Spinnin’ Gears

All About the Fab 9-Inch from Currie Enterprises

When deciding to increase your truck’s tire size, there is a major component that needs to be addressed to maintain the engine’s torque, horsepower and fuel economy. Increasing your truck’s tire diameter will affect the engine RPM, which affects torque and power. When changing your rear tire’s diameter from smaller to larger or vise-a-versa, it will not affect the rear end differential gear ratio. The only way you can change the rear end gear ratio is to physically change the ring & pinion gear set. Changing tire diameter only affects the ratio of the driveline.

Hot To Figure Gear Ratio and Tire Rotation

It is amazing that only two components can make a dramatic effect on drivability and fuel mileage as the ring & pinion gears in a vehicles differential. The smaller the gear ratio the faster the vehicle will accelerate. There is a trade off, when running a smaller gear ratio the engines rpm will increase and fuel mileage will decrease. When deciding which gear ratio will be best for your application, the first thing is to determine what gear ratio does the your truck have currently have.

You can figure your gear ratio without having to remove the rear-end differential. Using a floor jack place it under the rear differential jack the differential up until the rear tires are off the ground. Always place jack stands underneath the frame rails safety precaution. Place a piece of tape at the 12:00 o’clock position on the tire sidewall. Then crawl underneath the truck and place a piece of tape on the driveshaft at the 6:00 o’clock position. Put the transmission in neutral. Crawl underneath the truck and rotate the driveshaft until the tire completes one full revolution. Count the amount of time the driveshaft rotated to equal on full revolution of the tire.

Examples:
3 ½ rotations of driveshaft = 3.56
3 ¾ rotations of driveshaft = 3.73
4      rotations of driveshaft = 4.10

How to figure ring & pinion gear ratio:
Divide the number of teeth on the ring gear by the number of teeth on the pinion gear.

Example:
37 teeth on the rear gear
9 teeth on the pinion gear
37 / 9 = 4.11 gear ratio

The higher the gear ratio means more torque and power, but less fuel efficiency.

Engine RPM x Tire Diameter = MPH
2200 RPM x 33” dia. = 72.60 MPH

Recommended Engine RPM @ 65-70 MPH Highway Speed
V6: 2,000 – 3,200 RPM
Small Block: 1,800 – 2,800 RPM
Big Block: 1,800 – 2,600 RPM
Diesel: 1,600 – 2,800 RPM

We approached the folks at Currie Enterprises in Corona, California to checkout their FAB 9 differential axle housing. The FAB 9 diff/axle housing features a 3/16-inch thick Hi-Form 50 steel plate body, a 3/8-inch thick third member mounting face surface, with 3-inch diameter axle tubes. The FAB 9 differential axle houses is unique with the first bend of the housing was centered with the centerline of the axle tube. Inside the housing are welded bulkheads that interlink the face, body and inner ends of the axle tubes are fused together as one unit. The FAB 9 differential was designed to incase traditional Ford 9-inch internal components. The FAB 9 design and construction contributes to increased, strength and durability. It also looks cool and is bulletproof at the same time.


The Eaton Detroit Truetrac is a fully automatic limited-slip differential that works with helica gears instead of clutches or cones. The Truetrac eliminates and parts that would wear and requires no special limited slip lubricant or additive.

As with a conventional differential, the Truetrac side gears are interconnected by pinion gear, which allow one wheel to slow down or speed up as required. Truetrac gears have spiral teeth, and the pinions are mounted in pockets in the case.

Torque flows through the differential’s ring gear and the Truetrac case drives the pinions. Torque then flows to the side gears and to the axles. The side gears are also in mesh with each other side to side and in turns, one side will speed up and the other will slow down. Just like an open differential. Because there are no clutches to release first, the differential action is smooth and quiet. When there is a difference in traction side to side, the Truetrac uses resistance of pressure angles between the teeth of the side and pinion gears, as well as thrust forces of the pinion in their pockets in the case, to provide the “braking” action needed to transfer torque from the low to the high traction axle.

With large wheels comes the need for better stopping power using a 9-inch housing kit from Wilwood. It features larger 14-inch diameter rotors that are cross drilled and slotted for cooling and 4-piston calipers, the multiple pistons mean better and more responsive braking.

Follow along as we documented the construction of a Currie FAB 9 diff/axle housing, 31-spline axles. The installation of a Motive Gear Ring & Pinion 3.73 gear set, with an Eaton Detroit Truetrac and Wilwood disc brakes.

Sources

Currie Enterprises
 (714) 528-6957
www.currieenterprises.com

Eaton Detroit Truetrac
(800) 328-3850
www.eaton.com

Motive Gear
 (800) 934-2727
www.motivegear.com

Wilwood High-Performance Disk Brakes
805-388-1188
www.wilwood.com

Text by Bob Ryder
Photos by Jason Mulligan

The 3-inch diameter axle tube inner surface were chucked up in the lathe and machined to accept the fitment of the axle housing/outer bearing race flange.

The axle housing/outer bearing race flange was checked for proper fitment.

A 9-inch differential housing was bolted to the Currie FAB 9 body to eliminate any movement while the axle tubes are welded to the body.

The axle housing/outer bearing race flange was then pressed into the Currie FAB 9 axle housing.

The axle housing was measured to make sure it was the correct distance from the Currie FAB 9 body to the outer bearing race flange outside surface.

After double-checking the distance from the Currie FAB 9 body to the outer bearing race flange surface, the axle housing was tack welded to the Currie FAB 9 body.

The outer bearing race flange was tack welded to the 3-inch diameter axle housing.

The axle was inserted, chucked up and steadied with a tailstock. Then, the individual splines 31 in total were cut horizontally into the axle by the milling machine’s cutter.

The axle’s hub face surface was machined straight, true and perpendicular, to the axle’s centerline.

The major internal components of the 3rd member of the differential include the Motive Gear ring and pinion gear set with a ratio of 3.73 Currie 3rd member and the Eaton Detroit Truetrac unit with inner axle bearings.

After pressing on the inner axle housing to the Eaton Detroit Trucktrac unit, the Motive Gear, ring gear was aligned with the Tructrac unit. Ruben then threaded and torqued the bolts to 89 ft-lbs. in a criss-cross pattern to properly seat the ring gear.

After the pinion bearing was pressed onto the pinion shaft, the Motive Gear pinion gear was slid into the pinion support bearing housing.

Gasket synch was applied to the third member housing face surface then the gasket was carefully applied.

Gonzalo lowered and aligned the pinion gear and support bearing housing onto the third member.

An air gun was used to snug up the pinion support bearing housing bolts. Then torqued to 60 ft-lbs.

The Motive Gear ring gear and Eaton Detroit Truetrac unit was then laid onto the bearing carriers of the third member housing.

Gonzalo installed bearing carrier caps then inserted the cap bolts that were torqued to their proper specs.

A very important procedure when assembling a differential third member is setting the ring and pinion gears backlash. The gears backlash is the adjustment that meshes the ring and pinion gear together. To tight and the gears will wear out setting the mesh to loose and it can cause failure in the gear set. A depth gauge was mounted on the third member housing. The plunger was set on the ring gear surface the ring and pinion were then rocked back and forth watching the play between the two on the dial indicator. The optimum tolerance to look for is, .004 of variation, it was checked on three different locations around the ring gear surface for consistency.  The lateral adjustments were made by, tightening or loosening the side adjusting rings. The dial indicator was between .008-.012 so we were good.

To check the gear mesh pattern, a thin coat of gear compound paste was applied to the ring gear teeth surface. A drill-motor was attached to the pinion shaft. The pinion gear was rotated using the drill-motor, which in turn rotated the ring gear after a couple rotations an impression or pattern was left in the paste on the ring gear. By moving the pinion gear in or out would adjust the gear mesh pattern.

The side adjusting rings were adjusted to find the optimum gear-meshing pattern.

After the gear backlash and meshing pattern were dialed-in the cap bolts were torqued to their specs.

The pinion gear seal and cap were slid onto the pinion shaft followed by the pinion yoke that was slid onto the pinion shaft splines.

An air impact gun was used to tighten the pinion gear shaft gland nut securing the pinion yoke.

To secure the third-member onto the differential housing, mounting studs were threaded into the housing and tightened.

Sealant was applied to the housing surface. Then the gasket was installed and sandwiched between another layer of sealant too prevent from leaks.

The third-member was carefully lowered and aligned over the studs, then placed onto the differential housing.

Using Nyloc nuts to secure the third-member to the Currie FAB 9 differential housing, Gonzalo used a torque wrench to tighten the Nyloc nuts to their torque specs.

Wilwood offers this Explorer style internal drum parking brake, with their Wilwood disc brake kit.

The Wilwood disc brake cross-drilled/ball milled/vented rotor was secured to the Wilwood hat by hand weaving the safety wire.

The Wilwood 14-inch rotor and hat assembly was slid over the axle wheel studs.

The Wilwood 4-piston calipers were slide over the rotors then fitted with their semi-metallic brake pads for optimum stopping power.

A socket and Allen wrench were used to secure the disc brake pads detainment bar.

The Wilwood disc brake assembly will deliver excellent stopping power after installing the larger wheels and tires.

A picture is worth a thousand words.

Tire Dia./Engine rpm/Gear Ratio Chart is very helpful to figure engine rpm.