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.
Another option when using a cast pinion yoke is to use U-joint caps instead of the weaker stock-style U-bolt retainers. This increases the clamping force and eliminates the possibility of distorting the caps. New billet yokes typically come with the proper retaining caps.
Along with balance, the length and diameter of the driveshaft directly affect the performance of the unit. Determining the required length for the driveshaft necessitates looking at several factors. The distance from the rear yoke to the transmission seal is the most important measurement because it determines the overall length of the driveshaft. Measure this length with the pinion yoke installed and the car at ride height. The pinion yoke influences the measurement, and changing from a cast-steel yoke to a billet pinion yoke can alter the length by as much as 3/4 inch.
Provide these measurements to the driveshaft shop and they can create the complete shaft with the required slip yoke and predetermined run-out for the slip yoke. For most applications, a run-out of 1 inch is more than enough to provide the play needed for suspension travel, so do not let a shop convince you to accept more run-out than that. Some transmission shops insist on running out 1-1/2 inches, but this could be disastrous and lead to driveshaft failure. With that much of the slip yoke hanging out of the transmission, there could be less than 3 inches of splined yoke in the transmission, thus creating a wobble in the yoke that would cause a heavy vibration at various RPM. Stick with the 1-inch rule.
Always measure the driveshaft length at drive height. If the vehicle is too low to crawl under it on the ground, jack up both ends and use jack stands under the rear end and front suspension; be careful to make sure all the stands are at the same height. The slightest variation in the suspension can throw off the measurement, resulting in a driveshaft that does not fit.
The function of “critical speed” (CS) factors into the length versus diameter rule. Critical speed is the RPM at which the driveshaft becomes unstable and begins to bend in the middle. This is also known as “jump roping.” The longer and smaller (diameter) a driveshaft is, the slower its critical speed. Critical speed is felt as excessive vibration, and if run at CS too long, the unit will fail. To calculate the critical speed, you must know the length, diameter, wall thickness, and material module of elasticity. Then, using the critical speed calculation formula, you can plug in the numbers to calculate the driveshaft’s critical speed.