I received this from our member Bucksarrow.
Now just waiting for the snow to go away and the track to open buck

 

for those that could not open the link. jim

Last fall after talking with Jim (amx1397) about switching my differential to a 3.90 I pretty much decided that I was going to do the switch. Jim sent me a list of the parts I needed It all started to come together.Jim sent me a differential.l First I got a differential. There is not a wavetrac available so, I disassembled the differential with help over the phone from Jim, after getting the center section out I sent it to Phantom Grip in Florida and they installed the phantom grip for $500. Next I needed the rear half of a non srt6 drive shaft, bought one at a salvage yard for$125 plus shipping. With the limited car you can use your stock driveshaft,( only the srt needs to use the rear ½ of the driveshaft) .

You can use your rear cover or , have a machine shop mill the mounting bosses on the new diff rear cover down to 1 and 3/64. You will use the stock rear mounting bolts. The mounting bolt in the front of the diff either has to be cut down to 2 1/4 X 10x1.5 mm or buy a the 3.90 diff in the reverse, and trplace the driveshaft bearing, install the drive shaft. Lining uo the notches. Now install the exhaust. You will need to get the TCU from jim at a cost of $600 you could get TCU from others but I know his is going to work.and is made for this diff. he does not want to use your TCU but will if you want , keep your old diff and TCU so if you ever decide to sell

Jim is working on a wavetrac for this diff ,I just could not wait.

My son and a friend along with myself installed the 3.90 start to finish 1 1/2 hrs. Feels like I picked up quite a few horses, butt dyno LOL, runs out real nice, took it up to 115 mph and no vibrations.

new one

Removing the x fire diff was a piece of cake. First of all remove the xhaust from X all the way back,remove the bolts holding the mid shaft bearing in place and remove the bolts holding the driveshaft to the diff, remove the driveshaft.

Place your tranny jack under the diff before unbolting. Lower the stand, nose dive and turn the diff slowly until you can get one of the axles out. Then turn the diff the other way and remove the other axle lower the diff to the floor.

buck

 

We finished our research with the wavetrac, the crossfire wavetrac fits our new to us differential with the 3.90 gears.
this means that if you want a 3.90 limited slip diff, you can use the wave trac that was made for the crossfire.

By math the increase is equal to 70HP ,,, 60 mph at 2600 rpm and 80 at 3400rpm this is with stock size tires
this means most srt 6 crossfires with some mods will run in the 11's with drag radials ofcourse. jim

 

If one were to install this in a manual NA car, how big of a difference would this make?

 

it would feel like an extra 50+ hp it would be very hard not to burn tires in first gear, in otherwords u could start off in 2 gear and it woiuld be like you got pushed of the line,, your gas mileage would go to sh**t . the cost for a 390 diff for your car would run between 5 to $700 i do no have facebook don't want it either 954 336 3405

 

Thank you and thanks for the clarification on the gas mileage too. I drive mine a lot and enjoy the 360+ miles to a tank.

 

for u if you want i can send u a pumpkin (rear end) 307 gears that would increase your mileage to around 400, the way you sound you drive, with 360 per tank,, you may get more than 400 miles per tank let me look and see if i have a307 for a MT. the cost would be around 250. + shipping to ship would be around 100. I have a shop that can install in chicago