Those crazy Aussies. They seem to think good handling and a soft ride are greater priorities than the ability to slap on a set of slicks and spray a snoot-full of nitrous at the drag strip. While your IRS-clad 2004-05 GTO might withstand an outing in the outback, it may not survive plans for forced induction, increased displacement, or sticky tires. Since our Project 2005 GTO was already making 340 lb-ft of torque (at the wheels) naturally aspirated and nearly 500 on the juice with more mods to come, it would only be a matter of time before our drivetrain's weakest link showed itself. Enter BMR Fabrication, whose '04 GTO has already gone low 10s on the jug. With BMR's complete line of billet rear end and chrome moly chassis components, we were hoping our Goat would have a fighting chance at surviving the big plans we have in store for it.
One weak link in the GTO drivetrain is the factory two-piece (yes, you read that right) driveshaft. While common on trucks (in a much larger diameter) and some lower-powered imports, a two-piece driveshaft has no business on a GTO--especially with the 364-cube torque monster found in '05s. It is all but two inches in diameter and uses rubber for U-joints in order to quell high-speed vibration. Unfortunately neither characteristic is ideal for strength or preventing wheel-hop. BMR offers two replacement driveshafts, which are of the traditional one-piece construction, using forged 1350 series U-joints and a cryogenically frozen front yoke. A custom-machined 300M billet pinion yoke adapter also comes with the heavy-duty driveshaft in order to mate to the Aussie differential. BMR offers a 3-inch DOM steel version capable of 6200rpm and a 3.5-inch aluminum version that is recommended for 6-speed cars. While both will withstand up to 900hp, the aluminum version, which we obtained for this install, is capable of revving to 7200rpm.
The stock CV shafts have been known to shatter quite violently, potentially causing further damage to the drivetrain and undercarriage. The BMR Heavy Duty CV Shafts use 300M billet measuring 1 3/8-inch in diameter, while the factory shafts are made of lesser grade metal with only 1 1/16-inch diameter. The CV joints are also made of billet, which help the axles exceed 800 rear wheel horsepower. Replacing the shafts, though, will put strain on the factory axle stubs that connect the shafts to the differential, so BMR recommends replacing those too with its 300M billet upgrade.
In order to combat wheelhop, BMR carries Harrop differential covers and rear cradle bushings. Wheelhop, especially in 6-speed cars, is the greatest cause of rearend failure, and by changing the geometry and rigidity of the differential's movement it can be significantly reduced. With the Harrop differential cover, the factory snout and single mounting point is traded for an overhead bracket that connects to two polyurethane bushings on both sides of the cover, which will disperse the stress more evenly on the thicker unit. Besides reduced flex, the diff cover also increases the fluid capacity of the rear. The polyurethane cradle bushings are an obvious improvement over the spongy factory rubber bushings, providing a much firmer interaction with the rear subframe at the cradle's second and third mounting points.
While the unique design of the subframe and rear suspension in the Aussie-built GTO does make it quite hard for aftermarket manufacturers, BMR went out of its way to fabricate a trick set of connectors that still allow access to the rear cradle. Using rectangular tubing, which neatly hugs the undercarriage and provides four separate weld points, the connectors mount to the front structural areas and to a custom bracket. The connection to this bracket is made via two 3/4-inch bolts, making the connectors half (weld-in) and half (bolt-in), however this is necessary as it would otherwise permanently affix the rear cradle to the body. The subframe connectors should reduce chassis flex quite significantly, and let the suspension do all the work.
While stiffening the chassis it seemed as good a time as any to replace the factory strut tower brace, which is little more than decoration, with a more serious BMR piece. Measuring 1 5/8-inch in diameter, BMR's unit dwarfs the factory 1-inch front brace. As with virtually all of BMR's products, including the subframe connectors, the strut tower brace is available in a variety of powder-coated finishes. Red was chosen for the brace and connectors as well as a driveshaft loop since the GTO was already running low 13s bone stock. The loop had never been test-fitted with the optional 2005 sport exhaust, so we weren't sure whether there would be interference issues. Though BMR says the combination of the loop and larger driveshaft is sometimes a problem with certain exhaust systems, it was worth a try for the added insurance and NHRA approval.
Chassis support tends to add a little bit of weight, so we also tried to offset that with BMR's Lightweight Skid Guard. Any Goat owner who has ever tried to change his or her own oil knows that the nearly 16-pound factory shielding for the oil pan and filter could use the Atkins diet. The laser cut aluminum replacement is only five pounds and looks quite sexy. While most people won't be seeing it, there is something to be said for having a handsome undercarriage.
Knowing the strength of the T-56, the BMR upgrades should be all our 6-speed GTO needs to maintain a happy and healthy driveline, not to mention improved handling and launching. We plan to assault the Goat at a later date with numerous other upgrades and drag radials, so the extra support we gain here will be critical down the road. As a C5 Corvette specialist, East Coast Supercharging was all too understanding. Having replaced more than its share of output shafts and transaxles on Corvettes, we felt safe in the skilled hands of technician Mark Scharago. C5s are said to be particularly challenging, and we knew the GTO would be no different. However, a confrontation such as this always yields significant knowledge ... something we are happy to share with you.