Whether you're transplanting a big-block into a small-block chassis, or an LS1 into a wimport, ordering up a custom driveshaft is a must for any engine swap project. In addition to changing the distance between the transmission and rearend, engine swaps bring hundreds of extra horsepower to the party. As one of the most highly stressed components in the entire driveline, if the driveshaft snaps in half, you'll be putting down exactly 0 hp to the rear wheels. How's that for parasitic driveline loss? Moreover, increases in horsepower, vehicle weight, and tire grip exacerbate the stresses that the driveshaft endures. With a 775hp big-block, a drag-style four-link, and sticky Mickey Thompson meats, our '93 Mustang project car is guilty of all three offenses.
For assistance in solidly linking together Project Fox's stout Phoenix TH400 trans and built 8.8-inch rearend, we called up Strange Engineering for some advice. After taking a few quick measurements and discussing the needs of our particular application with Strange, they got busy building us a custom 3-inch chrome-moly driveshaft and had it on our doorstep in less than a week. From our discussion with Strange, it became very obvious that a driveshaft is more than a simple piece of round tubing spinning inside a trans tunnel. There's actually quite a bit of science involved when it comes to minimizing vibrations, and maximizing strength and durability. Selecting the right driveshaft for any application requires understating how tubing diameter, material, and wall thickness-in addition to U-joint and yoke design-all impact driveshaft dynamics and strength. Rather than merely stabbing our new Strange driveshaft into Project Fox and marveling in our ability to pull off such a challenging feat, we'll take a crack at explaining the technical aspects of driveshaft design.
While the car was on the lift, we figured it was also a great opportunity to set up the pinion angle. The procedure for dialing in pinion angle differs depending on rear suspension design, but nonetheless, it can be accomplished with a few simple handtools and is critical for keeping rearend wrapup in check under acceleration. As always, thanks to the good folks at Bill Buck Race Cars in Austin, Texas, for helping us out.
Measuring for a driveshaft...
Measuring for a driveshaft is an easy two-step affair. The first step is recording the distance from the lip of the tailshaft housing to the front of the pinion yoke. The second measurement should be taken from the lip of the tailshaft housing to the end of the output shaft. Measurements should be taken with the car on the ground, or with jackstands placed underneath the front and rear suspension. We did just that, but later placed the Mustang on the lift for illustrative purpose.
Factoring in the length of...
Factoring in the length of the trans slip-yoke, the prior measurements help determine the correct center-to-center distance between the driveshaft's U-joints, which in our case happened to be 47 1/8 inches. Since Fox Mustangs aren't blessed with particularly commodious trans tunnels, Strange set us up with one of its 4130 chrome-moly 3-inch driveshafts, which is still plenty strong for our application. The tubing has a wall thickness of 0.083 inch, and Strange limits runout to 0.008 inch to ensure concentricity throughout the length of the shaft. At 22 pounds, it isn't exactly light, but we felt the bulletproof ruggedness of a chrome-moly shaft was worth the weight penalty.
Many factory driveshafts came...
Many factory driveshafts came equipped with 1310-series U-joints, which become marginal at around 450 hp. Strange uses larger 1350-series joints on its shafts for increased torque capacity. A 1310 U-joint measures 31/4 inches wide, while a 1350 joint measures 3 5/8 inches wide. Furthermore, 1310 joints have bearing cap diameters of 11/16 or 1 1/8 inches, while 1350 joints have larger 13/16-inch caps.
Strange offers transmission...
Strange offers transmission slip-yokes for every conceivable application in both mild steel and 4130 chrome-moly. They set us up with a chrome-moly unit to mate up with Project Fox's TH400, which has been heat treated for improved tensile strength. Much like balancing a tire, all Strange driveshafts are spun up to high speeds on a balancer, and weights are welded into place at just the right spots to smooth out vibrations. While most stock driveshafts are only balanced to 3,500 rpm or so, Strange balances its shafts up to 7,500 rpm depending on their intended use. Excessive vibration leads to both breakage and parasitic power loss.
Pinion angle is simply the...
Pinion angle is simply the angle of the pinion yoke in relation to the driveshaft. The easiest way to measure it is by laying a magnetic angle finder down on the driveshaft. They're available at most hardware stores for about $15.