We Swapped To A Tremec Magnum Six-Speed In Our Home Garage

We Swapped To A Tremec Magnum Six-Speed In Our Home Garage

You know what they say: “Your car runs the best it ever has, right before it breaks.” Boy is that the truth. We had just finished installing our Paxton NOVI 2200SL supercharger kit on our 2005 Mustang GT project car late last year. And if you’ll recall, we had our hands full with some dyno woes thanks to our stock drivetrain.

In other words, our stock clutch with plenty of drag passes and more than 75,000 miles wasn’t too happy trying to hold more than 500 horsepower. That incident sparked a symphony of drivetrain upgrades to hold the power. A SPEC twin-disc clutch assembly found its way mated between the stock engine and the factory five-speed manual transmission, and a one-piece aluminum driveshaft bridged the gap between the transmission and the factory rearend.

Suffice it to say, we’ve seen these used in hobby-type drag cars with 1,000 horsepower. – Jeff Kauffman, Silver Sport Transmissions

If you’ll recall last year, we took our project car back to the drag strip for post supercharger-install results. You might also recall that we completely obliterated the factory transmission’s input shaft on our first and only pass. Now that you’re up-to-speed, followers of our ’Stang project will know that this segment of the build series is all about our TREMEC Magnum XL six-speed manual swap from our friends at Silver Sport Transmissions.

Now begins the journey to installing our new brute transmission — in a home garage, on jack stands with only hand tools, in just three days. Yes, you read all of that correctly. No lifts, no fancy air tools, and no BS. Just some good old tools and a few tricks up our sleeves.

To Swap Or Not To Swap…

Moments before catastrophe would claim our drive home.

Let’s be honest, once that 10-spline input shaft broke in our factory transmission, it was extremely difficult for us to justify the cost of rebuilding that five-speed to handle what we plan on throwing at it. Not because it’s not a bad transmission, but because of our intended purpose with the car. As we’ve mentioned in previous articles, the goal of our 2005 GT is a high-10-second street/strip build, and we weren’t exactly content with a few of the factory transmission’s features. For starters, like many of you, we weren’t fans of the remote-style shifter.

Here's a trophy (and new desk ornament) from drag racing. Our speculation is that the amount of torque we are now making post-blower install, coupled with a twin-disc clutch assembly on a factory transmission, was what destroyed our input shaft. Like many before us, ours infamously snapped at the Three-Four shift down the strip. Hey, at least it was a clean break! Or maybe not...

It’s also important to keep in mind that the TR-3650 five-speed manual transmission is an OEM transmission, not an aftermarket unit. The 3650 has as a high First gear (3.37:1), a body-mounted remote-style shifter, and a smaller overall surface area (on the gears) than anything the aftermarket has to offer.

“In the TREMEC world, you have OEM and aftermarket applications,” Jeff Kauffman, VP Sales and Marketing for Silver Sport Transmissions, said. “From our perspective, the integrated bellhousing design limits its use on the Ford modular engine and prohibits the use of a scattershield if needed.”

Happy Marriage

When it came to making a choice of rebuild versus swap, the choice couldn’t have been easier. See, Silver Sport Transmissions is in the business of providing enthusiasts with complete five- and six-speed swap kits. Since the company is a TREMEC Elite Distributor, it’s afforded the privilege to completely engineer swap kits in-house, which are designed to work for specific applications.

Jeff shared with us that SST’s affairs usually center around classic muscle cars, street rods, and trucks, but the company sells TREMEC five- and six-speed manual transmission swap kits for just about every late-model car around, including our ’05 Mustang GT. From bellhousing to driveshaft and everything in between, including all necessary wiring and electrical components, Silver Sport offers everything you’ll need to complete the swap in a 2005-2014 S197 Mustang.

All of our cards laid out on the table. Well, most of them, that is.

Pictured above is just about everything that SST includes in its 2005-2014 S197 Mustang PerfectFit Kit. What’s in the box includes: a TREMEC Magnum XL six-speed manual transmission, a QuickTime SFI-approved steel bellhousing and scatter shield, a steel crossmember, a rubber isolator mount and TREMEC shifter handle and shift ball.

Silver Sport gives you the option to include a clutch assembly, flywheel, driveshaft, and any necessary wiring harnesses (such as the reverse-lockout module) for an additional cost. Keep in mind, these options are available from Elite Dealers like SST only. For our installation, we opted for the wiring harnesses, as we’ll explain more in detail during the install.

Bigger, Better, And Stronger

We could write an entire article on the benefits of the Magnum XL (and we have here.) For now, we’ll stick to the ‘need to know’ details, and the first thing you need to know is, bigger is better in this case.

Bigger gears equates to more face-width; more than 20-percent over a stock T56 to be exact, and that’s one of many reasons why the Magnum XL shines over its Borg-Warner predecessors. Because our ’05 Mustang is a street/strip build, the benefits of the Magnum XL made the most sense for us. Double-overdrive Fifth and Sixth gears keeps cruising speeds to sub-2,000 RPM on the highway (at 70 MPH with our factory 3.55:1 gears and our 2.97 First gear choice) which returns favorable fuel economy with a supercharged V8.

Triple-cone synchronizers adorn the first four gears, and double-cone synchros rest at the final overdrives and (constant-mesh) reverse. The benefits of having triple- and double-cone synchronizers allow the Magnum XL to shift at more than 7,800 RPM effortlessly thanks to a single rail-shift design which removes any unwanted notchy feeling.

Like the Viper-spec TREMEC transmissions, the Magnum XL features a stout, 26-spline input shaft and a 31-spline output.

Most of us are well aware that the Magnum XL is rated for more than 700 lb-ft of continuous torque, but Jeff said, that’s just the beginning.

“Continuous torque means the transmission will withstand a stated input for a prescribed amount of time,” Jeff explained. “This means that the 700 lb-ft of torque rating is not a peak rating, and they will withstand much higher shock loads for brief periods. Suffice it to say, we’ve seen these used in hobby drag cars with 1,000 horsepower.”

Just Think Of It As A Clutch Swap

You haven’t lived until you’ve removed a transmission on your back in a home garage — twice!

Look familiar? We’re back at our buddy’s house in his home garage for another install on the ’Stang. Much like last time, we’ll be removing the transmission and essentially performing another clutch swap with essentially just a few minor wiring details added. We jacked up our ’Stang in the air as high as we could without compromising our welfare. You’ll want to get the car high enough to slip a low-profile transmission jack in there, but not so high as to cause concern for safety. Teardown begins by removing the exhaust from the exhaust manifolds back, followed by the driveshaft (and in our case, our driveshaft loop.)

Since we were removing the old transmission for good, we decided to drain the factory five-speed of its current fluid to make it lighter and easier for removal. Have a drain pan near by for this one, as it's going to come out fast. Oh, and don't forget to unbolt the shifter from the inside.

From here, you’ll want to remove the factory starter (that top third bolt is always a pain, so make sure you get clever with your extensions) then support the transmission with a jack stand or a transmission jack and begin removing the factory crossmember. Verify you’ve unplugged all of the factory sensors before you begin unbolting the transmission for removal – the last thing you want is to rip them out while you’re dropping a 150-plus-pound transmission.


After we removed the factory transmission, we assessed the damage and concluded that we probably only damaged the input shaft itself. We suspect the cluster gear was saved, since it had looked like the input shaft took most of the brutality, but we weren’t about to tear open the vulnerable five-speed to find out. With that, we pressed on and assessed the rest of the drivetrain assembly. The photos below show a severed input shaft placed perfectly in our clutch.

Removing chunks of input shaft from your clutch assembly is not fun, nor easy. Check out the twisting force on that input shaft.

Check out that factory five-speed transmission. After numerous drag strip passes, 75,000 miles and thirteen years in operation, we’re surprised it lasted this long.

Comparing the old with the new. Our new TREMEC Magnum XL six-speed manual transmission from Silver Sport Transmissions may look to be the same size as our factory five-speed, but that is only a trick on the eyes, as the new Quick Time bellhousing isn't installed on it yet.

Once the factory transmission is out of the way, we needed to remove our SPEC clutch assembly and replace the current disc-pack (which is meant for a 10-spline transmission) with the new 26-spline discs that SPEC sent us out. As noted in our article installing our clutch the first time, this is why we chose a modular twin-disc setup – you can replace just the discs without the need to buy a new flywheel and pressure plate assembly.

Reassemble the clutch in the order you removed it, and make sure to tighten down the flywheel bolts (70 – 80 ft-lbs). Next, you’ll need to clock the new bellhousing to ensure proper transmission alignment.

In goes the new Magnum XL transmission.

While it’s a good idea to get most things buttoned up before you start modifying the sensors, don’t bolt everything down permanently in case you find yourself in need of clearance. It’s also a good practice in the event any of the changes you made need modifying or undoing. In this instance, you’ll want to do things one at a time so you can back-track in the event that something doesn’t line up.

For this installation, our friends at AmericanMuscle came in clutch (no pun intended) with a Driveshaft Shop one-piece aluminum driveshaft on the same day as our installation. The driveshaft was designed in-house by DSS specifically for '05-'14 S197 TREMEC Magnum XL six-speed swaps which use a factory-style 8.8-inch rearend. The driveshaft is constructed of 3.5-inch aluminum and uses a 1350-style 31-spline yoke as the Magnum XL ditches the pinion flange that the factory ’Stangs use. Did we mention this thing is rated at more than 2,000 lb-ft of torque? And, our Steeda Autosports double driveshaft loop still clears everything with a bit of modification – we had to clearance those two bolts at its flanges where they mate to the transmission's crossmember.

Closing The Gap

You won’t need a speedometer calibration box from Ford for this job, but you will need an updated calibration from your tuner. Because our new Magnum XL from SST uses different gear ratios than our factory five-speed, the speedometer can be out of bounds by more than twenty percent. To remedy this, your tuner can provide you with a custom calibration that will fix the issue, allowing you to receive the correct speed input from the VSS. In our case, Brent White from Brenspeed was able to send us a new version of our previous tune file.

After you’ve buttoned up the transmission and the driveshaft enough to get them in place, you’ll need to modify a few of the existing sensors and install the new reverse-lockout module. SST does include new sensor connectors in case you need them, as the ’05-’14 Mustang models vary in terms of sensor styles.

For example, our ’05 GT uses a two-wire vehicle speed sensor, but an ’07-’14 GT500 uses a three-wire VSS. Because of this, SST includes a new three-wire sensor connector in the event that you need one. For all Three-Valve models (2005-2010), we recommend using your factory two-wire VSS connector — it is much easier than converting to the included three-wire connector.

The reverse lockout module is a bit more tricky. It uses a three-wire system in which one wire sends a signal to the ECU, one is grounded and one is used to return a signal from the ECU. This is not the case for a Three-Valve PCM, so the last wire (return signal) will need to be grounded in order for the RLM to work properly.

Jeff recommends that you put at least 500 miles minimum of easy up-shifting and down-shifting on the transmission and clutch assembly, as the gears themselves need to create a consistent wear pattern. Be sure to use a transmission fluid of your choice which meets DEXRON III only, and be sure to fill your new transmission with four quarts of fluid, as they are shipped dry from SST.

If you’re in the market for a new transmission for your street/strip build, you may find Silver Sport Transmissions is your one-stop solution for drivetrain components. The company offers more than its PerfectFit kits for classic and modern musclecars, so we implore you to check out what’s available here.

Article Sources

About the author

Harrison Noble

Living in San Diego for most of his life, Harrison was exposed to a variety of cars at an early age. His passion for anything that is fast, or has a V8, brought him to Power Automedia.
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