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.
OUR BUILD
Our car is going from relatively stock to a goal of a 600whp+ that can reliably be tracked, drifted, drag-raced, and comfortable as a daily driver. This makes it an easy decision to jump straight into a Ford Explorer 8.8” differential rather than trying to make the stock differential work, only to have a borderline solution with questionable durability above 550whp.
The Ronin Ford Explorer 8.8” Standard Differential Kit is reliably capable of a far greater 600-800whp (conservatively) and consistently runs 8’s in the ¼-mile. They also offer a “Grande” upgrade kit that replaces the FD outer CVs with Explorer CVs to further improve durability and power potential to over 1,000whp.
Since the all-in parts cost of upgrading to the Ronin Explorer 8.8” differential kit costs less than keeping the OEM diff, bracing it, and upgrading to an aftermarket clutch-type LSD (after selling the OEM diff, axles, and PPF) there really is not a reason to ever upgrade the stock LSD with the OEM cast iron diff unless you are competing in an event that requires the stock drivetrain.
We decided to install the Detroit TrueTrac LSD into our Ford 8.8” housing, which is one of our all-time favorite street/track limited slip differentials. We also will be using Driveshaft Shop’s driveshaft and their entry-level bolt-in 800-1,000whp “Direct-Fit” axles that utilize the OEM wheel hubs, to make the installation more ‘bolt-in’ without having to source used Explorer CV joints. Stay tuned for the next installments to see the installation of the Ford 8.8” swap.
Least expensive way to reliably handle these power goals:
Up to 400whp – Turbo II LSD & Differential Brace.
Up to 600whp – Ronin Ford Explorer 8.8” diff kit with stock LSD.
1,000whp+ – Ronin Ford Explorer 8.8” diff kit with Ronin “Grande” or DSS “Direct-Fit” Axles.
1,400whp – Ronin Ford Explorer 8.8” diff kit and DSS “Pro-Level” Axles.
Average Power Limits of FD RX-7 Components**:
** The power and torque limit depends on many factors such as if the car is drag raced, road raced, drifted, or driven on the street, the tires, drivetrain bushings, suspension bushings, play in the ball joints, clutch, shocks, wheel hop, etc…
300-400whp – Stock Torsen LSD.
400whp – Power Plant Frame (PPF) and Transmission Output Shaft.
400-550whp – Stock Inner Axle Stub and Axle Bar Shaft (400tq limit).
450whp – OEM FD Iron Diff Housing.
450-550whp – Turbo II S4 (86-88) Clutch-Type LSD.
550whp – Braced/Cradled FD Iron Diff Housing (with clutch-type LSD).
600whp – Stock Axle Outer CV Joint.
600-700whp – Limit of 2003-2004 Mustang Cobra “Terminator” Aluminum Diff Housing (31-spline, diff mounting ears fail).
600-1,000whp – OEM Mazda Subframe Distorts.
800whp – OEM Wheel Hub.
800-900whp – Limit of 2002-2005 Explorer Aluminum Diff Housing (Single Ear).
800-1,000whp – Driveshaft Shop “Direct-Fit” Axles (with OEM Wheel Hubs).
800-1,100whp – OEM Subframe with Ronin Diff Mount starts to bend (requiring Ronin Subframe Supports to brace it).
900whp – Braced/Cradled FD Iron Diff Housing (with Spool and upgraded axles) – on borrowed time.
1,000whp – Braced/Cradled FD Iron Diff Housing (with Spool, upgraded axles, and a custom rear cover that retains the main caps) – on borrowed time.
1,400whp – Driveshaft Shop “Pro-Level” Axles.
1,500whp – Limit of 2007-2010 Explorer Aluminum Diff Housing (Dual Ears).
5 comments
Awesome update and you convinced me that upgrading the diff is the right move. This car is going to be a monster!
I love seeing all the work and choices going into this! I’m excited to see it running!
I noted in a separate video that the ronin front bridge once welded in may interfere with the existing brake lines. Appreciate any update on this as you install.
The Eaton Detroit TrueTrac is also used in both of the Project Mustangs!
https://motoiq.com/project-grey-mustang-5-0-part-5-putting-the-power-down-with-eaton/
I might note that this article applies to many other cars besides FD’s