For reasons I do not fully understand, Mitsubishi designed and released a very conservative and heavy 3 piece driveshaft into production on the AWD (twin turbo) models. By doing so, they were able to stick to relatively short and cheap steel shafts each with a natural frequency well outside the possible operating ranges (good for durability and NVH). These are all good things, but substantial cost was then incurred with the additional bearing supports, universal joints, and a lobro joint. I would think a two piece steel design would have been more production feasible and acceptable to the bean counters, but what do I know.
So why change the three piece driveshaft? Because it is damn heavy, 44lbs! (including the bearing supports). Many performance enthusiasts (of all makes and models of cars) upgrade stock steel driveshafts with aluminum or better yet, carbon fiber in hopes of reducing weight and improving durability. Carbon fiber with its extremely attractive strength / weight ratio makes it the popular choice for those that want the best.
Mark Veldhuis at Performance Shaft Technology (PST) fabricated a limited production run of 15 (16 including Jason's, aka "shovel", first off prototype) for our cars (both 5 & 6 speed). Below is a comparison between my original three piece shaft and the new PST carbon fiber one piece for my 5 speed car.
As you can see, the one piece carbon fiber is physically larger with a 3.25" OD while the three piece measures at 2.56" OD on the front and mid shafts and 2.95" OD at the rear shaft. The larger diameter gives the carbon fiber shaft much of its strength and resistance to bending. PST wanted to go even larger in diameter but couldn't fit it due to the close proximity to the rear suspension X brace and the power cylinder for 4 wheel steering (both are located near the rear differential). The PST shafts feature a 15o weave pattern to improve strength (necessary due to our extreme length). Earlier carbon fiber shafts incorporating a 45o weave pattern were only able to safely replace the front two shafts.
So how much weight did I save?, how about 31 lbs! (44 - 13 = 31), most of which is rotating weight.
Here are some more detailed shots of the old and new driveshafts.
At first glance I was worried over the extra length on the slip yoke. However, PST explained that this was the standard size they receive for our application and there was no need to cut it down to size since the transfer case easily swallows the extra length (I also checked it myself). Going to the back end (rear diff), the new U-joint flange required new holes to be drilled into the companion flange. The bolt pattern is slightly wider than the original and PST uses a slightly smaller 3/8" bolt as opposed to the stock M10 (nearly same size). Note that the driveshaft locates itself off the high precision raised ring and not the bolt holes. The companion flange has a mating female ring to keep everything concentric.
The drive shaft wasn't shipped with the bolts for attachment to the companion flange, so I had to purchase them. PST recommends a high grade (grade 8) 3/8" x 1 1/4" with lock washer and nut or a flanged / self locking nut. I also purchased a new oil seal and companion flange nut as shown. The companion flange washer is the only item I reused. Another difference from the stock driveshaft is the addition of zerk fittings at both U-joints that will require periodic greasing for long efficient life (zerk / grease fitting is the small gold colored object near center of the right pic).
In order of expense, here is my total investment:
|$790||One piece CFDS (group buy from PST)|
|New companion flange (modified for new bolt pattern by PST)|
|$12||Shipping & handling (CFDS and companion flange sent UPS ground to my house)|
|$6.51||New rear differential oil seal (MB569924)|
|$2.86||New companion flange nut (MA143517)|
|$1.70||4 new grade 8 bolts - 3/8" x 1 1/4"|
|$1.02||4 new grade 8 nuts - 3/8"|
|$0.51||4 new lock washers|
That puts the grand total at $864.60. I actually had to purchase the oil seal twice, it is easy to damage if you don't have the proper tools (more on that below). You will also have to spend a little on topping off or replacing the transfer case and rear differential fluids. I also opted to keep the original oil seal on the transfer case.
If you decide to install a new oil seal(s), I found that a 2 1/2" ID tube worked well to drive the new rear differentail seal in place. It will only set you back about $1 at your local auto parts store (look in the exhaust pipe section). It does take some patience to fully and properly seat it, just take your time and don't allow it to get cocked or you may be buying another seal.
The Service Manual also recommends using axle grease to help break-in, prevent rust on the flange, and help the front lip collect dirt and hold it away from the sealing surface. Also apply some grease to the back side of the washer. The following schematic was taken from the Service Manual.
As far as assembly torques are concerned, I recommend the following values:
|Driveshaft to companion flange (4 bolts)||40 ft lbs (each - use grade 8 bolts)|
|Companion flange to rear differential (1 nut)||137 ft lbs (I used brakes to hold the wheels)|
Recommendation to anyone that removes a stock driveshaft: When you remove the stock driveshaft, carefully note whether shims / spacers were used with your rubber isolators and save them with your shaft. Not all cars will have shims, but if you do keep each shim with the corresponding bearing support hole (zip tie or wire them to the correct locations). You never know if and when you may want or need to reuse your old shaft. If you mix up the shims or don't install them at all, your driveshaft angles will be messed up resulting in excessive wear and noise potentially leading to an early failure.
For more details or purchase inquiries, contact:
Veldhuis @ Precision Shaft Technology
1717 Overbrook Ave
Clearwater, FL 33755
Email Mark @ PST
since August 7th, 2002
Last Updated: 08/08/02 07:50 PM