The 2004-2006 Pontiac GTO comes from the factory with a 3.46 gear ratio limited-slip differential. Limited slip means it is not a full true locked Posi rear end. Both rear tires are not always spinning at the same speed. They will both spin at the same time and same speed when enough torque is placed on the gears, such as pulling a holeshot or just stomping on the gas to squeal the tires.

You can download the “Rear Drive Axle” section of the service manual below. The differential is the same for all three years, 2004, 2005, and 2006 Pontiac GTO.

The rear ends on these cars are pretty sensitive to which fluid they have in them. I’m going to show which fluid works the best and how to change it.

Step 1:

Torco is by far the best differential fluid for these cars. I’ve tried stock and other synthetic brands, Torco RGO outperforms them all! If you use any brand other than Torco, I cannot guarantee your differential will not still clunk or make noises you don’t want it to. Only use Torco RGO 85w140!

And yes I realize Type F friction modifier is for Ford, Chrysler, and Toyota. But the Pontiac GTO rear end also uses a Type F friction modifier.

Step 2:
Drive your vehicle for at least 10 minutes to get the differential fluid warm. You want as much of the fluid to come out as possible when you drain it.

Step 3:
Jack up the rear end of your vehicle and place it on jack stands and look at the differential. Use the diagram  below:

• Number 1 – Differential Fluid Fill Plug
• Number 2 – Differential Fluid Drain Plug

Remove the drain plug bolt (Number 2) and drain the fluid. Be patient, let as much fluid as possible drain from the differential. You can loosen the Fill Plug (Number 1) bolt to let air into the differential for a faster drain if you wish. After the fluid has completely drained, reinstall the drain plug (Number 2) and tighten it to 20 ft/lbs (foot-pounds) using a Torque Wrench. Now completely remove the fill plug (Number 1) for the next step.

Step 4:
Using a funnel, pour no more than 2 ounces of Torco Type F Friction Modifier into the differential. Do this first.

Next, pour the Torco RGO differential fluid into the fill hole (Number 1). You will use just under 2 quarts. Fill the differential until it starts to drip out from the fill hole. When the fluid is coming out of the fill hole, the differential is full. Re-install the fill plug (Number 1) and tighten it to 20 ft/lbs (foot-pounds) with a Torque Wrench. Take the car off the jack stands and take it for a drive.

A quiet differential is a happy differential. Hope this service tip has helped. Thanks for reading!

The post 2004-2006 Pontiac GTO Differential Service first appeared on Mark Quitter Racing.

Today’s project was to install a new 3.90 Ring and Pinion Gear along with installing a one-piece driveshaft. The factory gear is a 3.46, and also came with a 2-piece driveshaft. I am hoping to gain a little more on the bottom end as far as speed goes with the new 3.90 gear, as well as putting in a lighter and stronger one-piece driveshaft instead of running the horrible 2-piece one that the car comes with.

Special thanks to Kevin at Straightline Performance in Joliet Illinois for the installation. Great work as always!

PARTS LIST:

Motive Gear 3.90 Ring and Pinion

Drive Shaft Shop 3.5″ Driveshaft 1000HP 1-Piece Aluminum Shaft with Direct-Fit Rear Flange

THE INSTALL

Drop the pumpkin, and install the new driveshaft:

Remove the gears, and install the new ones:

Use an excessive amount of heat on the side adjusters to remove them. Do not be afraid to use a torch. This is probably the hardest part of the job and is the only way to remove the ring and pinion gear. Do not re-use the stock adjusters. Once they are removed, throw them away and install new side adjusters which can be purchased at GForce1320 Engineering.

Parts used from GForce1320 for the installation (minus the gears). Click on the links under the pictures to visit the page where they can be purchased:

Gutted the differential and all new parts installed:

RE-INSTALL THE DIFFERENTIAL:

Unfortunately, I have no pictures of the rear end re-installed. We ran out of time trying to get the car back together.

FINAL RESULTS:

The new gears and drive shaft definitely made a difference. The car pulls great and feels faster. And having a lighter, stronger driveshaft makes the rear end virtually bulletproof!

The only real concern I do have is the gears are noisy. Which is a repeated complaint about the Motive Gear 3.90 setup. Have yet to read or hear about a 3.90 gear install without any gear noise, so all is well.

Straightline Performance (815)485-7741

The post 3.90 Gear and Driveshaft Installed first appeared on Mark Quitter Racing.