Michael Johnson Associate Editor
November 20, 2006
Not that you need a steeper set of gears to do a burnout, but it certainly doesn't hurt. We're just glad car owner Mike Zamboni knew how to do a burnout. At the time of the gear swap, Mike's '05 Mustang GT had Falken wheels and tires, a C&L Performance cold-air kit, a DiabloSport custom tune, and a plethora of BMR Fabrication suspension pieces.

Horse Sense: Michael Zamboni, the owner of the '05 Mustang GT featured here, didn't wait long to dig further into the car. He added a ProCharger P-1SC in an initial attempt to go fast, but too many dragstrip passes spelled the end of the original Three-Valve. Michael now has a new engine ready for more boost, and that's good since he's stepping up to a ProCharger F-1 supercharger for a shot at the nines.

With the introduction of a new Mustang, there's something intimidating about getting into the car and making it faster. When the fuel-injected '86 GT was introduced, supposedly the age of hot-rodding was over. Obviously, that was incorrect, as EFI opened up a whole new performance world that still thrives today. Another intimidating factor was thrown into the mix when mass air meters became standard on all Mustangs in 1989. Then came electronic speedometers, which threw everyone for a loop, especially when it came to gears-every Mustang enthusiast's favorite swap. Each new Mustang always seems more difficult to modify at first, but when it's all said and done, it's still comprised of nuts and bolts, and it still has gears. Sometimes, the aftermarket has to spend time working itself into the computers to enable us to play nice with the electronics.

We've done a million gear swaps in these pages, but to the rookies among us who have no clue why anyone would want to change the gears in a Mustang, here's why. A lower (numerically higher) gearset gets the engine into its powerband quicker than a higher (numerically lower) set of gears. This is known as torque multiplication. With a lower gearset, the engine's horsepower and torque are multiplied at a more rapid pace when compared with a lower set of gears. A 3.73-4.30 gearset accelerates a car more rapidly than a 2.73-3.27 ratio. Consequently, a higher gear ratio allows your Mustang to run at a lower rpm on the highway. Finding the right balance between acceleration and highway cruising rpm is our quest with every gear swap.

With today's overdrive transmissions, however, it's fairly easy to step up to a steeper gear without worrying about over-revving the engine on the highway. Every Mustang enthusiast has a different view on which rear gear is perfect. Some want the everyday friendliness of a 3.55-3.73 gear, while some want to go all out with a 4.10-4.30 or lower gear. If your Mustang rarely sees highway use, you can get away with a low gear, but if you do both city and highway driving, a 3.73 gear would be perfect in our opinion.

Of course, what do we know? Mike Zamboni, the owner of the '05 Mustang GT on which we're performing this swap, initially went straight for the jugular with 4.30 gears. In our defense, Mike's main goal is to drag-race the car, with a bit of street action thrown in the mix. Mike has an enclosed trailer to haul the Mustang, so we don't think his thought process fits every Mustang enthusiast. Our best advice is to ride in a Mustang like yours that's had its gears swapped and see how you like it. One thing's for sure-you'll be hooked on torque multiplication before the end of First gear.

We're doing this buildup because Mike exploded the Mustang GT's rear with a 4.30 gear and another aftermarket differential prior to this round of modifications. When he exploded the previous differential, he wanted one that would hold up to his abuse. He chose Detroit Locker's Truetrac, and we enjoyed one of them on a daily basis in our former Fox coupe, so we agreed with his choice. We reported on this swap in the May '05 issue ("Swapping Ends," p. 151) using the same components, even though we chose 3.73 gears. Mike chose Moser 31-spline axles and a step back to 4.10 gears. He believed the 4.10s would complement his Mustang GT better than the 4.30s-hence the gear swap at this time. He must have been on to something because after the rearend buildup, he visited Bradenton Motorsports Park and ran consistent 12.50s on Mickey Thompson E/T Street Radials. Mike also blistered a 1.33 short time with the radials. We know what you're thinking, but we've seen the timeslip.

This swap took place at BMR Fabrication in Thonotosassa, Florida, which is just a short ride from our Tampa offices. When we arrived, the man turning the wrenches, Dave Piercey from Mustang Performance, already had the housing gutted. [Perhaps Johnson was late?-Ed.] Even though some prefer to lay out a mesh pattern on the ring gear to set up the rearend, Dave likes to use a barrel gauge, or an OTC pinion-depth tool. The tool component on the right in this image is the actual depth-setting tool, and the bearing is the new pinion bearing. The new pinion bearing race is already in place within the housing, which is done prior to measuring the new pinion's depth. Dave will start the process by installing the depth-setting tool in the housing.

With the depth-setting tool in place, the cylinder is installed in the same manner as the differential would be- torque specs and all. You can see the depth-setting tool just past the cylinder.

Dave uses the pinion-depth tool to determine what thickness shim to use. The shim we're checking slides on the pinion prior to its installation in the housing. This shim determines the pinion depth within the housing, and this is just one measurement to make sure the ring-and-pinion mesh together in silent harmony. Using an actual shim is something like using a feeler gauge.

Since finding the correct pinion depth is out of the way, Dave can prepare the rest of the rearend components for installation. One of the first things he does is grease the bearings using white lithium grease prior to their installation.

Dave uses a press to install the pinion bearing on the pinion. The majority of the bearings in this exercise must be pressed in using a press or some other type of special tool. This kind of swap is best left to those with these tools on hand. That isn't to say you can't do a gear swap at home, but it's next to impossible without a press and the special tools found only in a shop.

With the bearing installed, Dave places the pinion inside the housing with the correct shim determined by the pinion-depth-setting tool. The pinion is held in place by a nut on the front side of the housing.

Dave uses an impact gun to tighten the pinion to the housing. He doesn't go crazy on it, however, because the pinion has to be torqued in place. Rather, he uses a torque wrench to check the resistance, or preload. Dave goes by inch-pounds for this, and he likes 5-10 in-lb on used bearings and 20-25 in-lb on new bearings. Many people perform this function by feel, but Dave believes it's best to use the torque wrench.

With the pinion installed, Dave turns his attention to the ring gear and differential. Here, he's installing the ring onto the Truetrac differential. Some people use a press to draw the ring gear up to the differential, but in this case Dave uses the supplied bolts to attach the ring gear. He follows a star pattern, such as one used to tighten a wheel, to avoid warping the ring gear.

Once the ring gear and differential bearings are installed on the differential, Dave places it into the housing. It's awkward to put the differential into the housing, so he's careful not to drop it on his foot. Shims are on each side of the differential, and on the outside of each bearing and race. These shims play a big part in making sure the ring gear and pinion mesh well. Basically, the shims ensure the differential assembly is installed squarely within the rearend housing.

Dave threads in by hand a set of ARP studs for the main caps prior to installing the differential's main caps.

With the caps in place, he torques them to spec.

To make sure the correct shim is in place, Dave checks the backlash using a dial indicator. He uses a micrometer to measure for the correct shims.

Dave uses a press to install the GT's ABS exciter rings. He put the axles in a freezer and heated the exciter rings to make them easier to install. Heating expands the metal, and cooling contracts it. Mike purchased new exciter rings because the existing rings often become distorted when you take them off, making them useless.

Dave slides the axles into the axle housing. Be careful not to damage the outer seal when reinstalling the axle. When we performed this swap, the axle lengths were given to Moser, then the axles were made and shipped to our installation facility at BMR Fabrication. The '05-up Mustang axles are a different length side to side. Moser has the proper-length axles in stock.

Here are the C-clips used to keep the axles in place.

Dave gives the C-clips a little tap to drive them home. With the C-clips in place, pull the axles out to properly seat them and the C-clips.

The Truetrac uses this plate to encompass the differential assembly.

Then the Truetrac uses this little C-clip to add even more protection and hold the plate in place.

Car owner Mike Zamboni decided to use one of Trick Flow's rearend girdles to strengthen the assembly. He didn't want to have another differential failure, and the girdle goes that much farther with added rigidity. Dave applies a liberal amount of silicone to keep a differential leak at bay.

The Trick Flow girdle is installed the same way as a factory rearend cover, except it has torque caps for the differential's main caps. The Trick Flow girdle's torque caps offer increased differential strength by keeping the assembly from moving around within the housing under a load.

Dave torques the Trick Flow girdle's main caps to 5 lb-ft, then he locks down the jam nut.

To properly lubricate the GT's 8.8, we used Royal Purple's Max-Gear 75W90 gear lube. Max-Gear is designed to give maximum protection to heavily loaded gears while keeping parasitic powertrain losses to a minimum. Royal Purple's Max-Gear features proprietary Synslide additive technology with the highest-quality synthetic oils.

Using a 3/8-inch-drive ratchet, Dave attaches the fill hole's drain plug.

Now we can finish the job by reattaching the GT's driveshaft.

Mike slides the Falken Koblenz wheels back on the car to get it ready for the street. Or should we say...the burnout.