I thought this mod might be worth a dedicated thread, as so far the only mention of it has been by @Jianlun in his build thread, but I think overall it could be worthwhile, and it was a very popular mod for the previous gen cars as well.
What are you talking about?
The differential on our car is mounted to the subframe via four mounting points with rubber bushings. These bushings are quite soft, meaning that there is a fairly substantial amount of movement in the diff. Below is a video highlighting what I'm talking about - it's filmed on a previous gen car, but the diff setup is the same on the GR.
You can see in the left of the frame, the three bolts which attach to the diff, and how much they move around in their bushes as the car comes on and off throttle. This translates to a rather unpleasant feeling 'rubber band' effect when making quite hard or quick throttle changes, especially in lower gears where there's more torque. In fact, I was recently becoming convinced that a lot of the jerky acceleration issues I'd experienced were less down to poor throttle mapping/lumpy tuning, and more due to this physical slop in the driveline. The worst symptom that I experienced was exactly as I said - when accelerating hard and quickly in lower gears, especially over rough roads, it felt a bit like the car was being pulled forward by a rubber band.
Now obviously the differential bushings aren't the only rubber bushings supporting the driveline, and I had already replaced the transmission mount with the firmer STI one. That improved throttle response a little, but the main advantage of that was helping the shift feel. I wanted to try the bush inserts as I figure the diff was where there was the most movement that you would feel through the throttle.
So what are the Whiteline bush inserts?
ukwhitelineperformance.com
Quite simply, they are like poly bushes, but instead of replacing the whole bush, they simply fit into the voids of the OEM rubber bush. By doing so, they substantially reduce the amount of flex the bush can offer, and so prevent that rocking motion of the different as the throttle input changes. The kit is for the GT86, but apparently it fits the GR86 as well.
Whiteline apparently also do a complete replacement bush, but I imagine that is costlier, nosier, and harder to install, so probably better suited for track setups than road driven cars.
Installation
Installation is easy. Rather than go into full detail, I'll just post this excellent video which I used.
Again, this is from a GT86, but the setup being the same, I pretty much followed it to the letter.
There is only one difference - for each of the front diff bushings, there are two inserts: an upper and a lower. It would appear that the upper ones no longer fit the GR, due to a slight change in bushing size, as confirmed by @Jianlun in his build thread. I took his lead, and didn't even bother attempting to fit these ones, I just fitted the front lower inserts, and the rear inserts (for the bushes you see in the first video). I suspect that the difference the missing ones make is minimal, and Jianlun said that he gets no NVH, but he did when he fitted the full kit to his previous GT86, so perhaps that's a contributor.
Installation took me roughly an hour lying on the ground. including time to jack the car up/down. But I was taking my time, checking the install vid and also spending a bit of time spraying ACF50 over the subframe whilst I was there. I suspect You could do this in 30 minutes if you got a wriggle on, and even less if you have access to a lift. I will say though, I also have an HKS exhaust fitted, which means I don't have the big OEM backbox anymore - that might have helped by giving me more room to manoeuvre.
Results
So far, I have only taken the car for a fairly short drive, but I am sure there has been an overall improvement. The throttle/clutch doesn't feel much different when pulling away normally - it's a tad more direct on clutch take-up - but when pulling away hard, or when making quick adjustments to the throttle, I am certain it has reduced that rubber-banding effect that could be so unpleasant. I have often noticed that even when changing gear normally and not pushing on, I could feel the car lurching a little sometimes as I came off the clutch, as though I wasn't coming off it quickly or cleanly enough. That effect seems to be lessened now. I need to drive it more, and a bit harder to be sure, and it's not like a night and day difference, but the car does feel 'tighter' at the back - but only through the driveline. There's no noticeable difference to the handling or how the car feels or steers, other than when your manipulating the power.
As for NVH increases - I can detect no change. I can't hear any untoward differential noises. I have an aftermarket exhaust, but not a particular loud one, but there's nothing I can hear over that, road noise, of the radio. I might do a test of some full-lock maneuvering as I guess that's when the diff is noisiest, but so far, nothing untoward.
Conclusion
This was a popular and apparently worthwhile mod for the GT86 that I never got around to. I would say that it remains for the the GR. It's not completely transformative, but it certainly helps smooth out the power delivery, and time will tell if I think it solves the jerky throttle issues entirely - but initial impressions are that it has definitely improved it a little at least. It makes the car feel a bit tighter with no real downside in terms of noise.
For the cost (I purchased from Fensport, £54.02 including the GRZOO discount) and the ease of installation, I would say it's definitely worth doing for anyone just looking to tighten up the driving feel. And if you don't like it, it's easily reversible.
What are you talking about?
The differential on our car is mounted to the subframe via four mounting points with rubber bushings. These bushings are quite soft, meaning that there is a fairly substantial amount of movement in the diff. Below is a video highlighting what I'm talking about - it's filmed on a previous gen car, but the diff setup is the same on the GR.
You can see in the left of the frame, the three bolts which attach to the diff, and how much they move around in their bushes as the car comes on and off throttle. This translates to a rather unpleasant feeling 'rubber band' effect when making quite hard or quick throttle changes, especially in lower gears where there's more torque. In fact, I was recently becoming convinced that a lot of the jerky acceleration issues I'd experienced were less down to poor throttle mapping/lumpy tuning, and more due to this physical slop in the driveline. The worst symptom that I experienced was exactly as I said - when accelerating hard and quickly in lower gears, especially over rough roads, it felt a bit like the car was being pulled forward by a rubber band.
Now obviously the differential bushings aren't the only rubber bushings supporting the driveline, and I had already replaced the transmission mount with the firmer STI one. That improved throttle response a little, but the main advantage of that was helping the shift feel. I wanted to try the bush inserts as I figure the diff was where there was the most movement that you would feel through the throttle.
So what are the Whiteline bush inserts?
Rear Differential Mount - Bushing Kit to Suit Subaru BRZ and Toyota 86
Reliable suspension part to suit Subaru Brz. Improve comfort, control, and long-term durability. Shop Now!
ukwhitelineperformance.com
Quite simply, they are like poly bushes, but instead of replacing the whole bush, they simply fit into the voids of the OEM rubber bush. By doing so, they substantially reduce the amount of flex the bush can offer, and so prevent that rocking motion of the different as the throttle input changes. The kit is for the GT86, but apparently it fits the GR86 as well.
Whiteline apparently also do a complete replacement bush, but I imagine that is costlier, nosier, and harder to install, so probably better suited for track setups than road driven cars.
Installation
Installation is easy. Rather than go into full detail, I'll just post this excellent video which I used.
Again, this is from a GT86, but the setup being the same, I pretty much followed it to the letter.
There is only one difference - for each of the front diff bushings, there are two inserts: an upper and a lower. It would appear that the upper ones no longer fit the GR, due to a slight change in bushing size, as confirmed by @Jianlun in his build thread. I took his lead, and didn't even bother attempting to fit these ones, I just fitted the front lower inserts, and the rear inserts (for the bushes you see in the first video). I suspect that the difference the missing ones make is minimal, and Jianlun said that he gets no NVH, but he did when he fitted the full kit to his previous GT86, so perhaps that's a contributor.
Installation took me roughly an hour lying on the ground. including time to jack the car up/down. But I was taking my time, checking the install vid and also spending a bit of time spraying ACF50 over the subframe whilst I was there. I suspect You could do this in 30 minutes if you got a wriggle on, and even less if you have access to a lift. I will say though, I also have an HKS exhaust fitted, which means I don't have the big OEM backbox anymore - that might have helped by giving me more room to manoeuvre.
Results
So far, I have only taken the car for a fairly short drive, but I am sure there has been an overall improvement. The throttle/clutch doesn't feel much different when pulling away normally - it's a tad more direct on clutch take-up - but when pulling away hard, or when making quick adjustments to the throttle, I am certain it has reduced that rubber-banding effect that could be so unpleasant. I have often noticed that even when changing gear normally and not pushing on, I could feel the car lurching a little sometimes as I came off the clutch, as though I wasn't coming off it quickly or cleanly enough. That effect seems to be lessened now. I need to drive it more, and a bit harder to be sure, and it's not like a night and day difference, but the car does feel 'tighter' at the back - but only through the driveline. There's no noticeable difference to the handling or how the car feels or steers, other than when your manipulating the power.
As for NVH increases - I can detect no change. I can't hear any untoward differential noises. I have an aftermarket exhaust, but not a particular loud one, but there's nothing I can hear over that, road noise, of the radio. I might do a test of some full-lock maneuvering as I guess that's when the diff is noisiest, but so far, nothing untoward.
Conclusion
This was a popular and apparently worthwhile mod for the GT86 that I never got around to. I would say that it remains for the the GR. It's not completely transformative, but it certainly helps smooth out the power delivery, and time will tell if I think it solves the jerky throttle issues entirely - but initial impressions are that it has definitely improved it a little at least. It makes the car feel a bit tighter with no real downside in terms of noise.
For the cost (I purchased from Fensport, £54.02 including the GRZOO discount) and the ease of installation, I would say it's definitely worth doing for anyone just looking to tighten up the driving feel. And if you don't like it, it's easily reversible.
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