Project FD RX7 Restomod: Part 14 – When to Upgrade the Differential

After upgrading the stock Torsen LSD, the FD’s diff housing/casing is usually next to break around 450whp, and has been known to fail at 350-400whp on drag radials.  The center of the stock cast iron diff housing is very thin to keep the overall vehicle weight down for handling.  While there are no issues at stock power levels, drag racing on sticky tires or running nearly double the stock output pushes the diff housing beyond its limit and they start to fail.  This is when bracing the differential becomes a must.

The Hokkaido-based Carshop Dream team that campaigned their “Lotas 7” FD RX-7 in the World Time Attack found out the limitations of the stock cast iron differential the hard way.  Despite using an aftermarket LSD, the diff housing could not handle the stresses of road racing with 600whp.

There are aftermarket braces and cradles on the market that try to address this issue with varying levels of effectiveness.  Aftermarket braces that support the front of the differential fundamentally change the design of the OEM drivetrain, and do far more than upgrading the diff bushings alone.

The Sikky PPF / Differential Brace that we installed reinforces Power Plant Frame (PPF) or can replace it entirely when using a traditional transmission mount.  This type of brace constrains the movement of the front of the diff and transfers the upward force/load of the differential snout directly into the chassis.  This limits and alleviates the stress on the diff housing and PPF caused by deflection, but the load is now directly transmitted into the housing which remains the weak link, where the diff is stressed internally where the leverage of the pinion gear attempts to tear the housing in half at the thinnest point near the front diff mount.

Differential cradles encase the whole diff in a thick and heavy steel structure to add rigidity to the weak cast iron housing.  Born from SCCA Super Street Modified competition, these cradles can be used in conjunction with a diff brace and together they can increase the differential housing and PPF’s power limit, but it will cost nearly $1,000 to do so.  This puts the overall price of upgrading the LSD and strengthening the factory differential at or above a Ford 8.8” swap, while still retaining the stock axles which become the next weak link.

There is not much information on how much power the differential braces and cradles can reliably handle, since most people who are serious about pushing cars above 450whp swap to the Ford 8.8” differential. Len at Bacon RotorSport Racing has pushed the limits of the stock FD RX-7 rear-end and has gone 7’s in the ¼-mile on the stock diff while making around 1,000whp. However, to get to this point he has blown up over 15 differentials on his journey to dial in his “secret sauce” clutch and hydraulic setup, along with adding braces, cradles, DSS “Pro-Level” axles, a spool diff, and custom cover that retains the main caps. This perfected stock setup ended up costing a lot more than an 8.8” rear-end and at best, can only last 30-40 passes while often blowing up after 5-10 passes, especially if he ever gets wheel hop (the pic above). He later upgraded to a Ford 9”.

Since the OEM differential housings are now so old and increasing in value, it does not make sense to push them well past their limit and risk destroying it.  Selling the OEM diff before it breaks makes the Ford Explorer 8.8” upgrade an easy decision if the car makes over 450whp and is tracked or pushed harder than a street-driven dyno queen.