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

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.

PLAN AHEAD.  BUY ONCE, CRY ONCE.

It is important to plan ahead and determine what your long-term power goals are.  This will save you a lot of money in the long run.  If there is ever a desire to use an expensive clutch-type LSD, change the final drive ratio, upgrade the axles, make more than 450whp, or drag racing and drifting with more than 350whp; upgrade to the Ford Explorer 8.8” differential before spending any money.

Unfortunately for most project car builds, modifications and power slowly increases over time and parts are upgraded as they break.  This is where projects become money-pits, throwing good money after bad upgrading the LSD, then bushings, then adding diff braces.  Once axles and differential housings start to break, this becomes a frustrating point where thousands of dollars have already been spent, and upgraded axles can cost a couple grand more only to be limited by the stock differential housing.

The cheapest solution for a reliable 400whp that might be able to push 550whp on the street is to brace the stock diff housing and find a Turbo II LSD. Anything above this becomes a borderline solution that might survive for a while, but it is only a matter of time before breaking something.  Once you need to upgrade the axles for a more reliable 550whp, the Ford 8.8” solution becomes the less expensive option (unless you can find a set of 300M axles), which makes the decision to upgrade a no-brainer.

Around the 400-450whp level, the transmission is also prone to breaking.  When upgrading the factory differential or swapping to a Ford 8.8”, it is important to also consider the transmission.  Paying attention to the gear ratios of the upgraded trans will determine the final drive ratio that is needed.  A Tremec T-56 Magnum, Nissan CD009, OS Giken, HGT, Sadev, etc.. will all require different final drive ratios depending on the transmission, power output, and usage.