Justin Fivella
October 11, 2013

Ask anyone who's owned a manual-transmission-equipped Fox-body and there's a good chance they have a story about grenading the stock T5 transmission. Some snap at the hit, others break on a good powershift, but most often they let go in the middle of a long Third-gear pull. Whether you've blown one or not, you fall into two categories: the ones who have blown a stock T5 and the ones who will blow a stock T5.

That's not to say the World Class T5 is a horrible transmission. Quite the contrary, as it's a lightweight, slick-shifting, synchronized five-speed box that can hold considerable power for its size. At stock(ish) power levels, the T5 is a great transmission, but bring the torque output above 350 lb-ft with sticky tires, and its days are numbered. Even the strongest of stock T5 boxes, the '93 Cobra spec units, were only rated for 310 lb-ft, so you can imagine our trepidation in feeding 530 lb-ft of torque at the wheels (that's roughly 600 lb-ft at the crank) through our stock T5.

The first few track outings with the stock components were anything but frustrating since forward progression was hindered by the worn differential and old T5. Any attempt at powershifting was met with protest, and forcing the issue would have ended in disaster. Instead, we launched soft and granny-shifted our way to some unimpressive e.t.'s. Miraculously, the stock T5 survived without spilling its guts, but a closer examination of the input shaft after it was removed revealed tons of free play. In other words, it was one good shift away from leaving the building.

One Bad Box

In search of an ever-elusive 10-second smog-legal timeslip, we decided another stock T5 just wasn't going to do. While thoughts of a six-speed box were tempting, we decided to save the additional 50-plus pounds and stick with the tried and true G-Force Racing Transmissions Street T5.

That's right, G-Force has been at it for over 20 years and builds transmissions for many top-level NASCAR and NHRA teams. Needless to say, they're damn good. Over a decade ago, G-Force saw the need for a reliable T5 transmission when Mustang owners began pushing their cars beyond the capabilities of the stock boxes. After tons of R&D, G-Force created the box before you, a fully built badass that's capable of 600-plus horsepower and 500-plus lb-ft in a 3,300-pound car launching on sticky tires.

Not one to leave anything to chance, G-Force redesigned the internals, and in turn, produced a T5 unlike any before it. In place of the wimpy stock components, G-Force uses high-nickel-content gears that are fabricated in-house, along with a heavy-duty, 26-spline input shaft and bronze/aluminum or steel shift forks.

Photo Gallery

View Photo Gallery

The high-nickel-content gears are softer than the stock gears, but actually much stronger. The increased physical size of the gears gives them strength, while the softer material helps prevent failure from shearing compared to the stock units. Because of the material, G-Force recommends a special Syngear oil that protects the gears from undue damage. This special stuff can be purchased from Syngear directly or through G-Force.

As for the rest of the internals, the mainshaft is also beefed up from the stock 10-spline shaft to a hardy 26-spline unit forged from 9310 alloy steel. The burly unit helps resist deflection and adds more mating surfaces for the clutch disc.

According to G-Force, gear spreading is a big problem with the stock transmissions. When the gears are loaded, the meshing teeth attempt to push apart from each other—the greater the load, the more severe the problem. The stock mainshaft only exacerbates this problem as it deflects under load, leaving only the gear tips (the thinnest area of the gear) in contact, which ultimately leads to catastrophic failure. As for why Third gear is the weak spot on stock units, G-Force explained that the third cog is not only the thinnest of gears, but it's also farthest from any bearing, which means this lack of support leads to extreme gear spreading.

Along with a total overhaul of the components, G-Force took it several steps further by offering multiple ratios and even different types of gear engagements. Since the Smog-Legal Killer will predominantly be driven on the street, we opted for the synchronized gears (like the stock units) to aid in smooth shifts and quiet operation. If you're after the ultimate race box, G-Force also offers dog-ring, straight-cut, and NMRA-approved helical-cut dog-ring gears for lightning-fast upshifts at the strip. The only downsides on the street are gear whine (which can be cool) and some stubborn shifts at slow speeds that may need a little double-clutching, especially on downshifts. Although neither are a big deal, we wanted to preserve the street-car traits in this little coupe.

Photo Gallery

View Photo Gallery

If that's not enough, G-Force also offers the boxes in different gear ratios with several different Overdrives to suit anyone's fancy. We opted for a taller 2.95 first (stock is 3.35) with a deep 0.63 Overdrive. The taller first is welcomed on the street and strip, where more vehicle speed can be achieved in First, thus prolonging the shift to Second until the car is well under way. The tall overdrive also comes in handy on the super-slab, where the tach sits below 2,000 rpm at cruising speeds.

G-Force offers several different First-gear ratios, and even has close-ratio boxes with ultra-shallow Overdrives for road-racing, or drag applications where Fifth gear is needed as another cog for acceleration, not as an overdrive.

Although there are other five-speeds on the market like the TKO-500/600, and six-speed units like the almighty T56, the lightweight G-Force T5 still undercuts the other transmissions by nearly 50 pounds and is more than up to the task at our power levels. As for the installation process, it's a simple remove-and-replace since it's still a T5 case.