Yahoo Autos Project Car Diaries: My ’69 Charger Gets Big Diff Energy and LSD Goodness
Since it entered my family in the 1980s, the weakest link of my 1969 Dodge Charger has been its 8-1/4-inch differential. That rear end was meant to be used behind small V8s like the 318 this car was born with — not the big block and manual transmission it's got now.
For those of you wondering, the differential's job is to distribute power from the transmission to the wheels. The name comes from its ability to turn the wheels at different rates, which is necessary when turning as the tires need to cover unequal distances. It's not at all uncommon to change a final drive gear ratio to tweak performance, or to offset the effects of an aftermarket set of wheels and tires and, possibly install a limited slip or a locker of sorts. In other cases, like this one, you'll need to move to something a little beefier.
Not all differentials are built the same — some are much stronger than others. And when you start building power and driving harder, the amount of abuse a differential will take becomes a major concern. While not a hard and fast rule, the size of the ring gear, which is 8-1/4 inches in the housing that's been in the Charger, correlates with strength. There's a lot more to consider, like housing construction, axle material, and spline count, but a bigger numerical value generally means stronger.
Amazingly, the small rear end I've been running has held up after years of abuse. But I'd always known I'd need to replace it at some point. I figured a snapped axle or carrier carnage would be my cue to put some proper equipment between the rear tires, but that wasn't the case at all. Instead, a ding in the driveshaft would put me on a slippery slope of making major upgrades to correct a "small" issue.
Bending my driveshaft pushed me to finally upgrade to a Chrysler 8-3/4 differential. This was my first step into the rabbit hole of making a long list of those debt-building "while I'm at it" modifications. A happy ending is a big ask when that happens. Yet somehow, I managed to land one, as it was the last thing standing between me and my dream car.
A Bent Driveshaft is Bad News
Last winter, I tore the Charger apart to paint it. While the engine was out, I discovered all kinds of signs that indicated a balancing issue with the engine. It turned out that the harmonic balancer had slipped, and I replaced it, along with the flywheel, to ensure everything was good to go. When vibration issues persisted at high speeds, I was a little more than frustrated.
Initially, I was stoked to have finally found the source of my problems when I discovered a ding in my driveshaft. I thought it'd be simple enough to fix until I called the local driveline shop. The folks over there let me know that I was pretty much screwed and that I'd be better off replacing it.
Since this is a Dodge, even that's pretty extreme. This driveshaft features swatched tubing and yoke sizes that have been discontinued. So, even though I had the yokes, finding a replacement tube to fit them to was too much trouble for my local shops to want to deal with.
Building a new driveshaft would mean replacing the yoke at the transmission and the differential. Because the 8-1/4 uses a crush sleeve behind the yoke to set pinion bearing preload, I’d also find myself tearing back into a differential that I'm aiming to replace anyway.
Instead of doing all of that, I decided to start the hunt for a new, stronger differential. I called up an old friend who happened to have an 8-3/4 lying around that he'd sell me for cheap. I also located a used driveshaft sourced from a local 1969 Charger project for pennies on the dollar that would tie it all together. So, that's the way I went.
Poor Man’s Overdrive
The 8-3/4 isn't the strongest differential that Chrysler's muscle cars of the 60s came with. That award goes to the Dana 60, which was the differential of choice for cars that rolled out of the factory with a 440 or a 426 Hemi and a manual transmission. The Dana is a heavy-duty unit that was better suited for the intense shock-loading that high-torque stick cars would drop on it. So, why'd I go for the 8-3/4 instead?
Aside from being more affordable, the 8-3/4 offers another major advantage. This rear end uses a drop-out center section. In other words, it gives you the option to set up multiple carriers, allowing you to essentially change your rear gear ratio relatively easily. For a street car like this, it's a viable alternative to overdrive, as I could have a carrier for regular around-town use and another for long trips down the highway. Obviously, it's not ideal as it means crawling under the car and swapping the pumpkin instead of just throwing the transmission into fifth gear, but I’ll take what I can get.
Don't let the point about it not being Chrysler's strongest rear hang you up, though. The insanely robust nature of a Dana 60 shouldn't overshadow how strong the 8-3/4 is. These rear ends could take a ton of abuse, and after finding one under a NASCAR-spec Hemi Daytona Charger, which is equipped with a manual transmission, I'm more than confident in its ability to survive this street car.
