The opinion duel rages on. Is there a best?
The stock 12-bolt’s Achilles’ heel is its C-clip axle retention--if the retainer or axle fails, the axle falls out. Note the custom driver-side steel cap retained by oversize 1/2-inch bolts, it minimizes ring-gear deflection.
The 9-inch third-member is removable after pulling the axles, so it’s much easier to swap gears than on a 12-bolt. For serious performance use, upgrade to an aftermarket nodular-iron third-member and front bearing retainer.
Axe the 12-bolt’s C-clip axles: Replace the stock axle flange (right) with a weld-on housing end (left) that accepts custom bolt-in axles. Bolt-on C-clip-eliminator kits are cheaper, but the more durable weld-on end accepts widely available axle bearings and seals.
Progressively higher numerical gear ratios require increasingly smaller drive-pinion gears. Ford compensates with thicker ring gears instead of varying differential-case-flange heights. Compare the U.S. Gear 3.00:1 ratio (left) with the 6.50:1 gearset (right).
On 4.10:1-and-up ratios, the Ford pinion’s tooth-end becomes progressively more massive. Compare the U.S. Gear Ford 4.11:1 pinion (left) with the Chevy 4.10 (right). With no diff case breaks, there’s more room for aftermarket 9-inch gearset makers to increase pinion size.
The 12-bolt Eaton posi’s secret is that it has clutches on both sides, outboard of the side-gears. Fairly durable right out of the box, the Eaton is both tunable and upgradable. Eaton experts like Tom’s offer up to seven stages of upgrades.
The differential case flange height in a 12-bolt varies to correctly mate the ring-gear to the drive-pinion. This saves weight but requires a costly case swap when installing gears that fall outside a given differential case range. Cases, from left: four-series (3.90:1-and-up ratios), three-series (3.07:1-3.73:1), and discontinued two-series (2.56:1-2.73:1).
The Ford’s stock Traction-Loklimited-slip diff is not as durable as the stock Eaton Posi used on 12-bolts. On the other hand,currently available Ford 9-inch limited-slip diffs all use four pinion gears, whereas Chevys use two.
Ford's 9-inch rearend has become almost a universal choice for many performance rear-wheel-drive applications. Are GM 12-bolt enthusiasts beating a dead horse,or does the General's best rearend offering remain a viable player? To find out, Car Craft polled a cross-section of rearend specialists and printed strong arguments for both sides. We found that there are as many opinions concerning the relative merits of these rearends as there are about Bill and Monica. And just like the world of politics, there is no ultimate truth--just many shades of gray.
Chevys Rule!
GM's design is more efficient than Ford's. A 12-bolt's pinion is 1.50 inches below the ring-gear centerline compared to Ford's 2.25 inches. For the 12-bolt, this results in a 3-1/2-percent-less horsepower drop through friction reduction and lower gear lubricant temperatures. The 12-bolt is also around 30 pounds lighter than the 9-inch.
The legend of 9-inch rears was built on the strength of the relatively few trick factory units, but they don't really have the internal brawn to back up their external bluff. Upgrading them with heavy-duty components costs more than it takes to build a 12-bolt, the vast majority of which already come with stout parts.
For example, the plain gray-iron third-member and caps used on most stock Ford 9-inch rearends are prone to catastrophic failure in extreme drag-race applications. Ford guys spend big bucks to upgrade to the 9-inch's coveted nodular iron carriers, but most 12-bolts come standard with a nodular case and caps. After installing a Ford nodular-iron case, the pinion retainer and ultimately the pinion bearings also need upgrading to avoid a cascade-failure scenario.
The 12-bolt's Eaton Posi is stronger--and more tunable--than Ford's OE-type Traction-Lok. Ford posis are supported by four spider gears (Chevys use two), but the thin, shatter-prone units need all the help they can get. Many other perfectly strong limited-slips, lockers, and spools are available for both rears, but they require an investment in costly aftermarket upgrades.
The Ford does have a huge selection of gear ratios. That may be critical for oval-track or road-racing, but to the average guy, what difference does it make if you can get a 3.50, 3.55, 3.56, or 3.60 gearset for a Ford, but just a 3.55 for a 12-bolt?
GM 12-bolt housing availability has been a problem, but that's changing. Strange has new '82-and-newer Camaro 12-bolt housings. KTRE offers new improved-design housings for nearly all GM rear-drive vehicles (even including oddball '73-'77 A-bodies). Tom's Differentials plans to introduce its own line of new 12-bolt housings later this year. All these new housings come standard with ends set up for bolt-in bearing-retainer axles instead of the 12-bolt's C-clip axle retention.
Speaking of axles, one of the Ford's supposed advantages is that its nontapered axles can be shortened and resplined. Original GM axles are tapered, so building a custom-shortened 12-bolt requires costly aftermarket axles, claim Ford boosters. This advantage is overstated. First, most Ford 9-inch stock axles have only 28 splines, compared to the Chevy's stouter 30-spline axles. Second, shortening and resplining a stock axle ruins the heat-treat, so this practice isn't recommended for extreme performance use. Third, stiff aftermarket competition has made custom axles affordable to the masses. With the housing availability problem just about solved, there's no reason to defect to Ford!
Fords Forever!
It's no accident that the 9-inch Ford has become the universal cross-platform rearend of choice. In production from the '50s through the '80s and installed in chassis ranging from Mustangs to big trucks, 9-inch assemblies are widely available for a reasonable price. Installation into a GM (or other non-Ford) application requires a custom-length housing and axles, plus the correct suspension-mounting brackets. Obviously, this costs bucks, but used GM 12-bolts are drying up while new-production aftermarket 12-bolt assemblies cost just as much as (if not more than) a custom-built 9-inch.
Unlike the 12-bolt, the 9-inch rear has a removable third- member. This makes it easy to swap gears, which is an important plus for dual-purpose street/strip cars. There are no failure-prone C-clip axles in this rearend; bolt-in, bearing-retainer axles are standard equipment.
A Ford's 9-inch ring gear is larger than the 12-bolt's 8.875-inch ring gear. The Ford's larger ring gear plus its greater offset relative to the pinion results in a tooth contact area that's slightly deeper than a 12-bolt's, making the Ford marginally stronger. Its pinion gear is supported in three places, compared to only two for the Chevy. More gear ratios are available for the 9-inch Ford than any other rearend.
Unlike the General's offerings, the Ford has no differential case breaks. The same 9-inch Ford differential works with any gearset from 2.47:1 to 6.14:1, so you don't need to buy a new diff when changing gears. Currently available Ford 9-inch limited-slip differentials all use four spider gears, versus only two for the 12-bolt. And, there are more different designs of aftermarket diffs available for the Ford than the Chevy; this gives the consumer more choices and the opportunity to select a tailor-made diff for the chosen application.
It's true that relatively few Ford 9-inch assemblies were built with a nodular-iron third-member and pinion-bearing retainer, but the average guy can get by without all the tricks. Ford boosters claim that just adding the good pinion support lets a standard 9-inch easily live through 450 hp. And for those really on a budget, any local machine shop with a dividing mill can shorten and respline a Ford's stock nontapered axles. All in all, the Ford is a reasonably strong entry-level unit for any make of car.
But if you're going Pro racing and do need the trick parts, aftermarket components based on the Ford 9-inch are available that can support anything up to and including a Top Fuel car. Nothing in 12-bolt territory even comes close. Aluminum or even magnesium third-members are available if you're concerned about the 12-bolt's weight advantage. If you want a rear that'll last virtually forever, the 9-inch Ford is your only choice.