1989 TriStar Syncro Project


Part of the reason why this TriStar was for sale is because the 3 / 4 gear slider hub was starting to fail.  I purchased a good used AAN gearbox with locking differential and an ADH front locking differential in Germany with the intention of getting them rebuilt and reinforced for a TDI motor with at least 50% more torque than the stock 112hp 2.1L motor.  The stock 3rd and 4th gears are traditionally weak and will fail if there is a lot of power applied to the transmission over time.  Perhaps the 112hp motor is the reason for this transmission failing so quickly?  A company in Australia makes stronger (big teeth!) 3rd and 4th gears with a slightly taller ratio to slow things down at highway speeds which is perfect for the TDI motor.

  Final Drive R G 1 2 3 4
Original 34/7 = 4.857 6.03 6.03 34/9 = 3.778 33/16 = 2.063 49/40 = 1.225 41/48 = 0.854
Modified 34/7 = 4.857 6.03 6.03 34/9 = 3.778 33/16 = 2.063 27/23 = 1.174 20/26 = 0.769

New and old 3rd gear.

New and old 4th gear.

I had Darryl at AA Transaxle in Duvall WA do the transmission rebuild.  After meeting Darryl all I can do is highly recommend him to anyone as he really knows his stuff!  His shop and his work method is highly organized, super clean and very efficient.  My transmission that I gave to Darryl for the rebuild was starting to show it's signs of age.  No serious problems with the transmission but I got it rebuilt at the right time in it's age before anything serious happened to it.  Cost for the rebuild was $1795 and $225 per gear set for a total of $2245 USD.

Syncro Transmission on assembly stand Syncro Transmission in pieces

A drive shaft decoupler essentially makes the 4WD Syncro into a 2WD when it is in the open or disengaged mode.  I feel this is an important upgrade as it greatly reduces the wear on the transmission during normal street and highway use where 4WD isn't critical or desirable to have.  The decoupler can be engaged by pulling a knob on the center console that controls a vacuum operated actuator.  This returns the transmission back to it's stock function by letting the viscous coupler in the front differential decide if it needs to be in 4WD mode or not.  I chose to install a 'sabotaged' VC that is welded so that it always transfers 100% of the torque to the front differential when in 4WD mode.  This effectively makes our Syncro into a user selectable 4x4 with user selectable front and rear locking differentials.  When all three knobs are pulled on the dash - all four wheels have equal power.  The only way of getting stuck is to have not enough traction on all four wheels!

I have chose to run tires that have a 14% larger diameter than stock.  They are the largest size you can fit on a stock Syncro without doing permanent modifications to the wheel arch openings, this is important on a TriStar has they come stock with plastic wheel arches.  I am running Continental Sportiva AT in 225/75R16 format on some aftermarket 6.5x16 ET23 alloy wheels for street originally designed for the Audi A6.  These tires are about 74cm (29.2") tall and accomplishes two things for us.  They will give us 10cm more ground clearance and they will decrease engine RPM by 14% at any given speed as compared to the stock tires.  The slower engine RPMs are important for the TDI motor because it makes all of it's power below between 2200 and 3800 RPM and redlines at 4400rpm.

We have upgraded the suspension for a few reasons.  The 74cm (29.2") tall tires will hit the wheel arches on a TriStar as it makes the openings smaller.  We also want increased ground clearance as we will be driving down logging roads and other areas that have suffered from erosion and have deep trenches.  The suspension lift we will be using will take the hub to fender static measurement from 47cm (18.5") to 52cm (20.5").  This lift kit coupled with the taller tires will increase our overall ground clearance by a very respectable 15cm (6.0") 

As a result of larger and heavier tires plus an increase in static ground clearance we need to upgrade our shocks and springs.  The larger tires will have a greater unsprung weight and will require more force to stop the suspension from moving.  Besides having slightly stiffer springs from the lift kit we also need to have shocks that can restrict movement more than the stock shocks could.   At the rear, we are using Rancho RS9000X shocks that have adjustable internal valving and about 5cm (2") more travel than original shocks.  At the front we are using nitrogen gas charged Old Man Emu shocks with 5cm (2") shock and upper control arm extenders.


2003 - 2004 David Marshall