Rear Axle Break In Procedures - The Great Rear Axle Comparo

Break-In Procedures
Since all three of these rearends began life with new gears, each had to be properly broken in before testing could be performed. Each rearend was outfitted with a set of brakes, filled with typical gear oil, and installed in our Chevelle. Each rearend was then driven for 40 miles on the freeway to ensure the gears were completely up to temperature and then allowed to cool overnight. This break-in procedure was repeated three times for a total of 120 miles. Then we drained the gear oil from each rearend and refilled them with Amsoil 75W90 synthetic gear oil for the test.

Chevy Chevelle Rear Axle Swap Adding Amsoil 75W90 Gear Lube

We chose Amsoil's synthetic 75W90 gear lube for its strong lubrication properties and thermal stability after breaking in each of the three rearends with nonsynthetic gear lube.

We chose Amsoil's synthetic 75W90 gear lube for its strong lubrication properties and ther

Rearend Weigh-In
We measured each rearend assembly with and without brakes. The Strange S60 is the heaviest of the three without brakes, but only by 20 pounds compared with the 12-bolt. Considering the massive 35-spline axles, huge ring gear, and 3-inch-diameter tubes, we're surprised it doesn't weigh more. The same Chevelle/Camaro 9 1/2-inch drum brakes were used on both the 12-bolt and the S60, while the Ford employed much larger 11-inch drums. The Ford drum brakes add 64 pounds, while the 12-bolt GM brakes come in at 39 pounds. If we level the weight playing field by using the same weight for brakes on the 9-inch, the Ford's overall weight becomes 213, which is lighter than the S60 by 17 pounds.

Any rear axle assembly is considered unsprung weight, which is defined as weight not supported by the vehicle's springs. From a vehicle dynamics standpoint, minimizing unsprung weight is important. But for a typical street performance car, the difference among all three of these assemblies is less than 30 pounds, which is less than 1 percent of total vehicle weight.

Chevy Chevelle Rear Axle Swap 383 Chevy Small Block Engine

This 383 has been in the car for several years and makes acceptable power. The Chevelle has run low 12s at 112 mph in this configuration with the 9-inch.

This 383 has been in the car for several years and makes acceptable power. The Chevelle ha

Our Test Mule
A backyard full of Chevelles made the choice for a test vehicle easy. This small-block '66 is equipped with a GM Performance Parts HT 383 short-block beefed up with a Comp XE282 hydraulic roller cam (230/236 degrees at 0.050, 0.510/0.520-inch lift, 110-degree lobe-separation angle) with 9.5:1 compression. For cylinder heads, it has a set of TFS 215cc heads with an Edelbrock Performer RPM Air-Gap intake manifold and a Barry Grant Speed Demon 750-cfm carburetor. The headers are a set of 15/8-inch Hedman headers backed up with a Flowmaster 21/2-inch exhaust system muffled by a pair of 40 Series mufflers. Directly behind the 383 is a TCI TH400 automatic with a 3,000-rpm stall speed 10-inch converter. For rear tires, we're running a pair of 26x10.5x15-inch Mickey Thompson ET Streets mounted on 15x8-inch Center Line wheels. We also used a giant seven-blade engine-driven clutch fan for these tests to ensure the engine temperature remained consistent.

Chevy Chevelle Rear Axle Swap Checking Pinion Angle

We checked pinion angle for each rearend by first removing the rear springs and then simulating ride height with the lower control arms at zero degrees.

We checked pinion angle for each rearend by first removing the rear springs and then simul

Test Day
Once all the rearends were assembled and broken in, we were finally ready for the actual dyno testing. This called for a trip to Westech to spin the company's SuperFlow in-ground chassis dyno to generate some accurate rear-wheel horsepower numbers for each rearend. The test procedure called for installing each rearend in our test Chevelle, bringing the rear axle up to 125 degrees F measured at the pinion bearings with a heat gun, stabilizing the engine coolant temperature at 180 degrees, and then running two pulls on the chassis dyno and averaging them into one run that would be plotted on a graph.

Chevy Chevelle Rear Axle Swap Adding Hotchkis Adjustable Upper Control Arms

To establish the pinion angle, we measured the position of the pinion yoke perpendicular to horizontal. All three housings required different upper control arm lengths to create the same pinion angle for all three rearends. We used the Hotchkis adjustable upper control arms to equalize the pinion angle at 2 degrees nose down for all three rearends.

To establish the pinion angle, we measured the position of the pinion yoke perpendicular t

All three rearends required different driveshaft lengths to accommodate the various pinion positions and U-joints. We already had a driveshaft for the 12-bolt, and Strange built a driveshaft for the S60 rearend. That left us needing a driveshaft for the Ford 9-inch, which we got from Denny's Drive Shaft. The Chevelle was also equipped with tubular lower and adjustable upper Hotchkis control arms. By adjusting the length of the upper arms, we were able to maintain a common 2-degree nose-down pinion angle for all three rearends. This eliminated the pinion-angle variable, since all three rearends position the upper control arm mount in different locations.

As you can see from the accompanying comparison graph, from 3,500 to around 5,000 rpm, there is only a slight power difference between the 9-inch and the 12-bolt, while the S60 and the 12-bolt looked almost identical. We also expanded the curve in the second graph because it was interesting to see how the S60 performed similarly to the 12-bolt up until around 5,000 rpm, then fell off after 5,600 rpm, and actually dropped below the 9-inch power numbers at one point.