The Best Of Both Worlds: Detroit Speed’s FN988 Hybrid Center Section
By Ron Ceridono
Even when Flatheads dominated in the early days of hot rodding, the side effects of a hopped-up engine meant that dealing with stripped ring-and-pinion gears, broken axles, and hubs with sheared keys was almost guaranteed. Things only worsened in the early ’50s as overhead valve V8s pushed the Flathead Ford and other early engines aside. The cure for rear end woes was often those wide and heavy assemblies found under Oldsmobiles and Pontiacs (both were out of production after 1964) until Ford unknowingly came to the rescue. Since its introduction, Ford’s 9-inch rear end has become the preferred performance axle assembly for modified vehicles.
Ford’s 9-Inch Rear End
Produced from 1957-86, the Ford 9 inch rear end is Hotchkiss style, which is to say the case containing the third member of the ring-and-pinion assembly is removable as a unit, making gear changes relatively simple. Of course, the real reason for the popularity of the Ford 9 inch rearend is its strength, primarily due to the relationship of the ring-and-pinion gears. To provide increased tooth contact the hypoid distance, which is the offset between the ring gear and pinion centerlines, is greater than that found in most rear ends. However, while this greater tooth contact does increase strength, the design has some drawbacks; a certain amount of efficiency is lost due to increased friction, and gear noise increases noticeably (often referred to as nine-whine).
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Why the Ford 9-Inch Rearend Became Popular Among Hot Rodders
To replace the venerable 9-inch rearend, Ford introduced the 8.8 axle assembly. It was first used in 1983 under Ford trucks and most V8 equipped cars from 1986 on. These rear ends are the Salisbury design, which has the third-member components mounted in the axle housing with a removable rear cover on the back side. This style of rear end makes gear changes much more involved compared to a Hotchkiss. Another difference between the 9-inch and 8.8 is the method of retaining the axles. The 9-inch uses pressed-on bearings and lock rings on the axles, which are held in the housing with retainers bolted to the flanges on the axle tubes. If an axle should break, the wheel will stay attached to the housing. By comparison, the Ford 8.8 axles are held in the housing by C-clips inside the differential case. That means a broken axle, along with the wheel and tire, can come completely off the car (there are C-clip eliminator kits that resolve that problem, and rear disc brakes act as retainers, to some extent).
Efficiency and Noise Issues
The obvious question is, why did Ford replace the 9-inch with the 8.8? There were several reasons—cost being one. The Salisbury housings use a cast center section with pressed-in axle tubes, all of which makes them cheaper to produce, but there were marketing considerations that influenced the change as well. The 8.8 is more efficient thanks to less hypoid angle than the 9-inch, which helps gas mileage ratings, a major concern for consumers. The 8.8 gears are quieter in operation, which is a customer satisfaction factor. New car buyers want the most miles per gallon possible while listening to the stereo, not the gears in the rearend.
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The FN988, A Hybrid Solution
When Detroit Speed & Engineering began investigating alternatives to the 9-inch, they found that the 8.8 rear end could reliably absorb an amazing amount of punishment, as was proven by Ford racers, notably the 5.0 Mustang crowd. So, the idea was born to combine the best features of the 9-inch and the 8.8 and create the FN988. The hybrid FN988 uses a 9-inch style Hotchkiss carrier, which makes setting up the ring-and-pinion considerably easier than the Salisbury design and makes swapping gear sets easier. In addition, the C-clips are eliminated as the axles are retained like a 9-inch.
Benefits of the FN988 Center Section
While the 9-inch Ford rearend is tough to follow, there’s much to appreciate about Detroit Speed’s FN988. It’s more efficient than the 9-inch thanks to reduced internal friction, which arguably may offer a performance and mileage increase. Certainly, the reduction in gear noise will be a welcome change. Since the 8.8 gear set is strong enough for all but the most abusive applications, it’s a good option for hot rodders, and Detroit Speed has made a good thing better with the new FN988. MR
Source
Detroit Speed & Engineering
(704) 662-3272
detroitspeed.com
Click on this issue’s cover to see the enhanced digital version of The Perfect Center Section For 8.8 Gears.
