Long-stroke aftermarket crankshafts and large-bore aftermarket blocks are commonplace in modern Pontiac builds. Powerful street engines commonly displace more than 460 cubic inches and are capable of generating 600 lb-ft of torque and/or achieving peak horsepower at an rpm that was once considered unrealistic. Combining such an engine with a manual transmission can make for great fun, but it’s generally far more power or speed than an original Muncie or Borg-Warner four-speed can handle, and complete transmission failure is often reality.
Tremec introduced a modified-for-GM-products TKO-600 five-speed manual transmission in 2004. It was conservatively rated at 600 lb-ft and featured an overdriven Fifth gear to reduce cruising rpm at highway speeds. It was instantly successful and is quite popular with Pontiac hobbyists.
So what’s a guy to do when he wants the advantages of a modern five-speed, but his Pontiac V-8 pushes the limits of a stock TKO-600? American Powertrain in Cookeville, Tennessee, has a solution in its 600Xtreme TKO upgrade. Follow along as we learn the details from a company insider.
This may look like an ordinary TKO-600, but it features American Powertrain’s 600Xtreme option, which improves the torque capacity and shifting ability for $1,200. It’s an excellent way to increase the durability and reliability of any Tremec TKO transmission.
The Tremec TKO transmission is very reliable and an excellent performer, and its versatility lends to its popularity. Based on the original TR-3550 Ford five-speed, Tremec modified and improved upon its design to create the aftermarket TKO, which is available with different capacity ratings, gear ratios, and spline counts. Companies like American Powertrain subtly modify the transmission case and/or shifter so it fits certain models like the Second-Gen Firebird. Also available is a complete installation kit that includes a custom crossmember, driveshaft, and all associated hardware.
With its 600lb-ft capacity rating, Tremec’s TKO-600 has been a popular choice for use with a high-performance Pontiac V-8. Our past experience with the TKO-600 has been nothing but positive. The gear ratios offer excellent acceleration and performance while reducing cruising speed to improve economy and reduce overall engine wear. Some hobbyists find that the TKO-600 doesn’t always shift smoothly when operating near 6,000 rpm. That wasn’t typically an issue since most Pontiacs built with stock components didn’t operate at such speeds. Aftermarket components have pushed the operating range and power output beyond the capacities of the stock TKO-600, and that can be concerning to some.
After a specific cryogenic hardening process, American Powertrain REM polishes the transmission’s gears and shafts to remove any minor burrs and blemishes. It creates a super smooth surface, improving strength and reliability and reducing noise while leaving behind a chrome-like finish.
American Powertrain picks up where the highly capable TKO leaves off. Its 600Xtreme TKO option increases the torque capacity of the TKO-600 by 350 lb-ft and improves shifting up to 7,500 rpm. And it accomplishes it without compromising synchronization or shifting performance at low speed.
The TKO is internally shifted, and during the 600Xtreme upgrade, the shift-rail mechanism (light blue in this photo) is disassembled and completely polished to improve shifting at high speed.
“We didn’t want to take anything away from the basic goodness of the TKO, which is our bread and butter transmission because it is so capable right out of the box,” explains American Powertrain owner Gray Fredrick. “Our goal was to tweak the box to get every bit of performance possible from it without negatively affecting driveability.”
So what makes the 600Xtreme special? Starting with a new TKO-600, a single American Powertrain technician in its craftsman-type shop completely disassembles and reassembles the transmission to ensure the build is consistent and thorough from start to finish. During disassembly, the parts are separated and sorted into those that are reused and/or reworked from those that are replaced.
The 600Xtreme-prepped TKO is assembled to “race tolerance” during the process.
The existing 4616-alloy gears, shafts, and shift forks are exchanged with identical pieces that have been cryogenically hardened to improve strength and performance. The controlled process includes slowly reducing the components to –300 degrees F for more than 12 hours with liquid nitrogen to improve molecular consistency, adding as much as 30 percent more strength to the original alloy without affecting its tolerances. “We specify that our parts be rapidly brought to a high temperature to maximize the effects of the cryogenic hardening process,” Fredrick adds.
Once the hardening process is complete, the gears, shafts, and shift forks are REM-polished to a chrome-like finish— it’s a micro-process that smoothes the contact surfaces of a component. It serves to improve the tolerances between components and eliminates the micro recesses on the gear faces, where transmission fluid can get trapped and pressurize as the gears mesh. “That can reduce efficiency and create noise,” says Fredrick. “REM polishing also reduces parasitic loss, increases durability by reducing heat and friction, and reduces gear noise from the already-quiet helical-cut Tremec gears.”
A steel bellhousing should be part of any serious performance build. The aluminum original isn’t designed to contain an explosion should the clutch and/or flywheel fail. Quick Time offers this unit (PN 8070) that fits and functions like an original, but is SFI-approved for maximum safety. American Powertrain supplied us with the optional unit for an extra cost of $500.
Improved strength is only half of the equation with the 600Xtreme. The TKO is factory synchronized for shifting speeds at or slightly above 6,000 rpm. While a quality clutch and pressure plate assembly, and smooth operating linkage, can increase that slightly, shifts above 7,000 rpm generally require additional work, and American Powertrain tends to the entire shift mechanism for clean and precise shifts at all speeds.
“The TKO-600 is an internally-shifted gearbox, and it uses linear solid-steel shift rails and lugs within the transmission to switch gears,” adds Fredrick. “The durable rods are constantly lubricated and do not bend or deflect like common external-rail shifters. With the 600Xtreme option, our technician hand-polishes each of these components to remove any rough edges that could create binding while shifting. The internal springs are also swapped for higher-rate units to improve the shifter’s rotational return rate.”
Aftermarket bellhousings typically require some type of adjustment to ensure the transmission is centered correctly. The Quick Time unit is dialed in at the factory for less than 0.005-inch run out. Ours proved within spec and it installed without issue. Refer to the supplied instructions for the complete install process.
The 600Xtreme transmission is carefully assembled to what American Powertrain calls “race tolerances.” Some of the small parts are upgraded during assembly, too. “The plastic shift-fork pads, which are quite suitable for everyday use, are replaced by solid bronze forks that will survive the abuse of aggressive shifting. Standard sprung-steel roll pins are replaced with hardened steel pins in many locations, and original stamped-steel slider keys are replaced with units constructed of solid billet steel.”
Once completely assembled, the enhanced TKO-600 is quality-tested for leaks, shaft endplay, shift quality, and in a few other critical areas. The job is then signed off on by the assembler, who assumes full responsibility for the quality of the transmission build. American Powertrain installs the appropriate performance shifter and ships it to its customer for installation.
Since we did a detailed install for the TKO in the previously mentioned story, we will simply discuss a few highlights here. Using a transmission jack and our four-post lift, the new TKO-600 was positioned in our Firebird’s transmission tunnel, the input shaft was aligned, and it slid directly into place.
We’ve had a stock TKO-600 behind the 467ci in our ’76 Bird for several years. We detailed its install in “Gearing Up for a Trans Swap” (HPP, Mar. ’05). It has always operated reliably and consistently without any real issue, but future plans to increase engine output had us wondering if its 600–lb-ft capacity would be sufficient.
Rather than take any chances damaging an otherwise totally functional transmission, we elected to simply replace our existing five-speed with a new TKO-600 that’s been upgraded with the 600Xtreme option. American Powertrain’s ProFit installation kit provides all the necessary components for quick and easy installation.
The entire assembly arrived on our doorstep within a few weeks of placing the order. It included a new 600Xtreme-prepped TKO-600 transmission, X-Factor solid-aluminum crossmember, and a 3.5-inch drawn-over-mandrel (DOM) seamless driveshaft that’s made for American Powertrain by Precision Shaft Technologies (PST). We also found a plethora of small parts, including a urethane transmission isolator, a speedometer cable adapter, a molded Weatherseal reverse-light wiring harness, and all associated hardware. The install went just as expected and took only a few hours to complete.
The stock TKO-600 remains an excellent choice for many vintage Pontiacs. It gives the distinct pleasure of shifting the gears of a manual transmission, while improving vehicle tractability at all speeds without compromising durability or reliability. American Powertrain’s 600Xtreme upgrade takes the TKO-600 to the next level, giving peace of mind in instances where output or rpm may exceed the stock TKO capacity. And best of all, if you already have a TKO-600 in your vintage Pontiac and are interested in the 600Xtreme upgrade, American Powertrain can take any existing TKO transmission and apply its 600Xtreme upgrade for a very reasonable cost.
It gives the distinct pleasure of shifting the gears of a manual transmission while improving vehicle tractability at all speeds without compromising durability or reliability
