Hi,

New to vettes so best to ask. I see people install poly mounts (ex: HPE mount on rear of diff), but is there a reason (other than perhaps noise/vibration) that the diff can't be solid mounted?

The reason I'm asking is that I've always run rod ends on my solid axle vehicles (F-bodies, trucks, etc.) and solid bushings on my IRS vehicles (E46 M3, Z4). So personal preference is to have the diff solid to the frame/body to remove all "give" out back.

Side question, looking around it seems like the diff and trans are only supported (at the back) by a single bushing. Is the "front" of the diff/trans supported by the torque tube/bellhousing)?

Thanks,
Chris

 

The C5, C6 and C7 drive trains are supported by the TWO engine mounts and the differential mount All mounts are attached to the front and rear K members.

The engine causes the entire drive train to flex. The mounts allow that flex to be controlled and to eliminate NVH into the cabin. Adding poly mounts limits that flex but there is still some controlled flex and increased NVH.

IF, you solid mount the differential to the K member, you will add unwanted stress to parts of the differential, trans, TT and the inherently weak Bell housing.

The Trans/Diff/TT, bell housing and engine are a ridged unit:

 

I suppose you could if you used something like a rod end to only constrain it from moving up/down and back/forth.

The differential end of the drive train needs to be able to rotate around the driveshaft a bit and also to be able to rotate in the plane of the axle shafts a bit. these are the directions it needs to move so there is no abnormal stresses applied in it as the car is driven.

Hard fixing it to the cross member would cause a lot of stress on that mount as the engine moved around or when the chassis flexed.

You can hard fix differentials in other vehicles because the differential is stand-alone and all the mounts are close together so there is almost no chassis flex between the mounting points.

 

Thanks guys, that all makes sense. So with the knowledge that the diff needs some "flex" but that I want to minimize unnecessary flex, what is the best way to approach the "problem?"

 

Order and install a PFADT Style differential mount.

See picture of the mount in #3 reply:

https://www.corvetteforum.com/forums...earancing.html

I would also look into new OEM engine mounts OR some better poly engine mounts.

 

aFe has them now.... http://store.afecontrol.com/afe-cont...-97-04-orange/

I have the Hinson mounts in the car atm, trying to decide if I want to try the Pfadt mounts or not

 

I would think that you would want matching engine mounts so that the torsional movement would be about the same on both ends.

If the diff mount is stiff and the engine mount is soft I would think that the engine would be wanting to move more than the diff causing more stress.

 

Agreed. The engine mounts should always be as stiff and/or stiffer than the trans mount.

 

Any issue with using solid motor mounts and pfadt trans brace?

 

I had pfadt motor and diff mounts in my car and I like them, it cured my wheelhop/drivetrain shake issues and helped keep the drivetrain alive... I put c6z drivetrain in the car about a year and a half ago so I no longer have the diff mount but the motor mounts are still there, they have been installed close to 7 years and still going strong

 

Yeah. Why not Pfadt both.

 

Came across a thread where a number of people talked about the poly bushing in the Pfadt motor mounts disintegrating.

 

^Interesting.

 

maybe with a lot of heat due to uncoated headers or poorly designed headers that sit too close to the mounts... you could always reuse the heatshields or make your own... both sets of headers I have had were coated and like I said no issues in close to 7 years

 

don't forget that the whole drive train is rather monolithic, effectively one long piece. So, thermal growth needs to be accomodated somehow. I get (envelope calc) roughly .12 inches growth when the temp of the drivetrain goes from 72 to 160 dgF (uniformly). So be careful solidly mounting the drivetrain. It needs a little "slop" in addition to damping vibration.

 

When you talk about "unnecessary" flex, remember that the engineers who designed this car spent a lot of time thinking about it. What goals, specifically, do you have, in going to a harder mount? Are you willing to put up with the added NVH? Remember, you're basically stuck with this decision until you replace the mounts, so maybe the first thousand miles you won't mind, but then ...

For a street-driven C5, I see no need for poly mounts. The car shakes enough as it is. For competitive events, you may want poly mounts. I have not yet seen a good reason for solid mounts unless you're doing something crazy.

 

That's what I was thinking. The OEM mounts even had some heat shielding.

 

Correct, the engineers who designed the C5 spend a lot of time thinking about flex but were ultimately focused on limiting NVH. The C5 has to appeal to a LARGE market segment, most of whom won't put up with much of any NVH (same reason the shifter has rubber in it). By contrast, every performance oriented DD vehicle I've ever owned (my 01 Z28, my E46 M3, my Z4) used spherical bearings and solid mounts in nearly every suspension and/or drivetrain location (Z28 had aluminum front and mid motor plates). So yes, I can live with added (as long as it's not every panel buzzing) NVH.


My experience has been that stiffer (poly or solid) mounts can often cut down on the "shakes", however they can induce "buzz" into the car. Rubber mounts have a lot of play in them, so the engine has a chance to torque over before the rubber becomes "solid"/stiff enough to resist movement. As a result, when the rubber does finally resist the movement the engine ends up "whacking" the motor mount and causes the vehicle to shake (think slide hammer). By contrast, stiff poly and solid mounts don't let the engine ever get moving in the first place so there is less "shake" in the vehicle. You certainly feel more vibration at idle/low RPM but that seems to smooth out past 1200-1500 RPM.

Again, this is just my experience.