Differential Swap from an RX8 to an MX5
Updated: Jul 26, 2021
The reason for this modification is simply: Acceleration. Any old-school hot rodder you talk to will say that the cheapest way to make a car move out of its own way a bit faster, is with a shorter final drive.
Those that are familiar with the third generation MX5 (chassis code: NCEC) will tell you that it shares quite a bit with the first and second generation RX8 (chassis code: SE3P). Suspension, brakes, some electronics, and some drivetrain pieces. Originally I found that the RX8 SE3P has 3 final drive ratios available and only one for the MX5 NCEC in the US Market.
RX8 SE3P Drive Rations:
Automatic transmission 4.3:1
First generation (till 2009) Manual Transmission 4.44:1
Second generation (2010 +) Manual Transmission 4.77:1
Automatic and Manual Transmission 4.1:1
After some researching, I found that there were people who had performed this swap and had documented a portion of it. I decided to jump into the deep end and go for the shortest ratio available. I used information from this post: https://miaterati.tumblr.com/post/116920440576/i-decided-to-change-the-differential-before
This modification will work twofold for making the car more reliable and faster. First, it would provide a noticeable boost in acceleration without spending large amounts of money on power upgrades. Second, the upgrade will use stronger components than those that are known to fail on the track on NCEC MX5's. Additionally this modification cost 1/4 that of a supercharger setup without cost of installation, of course I do all of my own work so that cost is not accounted for in the comparison.
I took to gathering a list of components I believed I would need:
1. Driver and Passenger rear knuckles from a manual RX8 SE3P. I found both on Ebay.com used from a 2004. Each side was roughly $65 for a total of $129.89 for both with shipping and tax.
2. 5/8" thick aluminum plate from cut2sizemetals.com, I got a much larger piece than needed. Roughly $60 with shipping and tax.
3. Rear wheel bearings for 2004-2012 RX8 manual front local auto parts store, roughly $36 each, $72 total.
4. Rear CV axle nuts from local auto parts $6 each. $12 total.
5. 4.77:1 Differential with axles from 2011 RX8 Manual. I sourced this from a private seller out of the Dallas area found through facebook marketplace. I made the drive from Houston and picked them up. $500 total.
The axles are thicker and have larger diameter spline into the hubs. This is the reason the rear knuckles must be used for this swap. The differential has an added cooling fin arrangement to the rear cover which will help keep it cool during track day conditions. It also comes equipped with a factory helical limited slip.
Photo found at https://nielex.net/maintenance_rfm3.html
The removable upper differential mount is different from the MX5 unit as shown above. I knew using my drill and a toothed burr specifically designed for aluminum removal. I was confident that I could perform modifications to my MX5 mount and make it fit without issue.
The exhaust has to be removed along with the powerplant frame. The transmission must be supported while this is out to ensure the valve cover does not contact the firewall. The cv axles removed, the differential will drop out with only 2 bolts removed from the upper mount.
Once removed, the modifications must be performed in order to fit the differential with the factory MX5 powerplant frame. The 5/8" block was cut and drilled to fit the new differential. The holes on the diff side were slotted in order to accommodate the wider RX8 differential.
I then worked on the rear knuckles. My MX5 has 100mm Long ARP wheel studs to ensure safe regular torque during track day use. Transferring the studs, meant that I would have to press the hubs out of both sets of knuckles and transfer the studs to the RX8 hubs. While I had them apart, I pressed in the new wheel bearings for good measure.
Once the rear knuckles were all together
again, I put them aside until the remainder of the differential install was completed. The axles are much easier to reinstall with the knuckles removed.
The upper mount was next on the list. I needed to cut a trough in the rear of the mount. The top of the RX8 differential has a raised portion that interferes with the MX5 unit.
Once the mount was cut to fit atop the differential, it was lifted into place and bolted to the chassis.
We checked the fitment with the powerplant frame and noticed we had to widen the holes in the power plant frame as the newer differential was not only wider, but the studs that bolt the differential to the power plant frame were further forward, causing interference when trying to mount it up to both the transmission and differential. After that minor issue, everything from the transmission to the differential was installed including the exhaust.
The final parts to install were the cv axles and the knuckles with speed sensors. At first I thought this issue was going to stop the completion of the project. The sensors in the RX8 knuckles for ABS are a larger diameter which meant the MX5 sensor didn't fit. The securing bolt for the sensor is also a larger 8mm diameter vs the 6mm MX5 sensor. I did note that the sensors were the same length. This gave me hope. I decided to take a step-bore drill bit and enlarge the holes to just over 8mm and then bolted the sensors into the knuckles on assembly. Please note that this has worked without issue since completion of this project.
Everything put together, the final test was the first test drive. The difference was felt immediately. My MX5 had punch like it had not had before. Comparatively it was as if it gained nearly 50hp. It is amazing what a short final drive feels like. The stock 2.0l 4 cylinder isn't exactly a powerhouse. Putting down around 145hp at the tires when tested, it won't be winning any drag races. This said, there was a notable difference in acceleration when I attended the next track day. At Motor Sports Ranch Houston originally V8 and turbocharged cars that were a LOT faster than I when exiting a corner with the 4.1:1 final drive now weren't so quick to leave me behind. I was nearly able keep up with a Porsche 987 Cayman down the straights. The gearing for the corners put me in the middle of the torque around 4000 rpm when accelerating from the apex.
If I were to rate this modification in terms of fun-to-drive, I would rate it a 4 out of 5. As far as sensation and overall pleasure, it is amazing. I will say that you will have more cabin noise from the higher revs on the highway. At 80 mph the engine speed now sits at 4250 rpms. That aside, it is amazing and a recommended addition to any NC MX5.
Total cost of parts was a little under $775 and a total time of installation of 6 hours between me and my wife. The limited slip 4.1:1 differentials are quite expensive and mine had particularly low mileage, roughly 30,000. I sold it and the CV axles for $1000 through facebook marketplace but saw these listed for $1300-$1500 on Ebay.
Total price for the swap -$225. So we made money on it.
Here the difference on YouTube: https://youtu.be/9iNmWfZupqs
Want to see me track it, follow me:
Additionally, thank you so much to my amazing wife who helps me break and fix things, go on these journeys and takes pictures. Follow her on social media:
Disclosure: I purchased all these items with my own money and the company had no prior knowledge to this promotion.