As the technology in Pontiac’s engines progressed over the years, so did its transmissions. Back in the glory days, manual boxes featured linkage-actuated clutches and three or four forward gears. The internals were simple and rugged, and the shifting a bit notchy, but they got the job done and provided a grin that can only be derived from manually slamming the shifter into the next gear.
Over the course of the next 30 years, three- and four-speeds gave way to the five-speed and six-speed transmissions with internal rail shifters and built-in reverse lockouts. The T56 six-speed was standard in Fourth-Gen Trans Ams and Formulas and late-model GTOs. It’s not uncommon to hear about a six-speed late-model LS car getting upper 20s in fuel economy or holding lots of rwhp with stock guts from Tremec, while still being a smooth shifter.
“How is this possible?” you ask. Let’s take a look inside a Tremec T56, more specifically an M12 out of an ’05 GTO.
This variant was shared by the other GM powerhouses of the day—the Corvette Z06 and Cadillac CTS-V—and has some upgrades inside straight from the Tremec factory that make it better than previous T56 transmissions. Out of the box, the synchronizer sleeves feature stepped teeth to make engagement feel lighter and more positive. The synchronizers also utilize high-density-carbon blocker rings and reverse synchronizers. Additionally, the M12 uses a one-piece countershaft, which is stronger than a two-piece.
[1] Because of the easy access...
[1] Because of the easy access to the adapter-to-bellhousing bolts, we decided to remove our transmission here. We also noticed a crack in our bellhousing, so we took extra precautions with the removal using our transmission jack.
Still, with all these upgrades, our intense downshifting and track duty combined with a few shifts during wheelspin resulted in the loss of Second gear. In our case, it was worn fork pads that prevented us from smoothly selecting Second gear and eventually caused the synchronizer to fail. We called Gray Fredrick from American Powertrain and explained our situation. We were soon provided with the correct package for our Goat.
With our heads and cam GTO used primarily for road-course racing and on the street, there was no need to go crazy with upgrades and break the bank. According to Gray, there are affordable solutions for each power hike you make with your Pontiac.
[2] With the transmission case removed, we discovered that the crack was actually a clean break (upper left), as the piece around the alignment dowel went along for the ride as we backed the transmission out and lowered it away from the car.
[3] The fluid was drained...
[3] The fluid was drained prior to disassembly.
“Unlike most packages out there, we include a vehicle-weight rating with ours because we can confidently say they can hold the claimed power level at the listed vehicle weight,” he says. “It’s easy to say a transmission can hold 500 hp, but is that 500-horse transmission going to hold the power in a car weighing 4,000 or more pounds? In our experience, it won’t.”
Because of these established power ratings, it was easy for Gray to suggest the Level 2 package, which is rated for a 495 lb-ft constant-torque load.
[4] Complete disassembly is...
[4] Complete disassembly is required to extricate the parts to send to American Powertrain, and is best done by a professional, like Greg Lovell.
[5] With the trans torn down,...
[5] With the trans torn down, we learned that our mainshaft received some heavy wear. Gray suggested we go ahead and replace it with a new one from Tremec to be treated rather than reuse this one.
[6] Gray took this photo before...
[6] Gray took this photo before sending our transmission pieces back to us. In it, you will find the parts you should expect to send out from your transmission. While you will receive the mainshaft partially assembled, your mechanic will still have some bearings to press-in, snap rings, and other assorted parts to assemble.
“This package is going to be the solution for 80-percent or more of enthusiasts out there. While peak loads and shock loads can climb well above this on the dyno, not many engines run these kinds of numbers on a sustained basis.
[7] This diagram illustrates...
[7] This diagram illustrates how the mainshaft and the input shaft and gear assemblies attach to the front adapter plate.
“The ratings are based on a 3,600-pound curb weight and street-legal radials. It covers the typical upgrades you would want, such as bronze fork pads and shims, but takes it a step further with cryo and REM treatments.” Those last two upgrades Gray mentioned add a new level of strength to the internal components.
You may have already heard the word cryogenic or cryo thrown around the hot rod industry for some time. It can increase strength up to 30 percent without affecting tolerances.
[8] This is the front mainshaft...
[8] This is the front mainshaft assembly, featuring (left to right) the Second and Third gears, synchronizer, gear blocker, synchronizer, gear blocker, First gear, and a tapered bearing. The billet synchronizer keys are considerably stronger than the factory parts, and are considered a necessity for high-performance applications.
[9] The First/Second shift...
[9] The First/Second shift fork was cryo-treated to increase strength.
[10] The Third/Fourth shift...
[10] The Third/Fourth shift fork is darker, as it’s not made from aluminum like the First/Second. This is because the 2-3 shift is usually the most violent and it’s not uncommon for the aluminum forks found in earlier variations of the T56 to actually break. Our GTO came from the factory with this stronger steel shift fork.
[11] Gray wanted us to point...
[11] Gray wanted us to point out the discoloration on the input shaft. This is normal and occurs after the treatment process.
[12] This is the Fifth/Sixth...
[12] This is the Fifth/Sixth driven gear. These are the overdrive gears that help these modern transmissions achieve excellent highway mileage with their 0.84:1 and 0.56:1 ratios, respectively.
[13] After receiving the parts...
[13] After receiving the parts from American Powertrain, Anti Venom’s Greg Lovell carefully taps the races for the countershaft and input shaft bearings into the front adapter.