Welcome to part 3 of the Mazdaspeed3 AWD Swap! If you missed part 1 and part 2 blog posts, then catch up by visiting these links. Lots of images in this blog as I get the CX7 rear subframe installed and figure out the correct control arms to use for the rear suspension.
Typically when I mention the AWD swap Mazdaspeed 3 to a fellow Mazdaspeed enthusiast, they assume I am using the rear subframe from a Mazdaspeed 6. While I understand their logic, incorporating one would require extensive fabrication.
Here’s why: the Mazdaspeed 6 has a very different chassis architecture vs the Mazdaspeed 3. It is important to note because it directly affects the subframe and chassis interface.
If it’s not the speed6 then what do I use? Good news! The Mazdaspeed 3 uses a chassis design based on a Ford global chassis used with various models in Mazda, Ford, and Volvo. Enter the Mazda CX7.
So I went hunting for a Turbo AWD CX7 model year 2006-2007…this is a great time to bring your buddies along for some junkyard fun!
Luckily I found one in a local junkyard that was still complete enough. Not knowing exactly what I needed from the rear-end suspension and drivetrain, I opted to take everything; driveshaft to differential, plus the ENTIRE rear subframe and suspension.
$380 later, we are driving home with our newfound treasure and ready to take on the swap! I was eager to see how this would bolt into the Mazdaspeed 3, so we went straight to the shop.
We wasted no time removing the speed3 rear subframe… literally six bolts and removing the brake calipers is all that is required.
Knowing the OEM fuel tank is in the way of the mid-driveshaft, we opted to just remove it right then as well. A handful of bolts and some fighting of the fuel tank filler and it’s out also. At this point, we are maybe 1.5 hours into this and the car is ready to accept its fate.
With both the MS3 and CX7 rear subframes out and sitting side-by-side, we took the opportunity to compare them. Checking the most important things first, we looked at the mounting points for the subframe to chassis. These all checked out visually and again after measuring to be the same…but this is where the similarities ended.
The trailing arm/hub assembly is very different between the two models. The CX7 appears to be much heavier duty and more complex. Doing some research, we found that the CX7 uses a different style of parking brake. The parking brake is actually a drum brake inside the rotor hat of the disc. Either way, the CX7 suspension looks heavy…which is not ideal for Racecar. The width also appears to be wider by a few inches.
The last noticeable difference is the addition of a rear differential – which is the whole goal of the project – so that is a good thing! That said, the OEM spare tire location in the MS3 will interfere with the fitment of the differential.
Not an issue for Racecar and my Sawzall! I cut the entire spare tire tub out since my new fuel cell will be going there.
We are ready to mock up the complete CX7 rear suspension with the spare tire tub removed. Knowing that the mounting points are the same, we installed the entire CX7 system to see how it fit.
The six mounting points lined up perfectly, confirming our initial measurements – it’s almost as if it was meant to be! Next, we tried to get the trailing arm forward mounting points bolted in but fought them, eventually giving up. We are confident the springs were fighting us, and the trailing arms would have bolted in had we removed the springs.
Moving on, we wanted to see how the track width looked before spending any more time on the trailing arms.
As we suspected, the setup was too wide – unless you want wider – which I did not want for a high-speed straight-line drag racer.
From the side view, the wheel’s centerline looks good, and the meaty 255/50R16 looks badass, but the CX7 springs obviously do not play nice. The monster truck ride height won’t work.
We planned a “hybrid” of CX7 and MS3 suspension parts, as advised by a friend, @junkiebuilt, that did a GEN1 AWD swap using a Honda drivetrain. We will use the CX7 subframe only and the MS3 trailing arm/bearing hub and control arms.
This combo was the ticket! The Mazdaspeed 3 control arms bolted into the CX7 subframe without issue, allowing me to retain my CorkSport Camber Arms and CorkSport Toe Arms, as well as the lower OEM control arm. Along with that, I get to keep the Mazdaspeed 3 trailing arm, which is not nearly as heavy or complex.
This setup also retains the OEM parking brake, my Mazda 5 rotor, and my MS3 calipers. Ultimately this is looking like a very straightforward swap with no fabrication. Don’t mind @farvaspeed6 looking at, um, something.
With the hybrid CX7/MS3 setup on the car, we wanted to see how things were lining up. The meats went back on!
Ah, much better this time. The wider stance is gone, and the wheel tire looks right at home. Surprisingly the tire tucks under the fender with just a tad amount of rubbing.
With the day of excitement winding down, I wanted to check on the last thing to see if this truly was a direct bolt-on swap. Unfortunately, the OEM MS3 rear wheel bearing is not the same as the CX7 wheel bearing. Being FWD, the MS3 wheel bearing does not have splines for an axle…duh. So I have to use CX7 wheel bearings on the MS3 trailing arm/hub assembly.
This is where my luck ended.
With the MS3 wheel bearing removed and the CX7 wheel bearing set next to the trailing arms…things look good. But they are ever so slightly different.
The CX7 wheel bearing has a slightly larger bore size, and the bolt pattern is somewhat different. I will never understand why Mazda went through the effort to make these so close but not the same. Either way, this was not a job a hand drill and grinder could fix. This needed proper measurements and machining.
I reverse-engineered the CX7 bolt pattern and hub bore. 3DP printed that to verify then off to the machine shop to get the one-off work done.
The existing bolt holes were welded closed, the ground flat, and the new holes were drilled following my measurements. The hub bore was also enlarged to match the CX7 wheel bearing.
With that one and only fab job complete, we had actually finished the CX7 to MS3 rear swap.
Minus the machining for the wheel bearings, the rear subframe, and the suspension swap was actually very easy and straightforward. This is great news because it could have been the death (or very expensive aspect) of the swap. Ultimately this part of the swap being so easy makes it a much more viable project for the average enthusiast.
Alright, that wraps up the rear subframe swap, a huge milestone for the build. There are plenty more milestones to overcome and those are coming up in this multi-part blog series!
I hope you are enjoying this series about the AWD Swap Mazdaspeed 3, stay tuned for more blogs to come!
Thanks for tuning in!
-Barett @ CS
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