Modified Mustangs & Fords
Differential Addition - Get Some Traction Action
Add a limited-slip diff to stop that one-wheel-peel
When we last visited our High School Hauler project, we had worked through much of the Mustang's electrical issues, beefed up the cooling system, and put a solid exhaust system under it so that we could actually hear ourselves think going down the road (that's where we do some of our best editorial project thinking actually). Along the way, we've been keeping tabs on the '70 coupe's performance gains (or losses) with the grand picture being a fun, streetable car that is ready for track action with the twist of a key. The owner is looking forward to hitting the dragstrip with each upgrade we bring to the table, and this month, we have a nice treat for when the '70 makes its next quarter-mile pass.
As any Mustang fanatic worth his or her T-shirt collection will tell you, the majority of early small-block Mustangs were saddled with relatively lame axles for performance duty. The typical 8-inch rear saw mostly 2.79, 2.80, and 3.00 gears bolted around an open diff placed between narrow-shaft 28-spline axles. While Ford certainly kept costs down, this setup wasn't much help in the traction department or for turning a decent e.t. Sure, you could option a Traction-Lok and better gears, but it was a rare occurrence until you got into the later years and big-block engines. It's commonplace to see a 31-spline 9-inch with a Detroit Locker and 3.91 gears added to the option list when buying a 428ci Mach 1, but certainly not on a '70 coupe with a 302 and C4 automatic.
Our previous exhaust system upgrade made the coupe much more livable as a driver, and the owner has been putting some miles on the car. Suffice it to say, the aged suspension and braking are the weakest links in the Mustang's chain of performance right now. The owner, like most of us, is itching for more power, too. We're looking at several upgrade paths for the near future, including a ôtop endö kit of some sort to wake up the small-block, and a freshening of the suspension with some sort of budget disc brake package up front. But more power and better handling aren't going to help a thing without an improvement in traction.
The coupe's stock 8-inch rearend no doubt has stock gears in it, in the 2.80 to 3.00 range, and as we've already proven in our last story, the differential is a stock open diff directing the 302's power to the right rear tire under heavy acceleration. Sure, we could ring up a number of axle builders and spec out a pipe-hitting 9-inch with 35-spline axles, a locker, and 4.10 gears, but besides the hefty price tag, it's plain and simply overkill for this coupe, even once we put more power under the hood. Instead, we called upon the guys at Randy's Ring & Pinion and Yukon Gear to help bring our existing 8-inch back to life with an upgraded center section, including a performance differential, more appropriate gearing, and stronger 31-spline axles.
Our previous best elapsed time at the quarter-mile drags was a 15.8 at 88 mph, with a 2.631 60-foot time. With the new Dura Grip diff spinning both wheels using the 3.80 cogs, we dropped that time to 15.38 seconds at 89.17 mph with a 2.22 60-foot time. Check out the swap in the photos and read on to see how we made it all happen.
|Part Used||Part Number||Cost|
|Randy's Ring & Pinion Third Member||REBF8-380SVO||$1,367.07|
|Yukon Gear & Axle Cut-to-Length Axles||YA F8-31-32.87||$160.99 each|
|Stifflers Bolt-In Driveshaft Safety Loop||DSL-M02||$78|
|Royal Purple Max-Gear 75W90 Gear Oil||01300||$17.99 each|