There are many reasons to run a high-pinion differential in a 4WD vehicle, and there are many reasons to run a Ford 9-inch axle. When used in the front of a vehicle, high-pinion differentials arestronger than a low-pinion axle when traveling forward due to the way the teeth mesh on the ring-and-pinion gears. This is why Ford, Toyota, and Jeep have used high-pinion differentials in the front of their vehicles for decades. Meanwhile the Ford 9-inch is incredibly light and strong, making it the axle of choice for everything from drag racing to circle track to NASCAR. This axle has been out of factory production since 1987, yet its popularity continues to grow.
Off-road, the Ford 9-inch does have its weakness. The pinion enters the third member incredibly low, which contributes to its strength but also results in a vulnerable driveshaft location and poor driveline angles. The solution for the low pinion came from the circle track world. Ernie Brookins designed a reverse-spiral Ford 9-inch third member and had Richmond Gear (a company whose gears are known for their strength and made in the USA) build the gears for him to go along with his quick-change gear style transmission. Mike Qualley from Trucksmith, a 4x4 building shop, recognized that it could be turned over and used for the rockcrawling community. Qualley teamed up with Chris Weivoda from Weivoda Automotive, a differential building shop, and they helped Brookins further refine the product to withstand the rigors of off-road abuse. Thus the True Hi9 was born.
 In addition to the obvious...  In addition to the obvious difference in pinion location, the True Hi9 third member also has significantly more ribbing and is thicker where the pinion support bolts to the third member. A fine-thread adjuster is threaded into the case that rests against the ring gear to keep it from deflecting under load. No other high-pinion differential offers this thrust block. |  The True Hi9 case has a smooth...  The True Hi9 case has a smooth radius with more material around the pocket that holds the third bearing the 9-inch is famous for. The cast in thrust block area that adds significant strength to the case. Further strength is added inside the case. |  On the left is a high-pinion...  On the left is a high-pinion Dana 60 ring-and-pinion. A True Hi9 ring-and-pinion is on the right. Both gearsets are 4.88s. The True Hi9 has a thicker ring gear, a larger pinion head, and the third bearing on the pinion. The only advantage of the Dana 60 is the 29-spline instead of 28-spline pinion stem. |
All high-pinion differentials run on the coast side of the gear when used in a rear application, and the True Hi9 is no different. We asked the designers why they did not have gears cut for drive-side engagement when used in the rear. "No gear machine currently exists to cut these gears," Qualley explained, "and to build one would easily cost half a million dollars." No other manufacturer has addressed this issue despite the fact that using reverse rotation gears in a rear application cause the pinion to try and push away from the ring gear. The True Hi9 is unique in that the third member contains a thrust block behind the ring gear that prevents deflection of the gears.
"Yes, standard-cut high-pinion gears would be more practical in the rear if they existed" Weivoda admits, "but we've found that running on either side of the teeth is equally strong as long as deflection is controlled."
The lack of both a thrust block and the third pinion bearing that the Ford 9-inch is famous for is the reason that other high-pinion third members on the market have restrictions in horsepower and tire size. The Hi9 can be retrofit into any Ford 9-inch housing, and the pinion enters the housing higher than any other offering, factory or aftermarket. Also the U-joint attaches to the third member closer to the axle than any other high-pinion differential, giving a longer driveshaft for an even better angle and less U-joint wear and abuse.
With the pinion located so high, proper lubrication can be an issue for a high-pinion differential that sees sustained high speeds. The oiling system on the True Hi9 consists of an extended shroud system in the housing and a ring gear scraper/cup to the pinion support. The scraper supplies oil to the pinion bearings primarily at low speed, and the shroud supplies oil to the pocket bearing and pinion bearings primarily at high speed. The "cup" supplies oil at all speeds.
 The diagram below shows the...  The diagram below shows the difference in ground clearance available with the True Hi9. The True Hi9 offers the highest pinion available and turns the liability of the Ford 9-inch into an asset. |  Rough castings are delivered...  Rough castings are delivered from the foundry to Weivoda Auto, where they are finished in an onsite machine shop. Precise holes must be drilled and tapped for the thrust block, bearing journals, and third member mounting pattern. |  With the product installed,...  With the product installed, you can see how the driveshaft is located significantly higher than the lower control arms, protecting it from damage. These photos were taken shortly after Rock Runner Racing won Supercrawl's Modified class. The True Hi9 third members were run all season long without a single failure. |
The True Hi9 is only sold as a complete third member in order to provide improved quality control, but you can supply your own differential to Weivoda Auto if desired. The third members accept standard 28-, 31-, or 35-spline Ford 9-inch differentials, including Detroit Lockers, ARB Air Lockers, and full spools. Currently both nodular iron and aluminum cases are available with gear ratios ranging from 4.11 to 5.38.
Those who lust for the ultimate can order a housing from a variety of housing builders, such as Currie Enterprises, Diamond Axles, Mad 4x4, Spidertrax, and Sunray Engineering. Custom housings can be built with a variety of options, including width, brakes, bolt patterns, and full-floating 'shafts to compliment their new third member. Others who wish to save money can bolt the True Hi9 directly into a junkyard housing. The Ford 9-inch housings are favored by many fabricators because of the ease in which suspension tabs and brackets can be added. Supercrawl Modified class champion Brian Errea and veteran Super Modified contender Ken Blume have both used True Hi9 third members and Ford 9-inch housings in competition with zero failures.
The True Hi9 isn't just for competition. Its oiling system was originally designed for the street and is capable of running at freeway speeds for sustained periods of time, a claim many recreational four wheelers substantiate every weekend.