proof 400. The case, bellhousing, and tailhousing are cast from 356-T6 cast aluminum. The updated design was developed to eliminate all of the crack-prone locations found in the original GM transmission case.
As we said, the rugged SuperCase and bellhousing are SFI-approved for competition, but what we didn’t mention was, the case features a steel inner liner that weighs only 29 ounces. This liner eliminates the need for any external shields required by many sanctioning bodies. Also, the SuperCase features a removable bellhousing that bolts directly to the transmission case instead of at the pump-bolt locations like other aftermarket cases.
Not only was the ATI case designed in-house, but it is also cast here in the good-ole United States. Once cast, the cases are sent to the ATI facility for machining. While this is an all-new design, fitment in the thousands of applications already using a factory Turbo 400 requires the SuperCase to be identical to OE – relatively speaking. To accomplish that, it was designed using a factory case that had been modified, epoxied, and tweaked to incorporate all of the latest tricks.
ATI still calls the transmission a Turbo 400, but there isn’t much left of the original GM offering. For instance, those with applications delivering up to roughly 850 horsepower, ATI’s engineers say a cleaned and inspected case might survive, but why risk a catastrophic failure? The SuperCase is much stronger and can alleviate any concerns. “The SFI SuperCase with an SFI bellhousing is a must for the safety of the driver. Why trust a 40-year-old part?” stated Harvey Baker, marketing and sales associate at ATI.
Building A Better Box
When talking about the internals, each stock Turbo 400 part gets replaced with an ATI part or modified to improve performance and reliability. For instance, a stock front pump is utilized, but it is ported and then precision ground and has new pump gears installed before it is put into service. Although the Turbo 400’s front pump is a great unit in stock form, Baker adds, “We offer both stock-style, blueprinted units and our Super Pump. The Super Pump is great for higher horsepower applications because those need more pressure than a stock pump can supply. You need the correct pressure for the transmission to work and to be able to handle the power. It’s also utilized as a bolt-in stator tube.”
The SFI SuperCase with an SFI bellhousing is a must for the safety of the driver. Why trust a 40-year-old part? – Harvey Baker, ATI Performance Products
The stock GM input shaft is a durable piece from GM and is acceptable in mild-horsepower applications. But, it is known to fail when placed in high-horsepower applications. To be safe, a new ATI piece is made in-house from either 300M or Vasco.
The center support houses the intermediate apply-piston and supports the direct drum. It has rings that seal it to the direct drum, allowing it to transfer fluid to the drum when applying Third gear and reverse. Many enthusiasts have noted this is a weak link in the original transmission when used in anything other than a stock application. All ATI transmissions come with a heavy-duty center support. The upgraded center support for the Turbo 400 features a bronze bushing that virtually eliminates premature sprag failure. It is highly recommended for competition applications.
The forward-clutch drum is — in most cases — an OE core with a new input shaft. For those wanting to lighten the spinning mass inside the transmission, aluminum versions are also offered. The forward drum in a Turbo 400 always rotates at least at the engine RPM. But, when shifting from First to Second, the direct drum goes from spinning 80-percent of the engine RPM in Low gear to completely stopped on the 1-2 gear change, and then from stopped to engine RPM on the 2-3 gear change. Since aluminum weighs one-third the weight of steel, the weight difference is a performance must. It’s also less abusive on the sprag and greatly enhances clutch life.
If you plan to subject your transmission to serious racing duty, the high-gear clutch (direct) drum can be made of billet aluminum and utilizes a 34-element sprag. If that’s not strong enough for you, order ATI’s 36-element sprag and in-house-made outer-race and clutch hub, capable of holding up to 3,000-plus horsepower. The extensively modified gearset is also an OE part with new pins and upgraded thrust bushings.
The valvebody is the road map for fluid, and each one receives extensive machine work to modify the passages in the OE form. If desired, the stock piece can even be replaced with a complete billet aluminum part.
Finally, output shafts are a must-have upgrade for any transmission that will see more than OE-delivered horsepower. “ATI’s part number 406027, is 4340. “We suggest using this over any old OE output-shaft in applications up to 1,500 horsepower. Again, why trust a 40-year-old part?” stated Baker. If you are looking for the ultimate output shaft, part number 406024 is 300M. This heavy-duty output is a two-piece design with an extended bearing surface. It is considered a must-have with high-horsepower hot rods and heavy cars.
Finally, we asked Baker about what fluids to use. “ATI suggests using its SuperF 20-weight in all of our Turbo 400 transmissions (it’s also great in any electronic transmission prior to the 4L-series) when seeing up to 2,000 horsepower. After that power number, then we recommend using our SuperF 30-weight.”
We understand that some enthusiasts might need to change or add fluid at the track, and might not have enough ATI fluid needed, but Baker assured us that you can use regular Type F or Dexron, as they are both acceptable alternatives.
While the ATI SuperCase could be considered overkill for a daily driver and should be considered for any hot-rod that is pushing some serious horsepower. How many horror stories have we all heard about a “rebuilt” transmission coming apart and causing severe damage to a car — or worse yet — the occupant(s) of the car.
If you’re building a transmission that will be receiving some serious abuse or having ATI Performance Products build you a stout transmission, by using a SuperCase and high-performance internals, your transmission will ensure your street beast has the best gear-selection fluid drive available to deliver the performance you crave.