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Rear Gear NVH

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I have not touched the rear gear on my 2012 Boss (40k miles) and it whined a little at 55 mph at little to no throttle input. I switched to a carbon fiber driveshaft and it was like flipping on a loud irritating switch. The whine is to the point of maximum irritation as it hurts my already tired ear drums when it happens around 55 mph under no load. I changed the gear oil to Mobile 1 gear lube (85W140) but this had no effect. I know Ford had lots of complaints with 3.73 gears so has anyone tried FMS-M420988373SF Ring and Pinion Gears, 8.8" 3.73 RING GEAR AND PINION SUPER FINISHED. Maybe this was created as a fix?

https://performanceparts.ford.com/part/M-4209-88373SF


Applying "Super Finishing" treatment to the gear faces. Super Finishing creates a highly polished non-directional surface that offers the following advantages:



    • Eliminates Break-in
    • Reduces Friction, Wear and Heat
    • Increases Durability & Component Life
    • Reduces Fatigue and Component Failures
    • Reduces NVH"
Any feedback would be appreciated if you have tried these finished gears.

Tim
 
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Cheapest option is to inspect the pinion angle. You may have not touched the rear gear but lowering the car or other suspension work may have altered it enough to add noise. IMO the carbon drive shaft didn't make it worse but amplified the problem due to it being lighter weight and resonance dynamics.
 
I’ve posted here before regarding this, and unfortunately...

There’s no fix to stop the noise. You can try “clocking” the driveshaft every 90° until you notice a change, but that may or may not work for you.

The 8.8 is just plain loud. The rubber bushings on the 3rd link and in the lower control arms help to mask this, but as they wear out (which is probably what you’re approaching) or if you replace them with something more rigid (in my case, the FFRP parts that contain poly bushings) it becomes... very audible.

I’d say it’s even more noticeable on the Boss Mustangs, on account of the 14 lbs of sound deadening that has already been removed from our cars.

If I were you, I would try the driveshaft clock (others have had limited success with this) first. After that, I would add Dynamat under the rear seats. The 3rd link mounts there, and that’s where most of the noise transfer comes into the cabin. If you’re still not satisfied, I’d suggest replacing the OEM rubber bushings in the LCA’s and 3rd link. Only after those would I consider swapping the gears.

Believe me, I supremely understand your frustration. I just got back from a 700 mile road trip through Appalachia, and my ears are still ringing.
 
Thanks for the feedback. I do not have adjustable upper or lower trailing arms so if the driveshaft angle is out slightly, which it probably is because of the lowering springs, I cannot adjust it. Do they make a adjustable upper control arm mounted in rubber bushings? Might try the RTR with the Max-C Dual-Durometer Bushings.

Tim
 
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I see a Steeda unit. It has poly bushings on the adjuster itself, but you can use the factory mounting plate to the chassis, which is rubber.

Be sure to update when you do something. I’m always curious of other people’s experiences.
 
I checked my drive line angles.

Transmission center line angle = +2.8 degree
Driveshaft angle = +1.8 degree
Rear Pinion angle = +2.1 degree

+2.8 - +1.8 = +1.0 degree Total Transmission angle
+2.1 - +1.8 = +0.3 degree Total pinion angle

+0.3 - -+1.0 = - 0.7 degree operating angle which is supposed to be 1.0 degree or less for happy u-joints.

The pinion angle is supposed to be -1 to -2.5 (not really sure for coil spring set ups) for all rubber mounting points which I have on my mostly stock Boss. I would think that must be for heavy acceleration only which does not happen all the time since I am not drag racing this car.

If I change the pinion angle to +1.8 (+1.8 - 1.8 = +0.0) +0.0 - +1.0 = -1.0 degree operating angle under normal driving but will go more positive during heavy acceleration which could cause some binding. I have a multi position lower control arm mount which allows me to move the connection point down to the lowest hole which increases the pinion angle slightly (+2.1 middle hole to +1.8 lowest hole).

I am waiting on the arrival of my adjustable upper control arm to check out these new angles to see if it affects the NVH any before I buy new rear gears. Received last week but this item would not fit without adjustable lower control arms as it was too long even adjusted all the way in.

The angles were checked with a digital angle finder and I used the bottom of the transmission drain plug and a flat on the bottom of the differential housing as the check points and the bottom side of the driveshaft. Note: on Mitutoyo digital angle finders make sure you check all points with the same surface as I accidentally turned it upside down on the axle housing location which changed the reading from + to -.

Hopefully my math and checking points and logic are correct? Updated 05 Sept 2019 but have not driven the car yet to see if the gear whine changed.

Tim
 
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xr7

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You want to get the angles to match, and you are close. The 1 degree operating angle is for the individual joints. You don't want a joint operating at zero degrees, it needs to move some to keep the needle bearings moving. At zero angle you will develop flat spots and grooves in the joints and the vibrations will shake your teeth out.
Spicer has an online drive line calculator that will help with this.
https://spicerparts.com/calculators/driveline-operating-angle-calculator
 
Bad news after the latest suspension adjustment.....at 60 mph it is still too loud but goes away above 61 mph and below 59 mph. I guess a gear change is in my future.

Tim
 
Bad news after the latest suspension adjustment.....at 60 mph it is still too loud but goes away above 61 mph and below 59 mph. I guess a gear change is in my future.

Tim

My advice is for you to pull the pumpkin cover and check the fluid and runout before you go off ordering parts. I only say that because I’ve experienced the same thing you are, thought the same thing you did, and upon inspection... nothing was wrong.

It is supremely annoying. I don’t believe anything is “wrong” however. Good luck, and I look forward to helping you out any way I can.
 

xr7

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Any thought of reinstalling the original driveshaft back in it? Or pulling the u-joints on the carbon shaft. I had a car that had a vibration, started at 65 and quit at 70mph. Pulled the drive shaft, could feel anything wrong, pop the joints apart, looked fine, put some fresh grease in reassembled and vibration was gone. I hate u-joints that you can't grease.
 
Any thought of reinstalling the original driveshaft back in it? Or pulling the u-joints on the carbon shaft. I had a car that had a vibration, started at 65 and quit at 70mph. Pulled the drive shaft, could feel anything wrong, pop the joints apart, looked fine, put some fresh grease in reassembled and vibration was gone. I hate u-joints that you can't grease.

You know, you might be into something with that. When I pulled my diff cover off thinking I was gonna have to replace the ring and pinion, my diff fluid was half a quart low. It’s like the car was either shipped without an adequate amount of fluid in it, and/or it wasn’t PDI’d when it arrived at the dealership. I’d bet a thorough inspection would turn up something noteworthy.
 

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