Deciding on which Ford differential is a better choice for your vehicle depends on the vehicle's intended use. Because differentials, or rear-ends, transfer power from the axle to the wheels, vehicle function and engine power both play a role in the decision. Differential size designation comes from the ring gear, which initially transfers the axle input torque. Housing unit strength, gears used and axle spline numbers are all considerations in differentiating between Ford 8- and 9-inch differentials.
As the stock rear-end on most Ford vehicles of the 1960s and 1970s, 8-inch differentials are lighter in construction and have a 28-spline axle. According to Ford Muscle, there aren't many cost-effective upgrade options for the 8-inch differential. However, they came stock with many of the same internal components, including spider gears, side gears and bearings. Ford Muscle maintains that the 8-inch differential will hold up behind the strongest 289 and 302 cubic-inch engines while being 100 lbs. lighter than the 9-inch differential.
Sometimes paired with high-performance car and heavy-duty trucks, the 9-inch differential is considered heavier and more durable. According to Ford Muscle, they provide a better transfer of power due to the availability of both 28- and 31-spline variations. The 9-inch differentials are also equipped with larger wheel- and axle-bearing housings. Overall, the 9-inch differential has more upgrade possibilities for heavy-duty uses. Another major difference, according to Ford Muscle, is the strength of the 9-inch unit casing itself, especially rugged duty "N" cases, intended for units with 31-spline axles and issued with trucks and vehicles with large engines.
Identify The Differences
A quick way to differentiate between the two, according to Ridgecrest, is to place a socket with an extension on the lowest housing bolts, where the axle meets the housing. If the extension bar hits the case, it is a 9-inch differential. If there is no obstruction, it is an 8-inch rear-end. Ford differentials normally have an identification tag and comparing the model number to a listing similar to the one found on drivetrain gives specifics on the unit in question.
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