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

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.

The FD RX-7’s Inner Axle Stub and Axle Shaft Bar tend to fail around the 400-550whp mark (400tq) depending on drag racing or on the street.  Upgrading to axle shafts made from 300M alloy steel was a solution back in 2010, but they have not been readily-available in nearly 15 years due to the price and the problem of the outer CV joints being the next limitation at around 600whp.

Drop-in axle solutions like the Driveshaft Shop “Pro-Level Axles” are very expensive, costing a few thousand dollars and will still be limited by the weak factory differential housing.  This is when the Ronin Ford Explorer 8.8” diff kit (which includes upgraded axle shafts) is without question the best choice, costing significantly less than the upgraded axles alone.

Changing the differential’s final drive ratio is an FD RX-7 is fairly limited and it can be extremely expensive, with new gear ratios priced at $800 to over $1,000.  At this point, the Ford 8.8” differential is a far better solution with countless gear ratio options ranging from 3.31 to 5.71 and typically cost around $225 but can range from $150-400 brand new.

The stock FD RX-7’s final drive ratio is 4.10 for manual cars and 3.90 for automatic cars.  Aftermarket gear ratios that are readily available are limited to 4.30, 4.44, 4.77 which are from the 04-08 automatic Mazda RX8, 04-11 manual Mazda RX8, and 2009-2011 manual RX8 respectively.  It is possible to source used differentials from the RX8 for less money and pull the gears out of them.

Lower final drive ratios are harder to find and might not even be possible.  On the RX-7 forums, there are threads that talk about the possibility of using 3.73 gear ratios from a 1989 MPV Sport (5-speed) and a 3.727 gear ratio from a USDM B2600 2wd truck.  However, it does not appear that anyone has used these successfully and most people end up swapping to a Ford 8.8” diff.