Mazdaspeed 6 forward differential mount
By Jess tremblay
Turbine Tech Inc, R&D manager
There has been much talk in the past few weeks about our latest forward rear differential mount for the Mazda speed 6. I’m putting this write up together to help people understanding what it does and the effect of such an upgrade on the car. This mount has primarily been built for our local customers and designed so that it would give the differential housing a proper support and thus, preventing it from breaking. We wanted to build something bullet proof, something that would never need to be taken off ever again. The Mazdaspeed 6 is a well-built and studied vehicle. It has a strong, powerful motor, sturdy drivetrain. Fuel economy is very good, the ride is plush and comfortable and still it handles like on rails. An excellent car overall!!
The dynamics behind acceleration
What is really happening under moderate or heavy acceleration? In simple terms, releasing the clutch pedal allows the crankshaft to transfer its energy to the gearbox from which the torque is sent to the front and rear wheels via the CV joints and driveshaft.
So, here is your mighty engine, being pushed backward and sideway due to the motion of the 3 axles. At this point, your engine mounts are stretched to their limits while the engine is being lifted up further and further back in the engine bay. In that particular situation, we will concentrate on the mounts. What are they actually doing? These soft & sweet rubber things with split personality start accumulating energy at a sick rate. The bad thing is that they do not have the ability to handle stored energy and must release it as soon as they can. That release order comes from your left foot when you press the clutch pedal to shift again. Those nasty little things changes from Dr Jekyll to Mr Hide and quickly propel the engine forward with violence! But wait, those mounts won’t just sit there waiting for your order. No way!! They’ll let that energy to go away when any occasion arise, which happens during any momentary traction loss from your wheels. Just like Jack the Ripper, those little suckers perfected their move along the years, operating in such a way that you have no clue what’s going on, until…
…. OOooOopps! Surprise!!
This customer went to have his differential mount installed. A purchase he made as a safety measure against the well known differential housing failures. Car is lifted up, differential dropped, rear bushings on the rearward mount installed. Then, minutes before installing the upgraded forward mount, a close inspection of the threaded knobs showed a beautiful hairline crack expanding nicely through the housing. Jaws dropped, our little fellow couldn’t believe his eyes! That was a close call!! The above picture is taken from a 2007 MS6 with barely 9686 miles at the odometer. This car has never been raced, driven in an excessive manner or abused whatsoever.
Above is a side view of the OEM forward mount. That ‘S’ shaped mount connects to the housing by the two screws on the left. When accelerating, the torque effect from the driveshaft is directed to the rear wheels via the differential. The rearward mount provides an anchor point on the vehicle for those forces that are trying to lift off the front of the car, just like it does with those 5000hp funny cars. Remember the above story about Jack the Ripper? Same thing goes with the differential mounts. The rubber bushing stores and releases energy as traction come and goes. The softness of the rearward mount bushings doesn’t help either. You must keep in mind that your engine is somehow rocking like a teen at a typical SOAD concert. The driveshaft has no choice but to send those repeated blows to the differential housing. If you aren’t too faint of the heart, you can picture yourself a little baby seal being shaken to death in the shark’s jaws. This is exactly what happens to your rear differential when you shift hard and traction is scarce. Not a pretty sight I agree.
The above picture shows a different view of the forward differential mount. The mounting point on the frame is about 3 inches offset from the two screws. With that in mind, it’s easy to understand how the compressive forces apply to the mount, trying to twist and rip off the attachments.
Notice the rearward horseshoe mount at the top of the picture. There are two mounts on that differential housing, which might have been a tad confusing for some people.
The rearward mount acts as a pivot point for the differential. This one comes with particularly soft rubber bushings, as demonstrated in the pictures below.
Mazda engineers are not dumb! They went with soft bushing so that shifting & clutch engagement would be smooth as silk. The car has certainly not been intended for heavy abuse or race purpose. This being said, the rear wheels being CV joint driven, the way the differential housing is connected to the frame doesn’t really impact on the quietness of the ride. However, bent or out of balance drive shaft, CV joints or any other rotating component will create vibrations that will be felt at a higher level whenever urethane bushings are used. The key to a smooth ride is to keep your car in excellent mechanic condition. Regardless, your car should always be kept in excellent mechanic condition.
All the cars we tested, either with partial or complete rear differential urethane conversions showed no increase in vibration during cruise or accelerations. However, if you perform such an upgrade, you must keep in mind that increased vibrations or harshness is always a possibility!!
Pictured below is the upgraded mount.
To solve the cracked housing syndrome, we’ve come up with a different design that would replace the OEM forward differential mount. It keeps the stock attachment at the left of the housing, but adds another anchor point, using some of the existing bolts that hold the two cases together.
The above picture shows the machined surface of the case where the bolts are. Those spot faces are roughly 1/16” deep. This is a standard manufacturing process that ensures a nice and flat surface for the bolts to sit.
The upgraded mount has been designed to precisely match those points so that the fingers are embedded into the case when the bolts are tightened. This provides a sufficient clamping force that holds the mount firmly in place. This design also keeps any radial loads away from the 8mm bolts.
Here is a close view showing the matching points. The mount now sits in place nice and high.
The thickness of the mount is the reason to why longer bolts are supplied with the mount. Lock washers are not an option here, but an absolute necessity.
The mount comes with split urethane bushing, 95A durometer. We recommend getting our optional bushings for the rearward differential mount as complement to the upgraded forward mount. The stiffer the bushing, the less energy it can store and release.
The rear bushings installation doesn’t require any special tools. The model we supply can be fitted without pressing out the stock bushings. You simply need to cut the soft OEM rubber and clean a bit with a knife before pressing the urethane bushes in place.
We are using a hydraulic press in that picture but any standard bench vise will do. With the help of a little grease, the bushing slips easily in place.
While the urethane bushing will not create additional vibrations, clutch engagements proves to be more direct with torque being transmitted faster to the rear wheel. The difference is especially noticeable if you blip the throttle on and off repeatedly with the clutch engaged. We never had to deal with a single customer complaining from increased harshness. However, don’t expect your car to be exactly the same as before. You wouldn’t expect a pair of Borlas to sound identical to an OEM exhaust, would you? Same thing goes for urethane conversion or any suspension upgrade for that matter.
The installation of the forward differential mount is a simple walk in the park, a 60 minute job at the most.
The rearward diff mount bushing installation takes a bit longer due to the remote location of the rearward mount. On a shop lift, with the proper tooling, it takes our technicians no more than 2 hours to have the rearward mount bushings installed. The complete removal of the differential is not required. It simply needs to be dropped a couple of inches. You can access the 4 bolts at the top of the housing with a 14mm wrench once the sway bars are removed. In fact, you don’t even need to remove the CV joints.
**IMPORTANT**
Detailed instructions are supplied for a quick and easy install. However, this is not for the backyard mechanic on ramps. The removal, modification and installation of any drivetrain, suspension or braking components are no room for practice and MUST be handled by certified and competent technicians!!
To be continued……