Let's face facts: General Motors just can't get things right in the rearend department. From the weak 9- and 10-bolts found in L98/LT1/LS1 F-bodies, to shattered C5 and C6 Corvette diff cases (and the absolutely F.U.B.A.R. rear of the early CTS-V), it's pretty safe to say that GM hasn't done an axle assembly correctly since the turbo Regal. The problems aren't just for owners who drag race, either: many such failures occur during spirited (or in some cases, totally normal) street driving.
This issue, GMHTP is focusing on the particular problems that plague our favorite musclecars, the LS1-powered Camaros and Firebirds. Though it's possible to substantially beef up these vehicles' factory 10-bolts to handle added torque or abuse, there are limits to the effectiveness of this-limits that are often well below the needs of many high-horsepower 6-speed cars, particularly those with grippy clutches and sticky tires. There are a few aftermarket companies out there that offer attractive solutions-namely, bolt-in, heavy-duty rear axle assemblies. We're going to perform separate installs of tough axle assemblies on two GMHTP project cars and compare and contrast the results in the hopes of providing enthusiasts with the background information they need to start shopping!
First up is Strange Engineering. When Strange was approached with a request for a suitable rearend for Rick Jensen's project 1SC-YA (a car whose stock bottom-end LS1 is already making good power and will be producing a whole lot more in the future), the company's S60 was the hands-down recommendation. A derivative of the renowned Dana 60 that can be found in many GM, Ford, and Mopar trucks built over the last few decades, this is one heck of a beefy axle assembly design that plays well into the needs of powerful Camaros and Firebirds. The Strange S60 comes totally assembled, carries PN PRSF05, and retails for $2,349. Beefy 35-spline axles are standard, and it can come ready for either version of ABS you might have, be it three-channel (add $150) or four-channel (add $69, or send in your old reluctor rings for Strange to press on). The company even goes so far as to include new brake backing plates. While there is no charge for 1993-1997 backing plates, 1998-2002 cars will need to shell out an additional $269 for new ones (or, you can send your old backing plates to Strange and they will be installed at no cost). A Truetrac differential comes standard, and a locker can be had for $70. As to the gears, a Spicer (the OEM supplier) set with your choice of ratio is included, and though we don't have them in our unit, softer, drag-only 9310 gears are also available. With that bit of background information in mind, let's get going on the installation!
Let's turn our focus to another GMHTP project car, this one owned by yours truly. Ever since completing the installation of its 383-cube LS1 stroker motor back in 2006, my project 2001 Trans Am has been in sore need of a strong axle assembly (and we've been hinting that one has been on the way for some time). The wait has paid off, big time: Moser Engineering is stepping in with one of its M9 Bolt-In Housings, which is a new, high-strength twist on the venerable Ford 9-inch. These stylish housings don't just look pretty, they are made to meet the demands of high-horsepower applications. Featuring 1/8-inch, laser-cut mild steel construction, the central part of the M9 is fabricated from one triangulated piece of metal-meaning it's not only very strong, but also very unique-looking. Our unit also includes an optional back brace (which extends from the center part of the housing and tapers toward the ends of the axle tubes), an item that adds strength and makes the M9 look even meaner. The price of $1,915 gets you a powdercoated housing along with axles, bearings, and hardware-plus Moser's new uniquely matched torque arm system, which the company claims is the "strongest torque arm assembly on the planet!" This fully adjustable unit is made from 4130 chrome-moly and is 100 percent TIG-welded. It also comes with a new transmission crossmember made from the same material. Though the system is technically not designed with street applications in mind, we're going to push the limits with this road-going T/A and see if we can get away with it!
Let's not forget that the 9-inch design uses a removable centersection (also called a "case," or "pumpkin"). Moser sells a nodular one with a spool for $1,195, and this includes a heavy-duty pinion support and 1350 yoke, as well as the gear ratio of your choice. Since this car sees primarily street duty, we chose a Truetrac diff to replace the spool, which ups the price another $305. Because the M9 is shipped in a less-assembled state, we'll spend some more time on the details of how it goes together than how it actually installs into the car (which basically mimics the installation of Jensen's Strange S60 that you just saw). Let's get the installation of this bad beauty going!
So far, we've discussed numerous differences between the two packages featured in this story (one of them being the state of completion each axle assembly is shipped in). But we also decided to do a bit more detective work to compare and contrast the two kits even further. Though by no means an apples-to-apples comparison-remember, each of these companies offers several different rear-axle assembly packages compatible with 4th-gen F-bodies-we'd like to give you, the readers, as much information as possible in hopes of steering you in the direction of the "right" axle assembly for your application.
First off, there is the issue of mass. Though aftermarket axle assemblies are far stronger than the F-body's stock 10-bolt, the added meat required to support this strength does incur a weight penalty. More weight to drag around means reduced acceleration; plus, an axle assembly is unsprung weight, so a more massive one will affect suspension behavior. With that in mind, we decided to weigh the stock rear and compare it to the Strange and Moser units. With all rears at the same stage of assembly-no brakes and no fluid, torque arm variations not factored in-the breakdown is as follows (keep in mind that these are dry weights, so differing fluid capacities can factor in to actual running weight):
Secondly, one thing you may not think of with regard to added mass is that it can also come into play in power loss. More massive axles and gears require more energy to get up to speed, adversely affecting rear wheel horsepower. Other factors, like internal friction, play in as well, of course. This friction may be attributable to bearing properties, along with axle fluid viscosity and flow behavior, and can also be influenced by geometric concerns like the diameter of the ring gear (which is intertwined with the properties of how the ring-and-pinion mesh). Though the physics involved may merit a full-blown engineering discussion in a future issue, for now we decided to do before-and-after testing of both of these project cars on TTP's Dynojet for a quick comparison. Our results are:
We're quoting the new and old axle ratios because a numerically higher rear-axle ratio necessarily reduces measured rear wheel horsepower on a dynamometer (again, for reasons that would best be described in a no-holds-barred engineering analysis). But you can see that the gear ratios used in both of our new rears were virtually identical, so this factor can pretty much be taken as a constant in comparing the percent loss difference between the two axle assemblies (though before and after dynoing on different days probably should not, even with our use of SAE correction factors).
A final note on the M9 is in order because of its fully adjustable torque arm system. Readers should be aware of what a well-constructed piece this is, and think about the fact that it was included in the price of our M9 assembly. Once installed, we adjusted this system to provide the Moser-recommended 1 degree of downward pinion angle (compared to the driveshaft angle) before hitting the dyno, and while no track testing of any sort has been done thus far, the system has performed admirably on the street, with the exception that some extra engine and road noise is now transmitted into the passenger compartment (thanks to the solid rod end connection between the torque arm and the transmission crossmember). We're confident this unit will help us put down some sick 60-foot times when the time comes to hit the strip. The system feels like it would perform admirably on a road course as well (the latter assessment is, of course, a bit subjective).
As to his new S60, Jensen is enjoying it very much. While it needed a quick Panhard adjustment to center the rear, and does transmit some gear noise into the cockpit, he's impressed with the ease of installation and can't wait for Raceway Park to open back up to finally do some high-rpm clutch dumps!
In conclusion: Both the Strange S60 and Moser M9 have duly impressed us so far as top-notch, high-strength rear-axle assemblies. While each has features and aspects better tailored to different customers and vehicle applications, both are high-quality choices. We look forward to hitting the races with these cars once the snow melts, and will keep you posted with our results!