Quit your whining! This is a popular phrase nowadays, where everyone has a beef about something. When it comes to the Ford 9-inch rear, the complaints are few and far between. One, however, is justified nowadays. While it’s about the toughest rearend out there, it can be a whiney if it’s not set up perfectly, or if the origin of your gear set is unknown. Now, there is a solution: The Detroit Speed & Engineering FN988 diff, an 8.8-inch gear assembly in a 9-inch-style dropout case.

The FN988 diff combines the best of two popular aftermarket pieces: the nearly indestructible 9-inch Ford, with the street manners and civility of the more modern 8.8 diff. Detroit Speed developed an 8.8-inch diff mounted in a dropout third member. This new design, which the company calls the FN988, uses standard Ford 8.8 components in a dropout case that bolts into any 9-inch format axle housing.

Recently, we did a story about Detroit Speed’s new Fox Mustang SLA suspension kit. This time, we’re headed to the opposite end of the car to explore their latest trickery.

According to Detroit Speed, the very thing that helps make the 9-inch so strong – its low-mounted pinion – can also create a lot of gear noise. Even fresh-from-the-shop, newly assembled 9-inch rearends with all-new components have some amount of the characteristic whine this venerable design is known for. The FN988 is an ingenious solution to a fairly new problem.

“When Ford quit making 9-inch gears, the aftermarket supported them and manufactured them. The aftermarket makes really strong gears, but we found over time that differing quality standards of gear manufacturers made it difficult to get high quality Ford 9-inch gears that didn’t make noise,” said Nate Peterman, Senior Project Manager at Detroit Speed & Engineering.

The advantage of the 8.8, which Ford replaced the 9-inch with, is the location of the pinion in relation to the ring gear. It is raised when compared to the 9-inch, which increases gear efficiency and reduces noise. Ford obviously saw value in it, and the strength difference is negligible for most street cars. The efficient operation of the ring and pinion leads to less noise and increased service life.—Nate Peterman

It’s Inside That Counts

When you take the cover off an 8.8 differential, you see two main caps holding it in place. Under hard acceleration, the pinion wants to shove the differential out of the back of the case. In a 9-inch, the caps and everything holding the diff in place are much beefier than on an 8.8 rear. The FN988 diff uses a 9-inch-style cap, so it has the adjuster rings like a traditional 9-inch.

“It’s really easy to set up,” said Peterman. “The traditional 9-inch-style spanner nut replaces the carrier bearing shims, so setting up backlash is a breeze. The forged steel bearing caps also add additional strength over standard cast-iron caps.”

The 9-inch offers a major advantage over the Ford 8.8 by having a center section that can easily be changed from the front. That is a huge plus. Two experienced people can generally swap center sections in a 9-inch in about an hour — and the gears in the new center section are already set up. This is important if you need to swap center sections in a hurry for open track, autocross, or drag racing purposes.

All the gear sets for the FN988 come from Ford Racing, so they’re high-quality OE gears. The standard differentials are Eaton True Tracs, a proven commodity in the racing world. All center sections are assembled with high-quality bearings by people who have been building rear gears for the racing industry pretty much their whole careers, according to Peterman.

Optional Ratios And Splines

There are many differential and gear ratios available for the FN988, and pretty much anything that was available for the 9-inch is now offered for the 8.8 To simplify all the options available, Detroit Speed has specified 31 spline axles as standard equipment, but it does offer custom gear ratios and custom differentials. Optional REM polishing and break-in is an available option on all center sections to provide the customer with a worry-free, ready to install unit.

“So if a customer calls in and they have a unique case, we’ll have them talk to our expert, and work out a package for them. We can build whatever’s needed,” Peterman said. “For ninety five percent of customers, the 31-spline is plenty, especially with street tires. The street tires aren’t going to hook up well enough before you’ll break an axle spline. The 33-spline and 35-spline upgrades are popular in the 8.8 for the drag race community.”

Fully assembled, the FN988 diff starts at $2,500.  One thing about the FN988 to know is because it is using an 8.8 instead of a 9-inch differential, the spline location in the diff is offset from the 9-inch, so it does require new axle shafts to account for this offset.

“That’s important for someone changing from a 9-inch center section. That’s just a cost that needs to be known going in,” Peterman said. “For a new build, obviously, it’s negligible because you’re buying axles anyway.”

The beauty of the FN988 diff really shines because the 9-inch-style rear has proliferated among the various rear suspension upgrade kits and complete chassis being sold today. Virtually every company will sell you a suspension set-up that accepts the 9-inch rear. The FN988 is an ingenious solution that offers improved strength and reliability, but with low noise.