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FG polyelast hybrid diff bushes


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I know this has probably been covered in some capacity 1000 times but I can’t quite get the information I’m after. 
 

I have a Fg manual xr6 turbo basically stock. It has the hybrid polyelast diff bushes in it. All the rear end was also changed at the same time every bush with super pro. It also has king springs and bilsteins. 
 

now don’t get me wrong the car drives mint but what I can’t get my head around is the thud when changing gears hard. You can get around this by being a bit more gentle between the shifts and loading up the tail shaft a little bit with the throttle but if your out and out changing gears hard I.e racing it will thud on changes. 
 

the information I have found is basically no matter what diff bush set up you have this is just how these cars are? Is this true. 
 

Does anyone know what is actually causing the thud? Driving around town and just cruising it’s sweet didn’t do it at all and if you get the shift just write it won’t do it but it’s pretty sh*t to be honest when you want to drive your car hard. 
 

I spoke to the shop they installed these and he said the bushes are all sweet and there was enough clearance on the out riggers from the way they pressed in the side bushes so I’m at a loss. 
 

I have been looking at getting the ims diff hat upgrade no issue spending the money if it fixes it but if it doesn’t I won’t bother. I can’t find any specific information from anyone who has a manual with either the ims diff hat or diff bush set up where basically they can drive it hard gear to hear and get no thud. 
 

Any feedback from ppl with the diff bushes I have or a set up that eliminates this would be awesome.

 

cheers 

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yes, the diff hat upgrade works to fix the problem in the diff bush area... also the tailshaft centre bearing can be toast to cause a similar thud.

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there's only so much lash the driveline can take, so if the sound emanates from the gearbox itself you can't eliminate it, but in terms of the rear end, yeah, it's heaps better.

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11 hours ago, k31th said:

also the tailshaft centre bearing can be toast to cause a similar thud.

 

This happened with my territory recently. I replaced the centre bearing and no more thud.

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My ZF-equipped car is exactly the same despite everything being all good in the rear. You learn to ease off for a split second before completely jumping off the throttle when you are up it.

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  • 9 months later...
On 17/06/2020 at 12:36 AM, EgoXR6Turbo said:

I know this has probably been covered in some capacity 1000 times but I can’t quite get the information I’m after. 
 

I have a Fg manual xr6 turbo basically stock. It has the hybrid polyelast diff bushes in it. All the rear end was also changed at the same time every bush with super pro. It also has king springs and bilsteins. 
 

now don’t get me wrong the car drives mint but what I can’t get my head around is the thud when changing gears hard. You can get around this by being a bit more gentle between the shifts and loading up the tail shaft a little bit with the throttle but if your out and out changing gears hard I.e racing it will thud on changes. 
 

the information I have found is basically no matter what diff bush set up you have this is just how these cars are? Is this true. 
 

Does anyone know what is actually causing the thud? Driving around town and just cruising it’s sweet didn’t do it at all and if you get the shift just write it won’t do it but it’s pretty sh*t to be honest when you want to drive your car hard. 
 

I spoke to the shop they installed these and he said the bushes are all sweet and there was enough clearance on the out riggers from the way they pressed in the side bushes so I’m at a loss. 
 

I have been looking at getting the ims diff hat upgrade no issue spending the money if it fixes it but if it doesn’t I won’t bother. I can’t find any specific information from anyone who has a manual with either the ims diff hat or diff bush set up where basically they can drive it hard gear to hear and get no thud. 
 

Any feedback from ppl with the diff bushes I have or a set up that eliminates this would be awesome.

 

cheers 

To the OP, if you still haven't had the IMS conversion done or are looking for info I can tell you that the more solid the side/front diff bushes are the less clunk you will get. Like you, I have the manual but mines in FGX form. Mine's far from stock but has the CAT600 solid side bushes and Nolathane rear. Has obvious diff whine audible through the cabin as to be expected and slight clunk when selecting 1st or reverse at a stand still but no clunk at all when changing hard through the gears. The design of the diff mounting is such that the side bushes are responsible for the rotational movement of the diff. If these mounts are solid (or close to it) there is very little movement permitted, even under high load. This means the rear mount is not left with much flexing load, therefore the bolt should never snap. On the other hand if the side mounts allow a lot rotation the rear mount will be very 'busy' and in higher power applications cannot cope and will pass the load on to the bolt eventually resulting in failure. The IMS upgrade is definitely something I've considered and is a worthy upgrade for high power, drag racing etc but I have found that with the CAT600 side bushes I do not need to go that far. But remember there is always a compromise, the less NVH the weaker, the more NVH the stronger really.

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