Inside Tremec's New TKX Five-Speed Manual Transmission and How it Compares to the TKO, T56

In the late 1800s Charles Darwin proposed the then-astonishing concept that evolution of species was based on the simple fact that the strong survive and the weak eventually disappear. That’s also the nature of automotive high performance parts. In the ‘50s, a transmission only had to handle 200 to 300 lb-ft of torque at low engine speeds and life was fairly easy. Fast forward to today and 500 lb-ft of torque is the new standard and shift points and power quickly escalate from there.

Given this powerful state of affairs Tremec decided to upgrade its classic TKO five-speed transmission for a number of reasons. Many have found that this transmission was lacking in several areas. While torque capacity was acceptable at either 500 or 600 lb-ft, shift quality suffered. Worse yet, as engines continue to make more power at higher engine speeds, the classic brass ring synchronizers employed in the TKO transmissions felt balky above 6,000 rpm.

To address these and other issues, Tremec has introduced the TKX. For those attuned to the previous line of five-speeds, the TKX is similar in many ways (like gear ratios) but it is a whole new transmission with regard to significant improvements. We’ll look at all the features starting from the outside and work our way deep inside this new gear box.

Much like the earlier TKOs, the TKX will be available in two basic configurations in either Ford or GM orientations using the same gear ratio options. The TKX is now rated at 600 lb-ft of torque across all the different versions with the wide ratio using a 3.27:1 First gear and the close ratio version employing the 2.87:1 First gear. Case mounting and rear crossmember locations also remain the same as the TKO, which makes this new trans a simple bolt-in swap versus the previous trans.

Tremec says its enhanced gear sizes and revised double-lined, carbon-fiber synchronizer package can now be safely shifted at 7,500 rpm and much of the stiff, notched shifter feel has been eliminated, and the gates now have special chamfers to improve the shift quality along with a special ball detent for each of the three rails.

An equally important part of the TKX’s upgrade was to reduce the size of the transmission’s envelope. Just looking at the TKX, it should be easy to see that it offers a much lower height from the input shaft centerline upward to the top of the case. The older TKO was tall with broad shoulders and often required major floor surgery or custom tunnels welded into cars like early Chevelles.

In addition to its compact nature, the TKX is a great solution for hot rodders who don't need the extra overdrive gear of the significantly larger T56 and T56 Magnum transmissions.

According to Paul Coffey at Modern Driveline, Tremec designed the new TKX case specifically to fit within the narrow and short confines of the ’68-’72 Chevelle tunnel. With a much smaller case configuration that also offers a trimmed-down shifter mounting area, Coffey says the TKX will fit both the ’68-’72 Chevelles as well as the earlier ’64-’67 floor pans without surgery. The only cut required may be for the shifter which is still in the transmission centerline as opposed to Muncie and Super T-10 shifters that are offset to the left. Modern Driveline offers an offset shifter to accommodate existing manual console and tunnel for the ’64-’67 GM A-body cars. The ’68-’72 cars have them available as needed.

For Ford enthusiasts, Coffey tells us the new TKX will fit within the floor pans of any second and third generation Mustang (’67-’73). The ’65-’66 cars will still require the floor support to be relocated as it was for the TKO. We’re not going to get into the details of any of these swaps as each will demand its own set of specific requirements for bellhousings, clutch linkage, driveshafts, and other details. Companies like American Powertrain, Holley, Modern Driveline, and others can help with conversion parts, complete kits as well as technical advice.

From the outside, the TKX presents a new and improved case design. From the front, the top is more rounded and significantly shorter than the TKO. The main reason for creating a new case was to add strength while also creating room for swaps. The old TKO was designed to be assembled from the top, much like the old Ford Top Loader four-speeds. The TKX eliminates that top “window”, improving case strength. The TKX is now assembled through the rear of the case.

The new mid-plate also adds strength. The original T-5 transmissions were designed for inline four cylinder and V6 engines and are weak partially because the case deflected under load. When this occurs, third gear on the main shaft would tend to separate from the cluster gear. The TKX adds a structural member called a mid plate between the main case and the extension housing that adds a support to the main shaft that minimizes deflection.

Moving toward the rear of the case, the TKX features two plates on top of the transmission that house the three rails used for shifting. The standard configuration is with the shifter in the rearmost plate but this mount can also be rotated to position the shifter forward. As an option, the forward opening can be configured for a shifter but will require optional parts to make that change.

Other advantages that are easily identified on the outside of the transmission include both a mechanical and electronic outputs that will accommodate almost any speedometer requirement. The vehicle speed sensor (VSS) output is located on the right rear of the tail housing while the mechanical speedometer output can be found just opposite that on the left hand side of the extension housing. The reverse light switch has also been retained and is also located on the driver’s side of the case.

Another point worth mentioning is the 31-spline output shaft dimension has also been carried over from the TKO versions. This larger output shaft diameter is common with the Ford C6 automatic should you be looking for a slip yoke.

The functional performance benefits of the TKX are hidden inside the transmission. A great way to improve shift feel and reduce the effort especially under high rpm is to increase case rigidity. As mentioned earlier, this was improved with the rear-loading design and enhanced with the addition of the mid plate. These improvements not only help to increase the transmission’s shift speed limit but also reduce what engineers call NVH – or noise, vibration, and harshness.

The TKO used brass synchro rings much like those used in Ford Top Loader and Chevy Muncie four-speeds. These perform well at lower engine speeds but suffer from reduced efficiency at engine speeds above 5,500 rpm unless seriously modified. A much more elegant solution had previously been applied to the T56 Magnum transmission using carbon fiber-lined/bronze synchro rings that allow quick gear speed matching which minimizes the time required to complete the shift. These hybrid synchro rings are combined with larger, stronger gears to handle engine speeds of up to 7,500 rpm which should instantly position this transmission as a popular upgrade for the Pro Touring crowd that regularly thrashes manual transmissions on road courses and autocross tracks.

So the TKX is really more of an evolutionary move for 21st Century manual transmissions, which is exactly what the performance world is looking for. It will bolt in place of an old Muncie four-speed, Ford Top Loader, T-5, or even Mopar four-speeds and offers the shift and feel of a modern transmission. We think Darwin would approve.

One quick way to evaluate torque capacity for a manual transmission is to compare the distance between the input shaft and the cluster gear. The greater this distance, the larger the gears and the more torque the transmission will theoretically be able to accommodate. This spec is just one of many factors that come into play but it is an important one.

As with the TKO, the output shaft is a 31-spline which uses a Ford C6 style slip yoke.

This Tremec cutaway image reveals the much larger gears and overall compactness of the transmission. The mid-plate is roughly where the bolts are placed in this cutaway.

This Tremec exploded parts view of the TKX clearly shows the mid plate position between the main case and the extension housing. Both the cluster and the main shaft are supported by this mid plate.

The callouts on this photo refer to 1: the forward shift position which will require additional parts to install, 2: the standard but reversible shift position, 3: Reverse light switch, 4: the mechanical speedometer output position, 5: the neutral safety switch.

Tremec recommends its own specific formula of High Performance Manual Transmission Fluid (HP-MTF). If this is not available, then either GM Synchromesh or Mobil 1sythetic ATF should be used.