Street Fight: Upgrading An SN95 To A TREMEC TKX And SPEC Twin-Disc

Street Fight: Upgrading An SN95 To A TREMEC TKX And SPEC Twin-Disc

You might recall a few years back when we took our black SN95’s 9-inch rearend apart in order to swap over to a more streetable differential. Instead, opting for a Detroit TrueTrac from Eaton, along with a set of their 3.70:1 Super Gears. Well, that improved drivability enough that the car was getting a lot more use. Unfortunately, on the last road trip we took with the car, as we were coming back down the mountain, a noise started to develop in the transmission.

That noise grew to a howl, which we were smart enough to know, was not a good thing. Besides the fear of being stranded somewhere remote, we also looked to the future of the car. If you’ve been following our small-block Ford engine project, Retro 5.0, you know we’re going to be putting some serious power to the tires, especially with version 2 already underway.

What that meant for us was we needed to put all the needed components for that eventual power level into the car now, so that it would check off a list item ahead of time. Now, in years past, a transmission and clutch capable of holding big power meant there was no skipping leg day, since every day you drove the car would be leg day.

Thanks to modern technology, that isn’t necessarily true anymore. The parts needed to hold stupid power now have impressive street manners, so installing them behind the mostly stock 5.0-liter currently in the car isn’t going to make it unpleasant to drive, or so they say. We’re about to find out as we install our SPEC twin-disc clutch and TREMEC TKX transmission.

While this job can absolutely be completed at home in your driveway, using a lift makes the task orders of magnitude easier.

Evolution Of A Technical Knock Out

For a long time, TREMEC has been synonymous with late-model Mustang performance. From the original aftermarket five-speed TKO transmission debuting the same year our 1994 Mustang GT was made, through the TKO-II, to the TKO-500 and TKO-600, the iconic T-5 replacement has been a staple. In fact, I remember my first experience with a TREMEC was hooning around the streets of Southern California as a youngin’ with a former TREMEC employee in his ‘94 GT with a TKO-II in it.

Back from the detour down memory lane, TREMEC has once again improved the TKO-line of transmissions, with the new five-speed TKX. Designed as a direct replacement for any previous TKO variants, the TKX’s extremely compact dimensions make it an almost direct swap for our 1994 Mustang GT’s stock T-5. We say almost, because there are a few other changes we need to make, which we’ll walk you through in a bit.

You can tell in this photo just how compact the TKX really is. Not only is it a direct fit for T-5s, it’s compact dimensions and light weight make it ideal for using in all sorts of different applications.

The TKX is available for Ford applications in four part numbers. The variables are 10 or 26-spline input shafts, a more-aggressive 3.27 or less-aggressive 2.87 first gear, and three available overdrive options (0.81, 0.72, and 0.68). In addition, there are three available shifter locations if you are looking to put this in something other than a Mustang, and comes with a factory-installed short-throw shifter.

Internally, there have been improvements in both materials and design, with heavy-duty multi-cone synchronizers and hybrid bronze-carbon synchro rings providing buttery-smooth high-RPM (7,500 rpm) shifts. The internal design and layout offer both increased strength and reduced noise. The gears are wider than previous models and made from ASTM 4615 steel (along with all of the gear shafts), while caged needle bearings are utilized throughout the transmission.

If you look at this cutaway, the most noticeable feature is the width of each of the gears in the gearset. By increasing their width, the internal strength is significantly increased.

The TKX’s compact design doesn’t mean weak, as the highly refined three-piece aluminum case design is the most advanced of any of the previous TKO designs. Using advanced modeling software, the external ribs maximize the case stiffness, and TREMEC advertises 600 lb-ft of torque capacity and a maximum engine speed of 8,000 rpm through the traps.

For our Mustang, we opted for P/N: 17765, with the 2.87 first gear and the 0.68 overdrive. Our reasoning being, that we have 3.70s in the rear, and picked those specifically for the balance of performance and aggressiveness. Since this is a street car, we really don’t want an aggressive first gear. Conversely, on the highway, we want the most overdrive we can get, as we don’t expect to ever see fifth gear in a performance scenario, so the benefits of the close-ratio 0.81 overdrive will never be realized.

Here you can see the available gearsets and input shaft combinations. We opted for the TCET17765 model with the shallower first gear and deeper overdrive since this will primarily be a street transmission.

Multi-Disc Changer

You might remember a few months ago, we published an article discussing SPEC’s modern clutch offerings. That article came from our discussion with SPEC regarding this project. After talking about the powerplant we were planning and the transmission we wanted to use, SPEC recommended their Super Twin clutch kit with organic friction material and a billet aluminum flywheel (P/N: SF05SST-O). Let’s discuss the reasoning behind that choice.

SPEC’s Super Twin clutch lineup combines a light pedal feel, tame drivability, and extreme torque-holding capabilities, ranging in capacity from 847 lb-ft to 1,631 lb-ft thanks to varying clamping pressure and friction materials. Since our original power numbers from Retro 5.0 were 829 horsepower and 673 lb-feet of torque, we felt that there was no need for any drivability sacrifices to be made, when the basic organic-material twin-disc clutch assembly gives us some extra wiggle room.

On the left you can see the SPEC billet aluminum flywheel, that is not only lighter than stock but comes with adjustable weights for 50 and 28-ounce imbalances, as well as a zero-balance option. On the right is the billet pressure plate for our SPEC Super Twin clutch, that is a shame to hide under the bellhousing.

Between the forgiving properties of the organic friction material and the light clamping pressure required from the pressure plate, thanks to the multiple clutch discs, the SPEC twin disc offers a clutch pedal feel that is far more similar to the stock clutch than even some other brands’ “stage 1” offerings. And while it has been mentioned that the mass of a multi-disc assembly can eclipse that of a traditional single-disc assembly, the lightweight billet-aluminum flywheel definitely helps offset that.

The aluminum flywheel in the kit is SFI-approved, so no concerns about safety compliance there. It’s much lower mass than stock also leads to a faster free-revving of the engine, though not to the point of being difficult to control by any means. Another really cool feature of the SPEC flywheel included in the kit, is the swappable weights on the flywheel itself. It comes with the correct weight for a 50-ounce imbalance preinstalled, along with a weight for a 28-ounce application in the package. If you are running an internally balanced engine, like we will be with Version 2 of Retro 5.0, you can simply remove the weight and be zero-balanced.

Swapping A TKX In Place Of A T-5

We mentioned earlier that swapping a TKX into our SN95 originally equipped with the venerable T-5 would require a few non-OEM parts to make work. First, I want to stress, these parts swaps are more administrative than anything. While we were extremely fortunate to have Doug Driggers and the professionals at Arizona Race Company to perform the swap for us, the job itself isn’t complicated or custom. If you can remove and replace a transmission, you are more than equipped to complete the swap yourself.

For a majority of these parts, our friends at Speedway Motors had the parts in stock on the shelf, so we were able to run down to their Phoenix showroom and pick them up. (They even showed us around the warehouse and let us use their order-picking system to pick our own order, which is a whole lot cooler than it sounds.) The first component you’ll need is a TKX-specific bellhousing. If you already have a TKO-specific bellhousing, that will work perfectly. In our case, we have the OEM bellhousing in place, so we went with TREMEC’s cast aluminum small-block Ford bellhousing (P/N: TCEP8639)

The TREMEC aluminum bellhousing is modeled after the standard Fox Body bellhousing. As such, it requires a 1982-’93 shift fork and will move the transmission forward 5/8-inch from the standard SN95 configuration.

Designed to fit all 6-bolt Ford engines and built with OEM design and quality, the bellhousing comes equipped with the clutch for pivot ball stud preinstalled. One note here is that the new TREMEC bellhousing requires the use of a 1982-’93 Mustang shift fork, which is important to remember when swapping into a 1994-’95 Mustang GT. Also required are specific-length transmission-to-bellhousing bolts, so make sure you have those before you get everything apart.

To mount the transmission, we used one of Stifflers TKX-specific crossmembers (P/N: TCB-M03) along with the TKX-specific installation kit (P/N: TCB-TKO). Here’s a chance to learn from our mistakes. We forgot to order a new urethane transmission mount so we had to reuse the stock 30-year old mount, and we’ll have to get back under the car to swap that soon. It can be done using the stock mount, but it’s not ideal.

The Stifflers TKX-specific crossmember makes installation easy. Unfortunately, we forgot to order a polyurethane transmission mount, so we had to reuse the stock rubber one.

Moving back from there, you’ll then need a 31-spline yoke for your driveshaft to mate with the output shaft of the TKX. These are common parts across the aftermarket, and we went with the Spicer 31-spline 1330 yoke specifically for TREMEC transmissions. Additionally, if you are using a stock driveshaft, you’ll need to either use a driveshaft spacer, or lengthen the driveshaft by 5/8 inch. Luckily, we already have a custom driveshaft in the car with enough extra length built in, and it fit perfectly.

One thing to note, is that 5/8-inch setback is caused by the Fox-length bellhousing, and only shows up in ’94-’95 Mustangs. While the minor 5/8-inch difference still puts the shifter into the stock rubber shift boot, it does place the shifter another 5/8-inch away from you. With the “SN95 lean” already a thing, this does slightly exacerbate the problem. But, don’t worry, I think I have the fix for that which you’ll see in an upcoming article.

On the left you can see the stock driveshaft slip-yoke, and on the right is the Spicer 31-spline unit. Swapping yokes is a straightforward process involving copious amounts of percussive persuasion.

The TKX shifter utilizes the familiar 7/8-inch-on-center shift handle bolt spacing, however, the Tremec shifter is threaded at 3/8-24 instead of the Mustang’s stock 8mm thread. What that means is that you will either have to run longer bolts with a nut on the other side to retain the same shift handle, or throw the handle on the drill press and open up the 8mm holes to 3/8 inch. We did the former to get home, and the latter once I was back at my shop.

The TKX utilizes the standard speedometer gear in this application, with a provision for an electronic speedometer pickup as well. We had a 23-tooth speedometer gear for the 3.70s with the stock driven gear (which we hadn’t installed since the rear gear swap), but the TKX comes with a 7-tooth driven gear instead of the original 8, so with the new math, we ideally needed a 21-tooth. Between the 19-tooth gear installed in the car, and the 23-tooth in a bag, we opted to stick with what was already in the car for two reasons – one, the 19-tooth gear was ever-so-slightly closer to correct (thanks to the 3.70:1 ratio of the 9-inch gears, vs the 3.73:1 of the 8.8-inch ring and pinion), and having a speedo that reads slightly high is better than one that reads slightly low, in our opinion.

The TKX shifter uses the standard 7/8-inch on-center bolt spacing, but uses a larger 3/8-24 thread, as opposed to the OEM 8mm thread. However, that is easily remedied by either using a captive bolt setup with your 8mm shifter, modifying your shift handle, or purchasing a shift handle with 3/8-inch holes.

First Drive With A Twin-Disc Clutch And TKX Transmission

With the transmission in the car, all that was left to do was install the billet clutch quadrant, new clutch cable, and firewall cable adjuster, and give ‘er a test drive. A few quick slack adjustments later and the car was driving just like it was on the way down to the shop, albeit with way less noise coming from the transmission. TREMEC recommends a 500-mile break-in period on the new transmission, and what better way to break the new twin-disc clutch and TKX in, than in major metropolitan rush-hour traffic?

Now, simply by chance, we happened to wrap up the installation right as Phoenix rush-hour started. Anyone who daily drives a stick car knows how bad bumper-to-bumper traffic can be with a stock clutch, let alone one rated for 847 lb-ft of torque. But, much to my surprise, the twin-disc clutch’s advertised light pedal pressure is 100 percent legit. The only stress I had to deal with on the drive was from moronic commuters, not anything involving my left leg or right arm.

I’m currently still short of the 500-mile break-in mark, so I haven’t revved it up and dumped the clutch yet, even though I really, really want to. But just wait, we’ll have some updates on that front soon. In the meantime, if you’re considering a transmission upgrade or an outright five-speed swap, the TKX is an absolutely stellar option regardless of where you choose to drive your Mustang – the street, drag strip, or road course.

The original T-5 and bellhousing. They’ve lasted 30 years and a little over 114,000 miles, with some pretty decent abuse thrown in along the way.

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About the author

Greg Acosta

Greg has spent nineteen years and counting in automotive publishing, with most of his work having a very technical focus. Always interested in how things work, he enjoys sharing his passion for automotive technology with the reader.
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