You might say we’re out for blood in 2014 with our Project BlownZ Camaro in the 275 Drag Radial ranks. A completely overhauled chassis (which we’ll be telling you more about in the coming weeks), and a brand new engine combination, both of which we hope will place us up there with the elite 275 cars in the nation. Of course, there have been a host of other off season changes on the car, as well, including the very ingredient that gets all of the power from the ProCharger F1-X-equipped, Late Model Engines-prepared 400 cubic inch LSX mill to the chassis — that being an all-new three speed Turbo 400 transmission from the drag racing driveline geniuses at Hughes Performance in Arizona.
We’ve teamed with Pete Nichols and the gang at Hughes to not only deliver us a top-flight piece for BlownZ this season, but also to take us (and you, the reader) on a guided walk-though of the construction of their T400’s, complete with a detailed look at all of the features they’ve built into their transmissions to create not only a record-capable piece, but a durable one, as well.
You may recall that in December, we published a story along the very same lines, guiding you step-by-step through a custom-built Powerglide and converter package build, that will find its way into our Project Fugly wagon this summer. But this time around, it’s all about the venerable Turbo 400, which has taken drag racing by storm in recent years as high performance drivetrain manufacturers likes Hughes Performance have focused their efforts on the once-OEM transmission fit for high-horsepower drag racing.
Before we get into the build, you may be wondering why the Turbo 400 has become a go-to automatic transmission for so many race teams, from fast bracket racers all the way up to 4,000 horsepower Pro Modifieds. Many recognize it as a production tranny from GM but what makes it so suitable this environment?
The Turbo 400 At A Glance
The Turbo 400 transmission, known officially as the Turbo-Hydramatic and commonly referred to as simply the TH400, was part of a family of automatic transmissions developed and produced by GM. It was designed with a three-element turbine torque converter mated to a epicyclic planetary geartrain, thus delivering three forward speeds plus reverse. Since its arrival in the mid-1960’s, the TH400 has been regarded as one of the strongest transmissions available, thanks to its production cast iron center support that suspends the concentric shafts joining the clutch assembly the geartrain and providing a brawny reaction point for first gear, making it quite popular in high performance racing applications.
“The gear set design in the Turbo 400, even in OEM form, is probably the strongest gear set design ever developed by the “big three” auto manufacturers. Even the factory gear sets can handle upwards of 2,000 horsepower without an issue, which is unheard of out of any other transmission. It’s just a fundamentally strong transmission. What makes the three-speed variant popular in a drag racing environment, particularly in a supercharged combination like BlownZ, is that it gives you better gear splits, or gear ratios. So on the gear change, the engine doesn’t see as much RPM drop, thus the engine spins for a longer period of time in its primary effective power band, compared to what a Powerglide, for example, would offer.”
As Nichols further explained, using BlownZ as a primary example, centrifugal superchargers are very sensitive to boost drop, and the three-speed Turbo 400 naturally lands itself to that with the lower RPM drop on the gear changes. This same concept applies, as well, to other power adder applications and even those running naturally-aspirated.
The gear set design in the Turbo 400, even in OEM form, is probably the strongest gear set design ever developed by the “big three” auto manufacturers. Even the factory gear sets can handle upwards of 2,000 horsepower. – Pete Nichols
“If you have a powerplant with a narrower operating range in terms of its effective power band, anything you can do to keep that RPM drop manageable, the faster the car is going to run,” says Nichols.
The two-speed Turbo 400, likewise, is necessary for applications where one needs to take ratio away, compare dot what the Powerglide can offer. “Numerically, the lowest you can goon a low gear ratio in a ‘Glide is 1.58:1, so if you need something lower than that, it’s where the two-speed 400 comes into play,” Nichols explains.
The venerable Powerglide, long regarded as the bulletproof, go-to drag racing transmission, has slowly gained some company from the Turbo 400, as Nichols went on to tell us. Much of this has can be attributed to a strong push by manufacturers to satisfy the needs of ever-higher horsepower race cars over the last 3-4 years, with aftermarket cases and components, like the build you’re about to see.
Today’s aftermarket manufactures have taken this original design much further — as we’ll be highlighting in the build-up in this article — developing custom cases and internal components to make a durable transmission even greater so.
The Bits And Pieces Of Hughes’ All-Conquering TH400
In this guide, we’ll be breaking down each and every part of this particular TH400 transmission and the processes and steps taken to complete its build-up. We begin of course, with the entire assembly broken down in an exploded view, with the most obvious of parts — the case itself — the first thing we’ll look at.
The basis for this build is Reid Racing’s TH400 transmission case, which Hughes Performance offers as an SFI-certified upgrade in all of their builds. The construction of this particular case is superior in strength to OEM cases, and in fact, does not require any external safety shields. The case features an extension housing with improved strength over stock, and has a roller bearing instead of a bushing for lower drag. The case is mated with a Reid Racing SFI-certed bellhousing that, likewise, with its impressive strength, doesn’t require an external shield.
Moving on to the internals, Hughes Performance uses a high static rear band in all of their TH400 builds, intended specifically for drag racing use. These feature a heavy-duty, carbon-based lining that creates a high co-efficient of friction for the utmost in holding ability while on the transbrake. Next are the forward, direct, and intermediate clutch packs — a combination of red racing (forward and direct clutch packs) and High Energy frictions (intermediate clutch packs). The red racing frictions, according to Nichols, provide outstanding grip and durability in high horsepower applications, having been used in combinations exceeding 3,800 flywheel horsepower. The High Energy frictions are likewise exceptionally durable and provide a cushion as they apply, resulting in less shock to the intermediate sprag.
BlownZ's Turbo 400 Build Sheet
- Part number 34-3XTH-195
- Reid Racing SFI-certified transmission case
- Reid Racing SFI-certified bellhousing
- Reid Racing heavy duty extension housing with roller bearing
- Billet steel solid input shaft
- Custom steel forward drum with oversize input spline
- Forged steel forward clutch hub
- Billet steel intermediate shaft
- Billet aluminum direct drum
- Pro Mod 36-element intermediate sprag
- Heat treated extra wide intermediate sprag races
- Billet steel intermediate pressure plate
- 6 frictions and steels in forward drum
- 6 frictions and steels in direct drum
- 5 frictions and steels in intermediate
- Billet aluminum planetary carriers with billet steel straight-cut planetary gears
- High static rear band
- Thrust washers replaced with Torrington roller bearings
- GM cast iron front pump with custom pressure regulator and modified converter feed circuit
- Pro Series transbrake valve body with reverse shift pattern (P-R-N-1-2-3)
- Deep cast aluminum pan
- Lokar NHRA-approved locking dipstick and tube spec
“Anything we can do to reduce rotating mass in the transmission serves to reduce stress on the intermediate sprag,” says Nichols. “This reduction in mass also serves to reduce parasitic horsepower consumption through the transmission.”
Another reason for the use of the billet drum is preventing a drum explosion in the event that it would go into an overspeed condition when shifting from high gear into neutral under load, where drum speed can nearly double. Cast iron, in this situation, would simply explode at the nearly 14,000 RPM it might see. The hard anodizing of the direct drum increases the surface hardness, giving it more longevity. Aluminum is a naturally “soft” material, and the hard anodizing reduces wear that aluminum is prone to inside the lugs where the direct steels are located inside the drum.
Hughes also uses a direct clutch apply piston and direct piston return spring retainer in their builds. “We use a specially calibrated set of direct piston return springs that help release the direct clutch pack as quickly as possible when the transbrake is released. Reducing the amount of time required to release the direct clutch pack will pay off with improved reaction times,” Nichols explains.
Inside the transmission are custom ratio planetary gear sets. Hughes offers several ratio options in their two- and three-speed builds, all featuring billet aluminum construction for the carriers and billet steel for the actual gears. The gears have straight-cut teeth for strength, proving themselves well in cars over 3,800 horsepower and 3,250 lbs. race weight.
That brings us to the planetary output carrier, or the rear-most gear set in a TH400 planetary assembly. The square notches in the outer shell of the gear set allow for parking pawl engagement, which is a design required because the gear set increases in size when the ratio is reduced, and thus the OEM lug design cannot be retained for parking pawl engagement. In the planetary reaction carrier — the front-most gear set — the rear band engages on the outside of the carrier, making for a smoother outer shell. The rear band is applied along with the forward clutch pack to create a reverse gear, and in unison with the forward and direct clutch pack, creates the transbrake feature on a TH400.
The planetary sun gear, meanwhile, rides between the planetary reaction and planetary output carriers, and is driven by the sun shaft through the large inner spline inside the gear. As well, the planetary ring gear rides in the back side of the planetary output carrier, and is driven by the intermediate shaft though the small inner spline in the ring gear. The output shaft also locates the planetary output carrier through the square-cut lugs in the outer shell of the housing.
The pump stator in the TH400 forms the rear half of the front pump assembly. According to Nichols, Hughes resurfaces each pump stator in every transmission build, which they consider a critical step in the process, as it will warp over time. “OEM machining doesn’t always leave a true flat surface,” says Nichols. “As such, leaks can develop between the pump body and pump stator if the pump stator is not truly flat. Fluid leaks between these components will result in a loss of line pressure, air leaks between them will result in cavitation of the front pump, and both conditions together can be extremely detrimental to transmission and torque converter function and longevity.”
TH400’s are notorious for generating excessively high converter charge pressure. When excess charge pressure is generated into the torque converter it can then be physically thrust forward from that high fluid pressure. – Pete Nichols
As Nichols goes on explain, “TH400’s are notorious for generating excessively high converter charge pressure due to the high volume of fluid that the pump gear set is capable of producing. When excess charge pressure is generated into the torque converter it can then be physically thrust forward from that high fluid pressure. Enough thrust force can potentially be generated that damage will occur to the crankshaft thrust bearing and crankshaft thrust surface inside the engine. Engines with relatively small thrust bearing surfaces such as the LSX and Ford modular platforms are particularly sensitive to this issue when using a TH400 behind one of these engines.”
For this reason, Hughes utilizes what’s known as a torque converter fluid feed restrictor inside the body of the pump stator, which allows sufficient fluid volume to allow the converter to maintain its fluid charge while reducing feed pressure. This acts to reduce the forward thrust motion of the converter. As well, a torque converter feed balance circuit, an additional fluid passage, is added to create a balance in the converter fluid feed circuit.
The Hughes TH400 uses a GM pump body that’s lapped to ensure the sealing surface is flat and true. From there, the billet steel intermediate shaft, made of a proprietary material, is installed. As we pointed out earlier, this is one of the most stressed parts of a TH400, with its shaft joining the forward clutch hub to the ring gear. That then brings us to the billet steel input shaft, manufactured of 300M steel, that’s a press-fit assembly installed into the forward drum. The shaft is designed with an oversize spline that delivers a more positive and stronger joint between the input shaft and the forward drum, preventing the spline from stripping inside the forward drum.
We then move to the all-important transbrake valve body assembly. Hughes’ Pro Series TH400 transbrake is designed for an exceptionally quick release to give racers an edge at the starting line. These have a reverse (P-R-N-1-2-3) pattern. Hughes Performance has found the original GM output shaft to be incredibly durable, and they’ve successfully mated these behind 3,800 horsepower, 3,250 lb. race cars without fault.
As we move to the underside, we take a look at the deep cast aluminum transmission pan that’s been chosen for this build. This pan has a 1/4-inch thick sealing rail and holds two additional quarts of fluid for maximum cooling capacity. It also has an O-ring equipped drain plug for easy fluid servicing at the track.
Having looked in-depth at many of the components going into this transmissions, assembly of the build begins with installation of the intermediate shaft into the rear ring gear which is located inside the planetary output carrier. The shaft is retained to the ring gear by a simple snap ring. The Hughes team installs the output shaft into the back of the planetary output carrier, also retained by a snap ring. The planetary sun gear is placed into the output carrier, which is then joined to the planetary reaction carrier. The low roller clutch then joins the mix as it’s installed into the reaction carrier. With these steps complete, the gear train assembly is ready for installation into the Reid case.
Once in the case, the center support, which is intended to support the gear train, the direct and forward drums, and the sun shaft, and the intermediate shaft, is installed. Gear train play is verified and set at this stage. As Nichols shares with us, “The center support also directs fluid flow to the direct drum for application of the direct frictions. We modify the center support to dual feed the direct frictions with fluid which exponentially increases the holding ability of the direct clutch pack.”
With the center support in, the intermediate clutch pack then follows. This pack, as Nichols tell us, is retained in the case by a large snap ring. The center support houses the intermediate apply piston, with fluid being routed by the valve body through the center support to apply the intermediate clutch pack through the apply piston. From here, we’re ready to install the direct and forward drums. This process is begun by installing the direct clutch pack in to the direct drum, which is then placed into the transmission.
“Care must be exercised to make sure that the inner teeth on the intermediate frictions align properly with the drive teeth on the outer sprag race which is located on the back of the direct drum,” says Nichols.
With the direct drum in, the assembly process is continued by placing the forward clutch hub into the forward drum, installing the forward clutch pack into the forward drum, and finishing the forward drum assembly with installation of the direct clutch hub and the snap ring that retains it. The entire forward drum assembly can then be installed into the transmission case, with care taken to align the inner teeth of the frictions with the drive teeth located on the outside of the direct drive clutch pack.
Says Nichols at this stage in the assembly, “Some finesse is required here as we also have to align the spline of the intermediate shaft with the spline of the forward clutch hub as we drop the forward drum assembly into the transmission.”
With the direct and forward drums installed, the main assembly of the TH400 is completed by installing the front pump, with drum end-play verified at this stage. With the rotating assembly of the transmission finished, the internal shift linkage, parking sprawl, rear servo, valve body, oil filter pick-up tube, oil filter, and transmission pan are installed, bringing the build-up to a crescendo.
Once assembly is complete, each and every transmission goes to Hughes Performance’s in-house dyno for testing. “We dyno test each and every transmission that is built at our facility, Nichols tells us. “We verify line pressure, shift function, transbrake function, and fluid flow through the cooler circuit during the dyno test, and also run the transmission up to proper operating temperature. The dyno test procedure also serves to break in the transmission so that it is ready to use and race once installed in the customers vehicle.”
Once it’s earned the seal of approval on the dyno by the Hughes staff, the transmission is sent out the door, arriving just in time to raise it up into place in our Project BlownZ Camaro for what we anticipate to be a record-setting year of racing, and the bulletproof Turbo 400 transmission will certainly be an important piece of the puzzle. As you’ve no doubt learned (and subsequently seen) in this build, GM’s Turbo 400, already a stout piece in it’s own right, has become a bonafide monster of a transmissions, capable of handling nearly anything a dry racer could throw at it. It’s quick, and it’s as durable as anything you’ll find, and that’s a potent combination. A special thanks to Pete Nichols and all the staff at Hughes Performance, who made this informative trip through the construction of their Turbo 400 possible.