the factory crossmember requires dropping the center section because the exhaust doesn’t clear if the entire unit is lowered. The transmission needs to drop about 3 inches.
If the drive angle is not between 1 and 5 degrees, the crossmember must be modified so the driveshaft has an adequate angle. Several methods can solve this problem, but it depends on the crossmember you are using. Two versions of crossmembers for A-Body cars are available: a tubular unit for open-frame cars and a formed steel unit for boxed-frame cars (all convertibles, El Caminos, most GTOs and Stage 1 Buicks, and some Oldsmobiles).
With late-model transmissions, the most common issue is the tailshaft sitting too high in the car. Because the crossmember sits on top of the frame, it cannot be lowered easily. You can cut and weld the ends to lower the entire crossmember or you can drop the center; either one achieves the same end. The crossmember, however, also has raised sections for the exhaust, so lowering the center is the better option.
If that’s not possible, a new crossmember is required. Numerous aftermarket crossmembers are available for open-channel frames. Some motor mount adapter brands, such as Hooker (Holley) and BRP Hot Rods, are complete systems, designed to work with the same brand’s transmission crossmember. If you have a boxed frame, the open-frame transmission crossmembers don’t work and you need a special crossmember. G Force Performance makes a heavy-duty crossmember that fits the A-Body boxed frame quite well and is perfect for LS swaps. BRP Hot Rods makes a boxed frame crossmember designed to work with its swap kit.
The keys here are driveline angles and keeping the tailshaft square between the frame rails. When fabricating crossmembers to support the transmission, use materials that are strong enough to hold the weight and torque of the transmission. Tubing (round or square) is a good material to use because it provides structural stability with less overall material thickness and weight. Flat-plate steel requires thicker material to achieve the same structural integrity. Angle steel is another excellent material for custom transmission crossmembers.
For the Buick, I opted for a G Force Performance Products crossmember because it’s the only one available for boxed frames. To mount it, you drill out the stock bolt holes to 1/2 inch.
The G Force crossmember has a dropped-center mounting plate, so the transmission sits lower in the chassis for tunnel clearance, about 3 inches. You can always adjust the transmission mount with the provided mount spacers. I used one spacer on this install.
Placing the nuts into the chassis can be difficult; this trick helps you install them: Roll half a thread onto a long bolt and then thread the nuts onto the upper bolts. These nylock nuts are a little easier to work with than other nuts.
The beefy crossmember has large hoops for exhaust clearance and fits the chassis quite well. It’s very strong and provides excellent support and strength, which helps maintain chassis strength. It is a little heavy.
I also installed the G Force sliding transmission mount so I had 2 inches of travel to mount the transmission to the crossmember. When sliding motor mounts have been installed, a sliding transmission gives you extra flexibility so you can correctly position the engine, transmission, and driveshaft for the best performance possible. A sliding transmission mount is especially helpful with sliding motor mounts because you can quickly exceed the reach of the factory slots in the crossmember.
I installed the mount and then slid the crossmember in place. You may need to remove the sliding section of the mount and roll the bolts into the crossmember slots because the bolts are actually studs and are made long for installs that need spacers.
Performance Project: Choosing a Driveline
When performing an engine swap, the driveshaft often needs to be replaced because it isn’t compatible with powertrain and chassis requirements. Most often simply shortening the stock driveshaft is not a suitable solution. Your LS engine swap requires a significant investment that includes preparation, fabrication, time, money, and effort. Why use the same old driveshaft that will never work like a properly designed custom unit?
In most cases, dropping in an LS engine increases the torque and horsepower output. Any time you increase the power output to the stock driveline, you must consider the impact on the stock driveshaft. Most factory driveshafts are balanced for a range of 3,000 to 3,500 rpm. Increasing shaft speed higher than 3,500 rpm can induce a parasitic effect. Steve Raymond of Dynotech Engineering said, “I have had several NASCAR teams tell us that our driveshaft saves them 3 to 7 hp on their chassis rolls dynamometers. That’s why balance is important and why we manufacture shafts for about 85 to 90 percent of the NASCAR teams.” The stock balance on the stock driveshaft is not good enough for anything but a stock engine.
Dynotech Engineering uses Balance Engineering’s driveshaft balancers because they are considered the best in balance accuracy. Dynotech suggests balancing a performance driveshaft at a minimum of 5,000 rpm, and as high as 7,500 rpm. This ensures a properly tuned driveshaft that reduces parasitic loss.
Both slip and pinion yokes are critical driveline components that physically connect the transmission, driveshaft, and differential. Break one of these and you often experience expensive car damage and loss of control. That said, a cast yoke often withstands up to 800 hp for most applications. But you can exceed 800 hp for certain cars such as lightweight hot rods with street tires because they put less strain on the driveline than a 4,000-pound Chevelle with slicks and 500 hp. You need to carefully consider this, however, and often it’s better to upgrade.
