Boost and Bushings
 

I've searched a bit and don't see many threads about boosted cars and bushing changes that follow. I'm specifically posting in here for FI owner feedback.

For reference, car is 14 FRS, HKS SC kit with their gas tune, Whiteline trans bushing, and Perrin shift bushing. South Bend 350 ftlb clutch, CSG MXP cat back and a uber hp adder Perrin air filter.

In my case, I'm getting wheel hop occasionally on launch. Especially in this three digit weather and driving for a while. I plan on adding some Whiteline bushings to the rear assembly. Unless there's something else to consider and STI makes it all in a stiffer durometer than stock. I see solid fit kits and urethane bushings. I'll go urethane as this car is still a commuter.

I also see Whiteline has kits to change about every bushing in the suspension assembly, but that's racecar level changes, so I'd have to do the front too. Not averse to it, just a lot more than fixing some wheel hop.

Don't mean to make this a Whiteline bushing thread, just appears they cover this car in that respect way better than others.

Rear subframe mounts + diff mounts should go most of the way to eliminating your issue. Can't speak to whiteline Vs others, but they do seem to have the most extensive catalogue

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Had to do rear subframe/diff bushing and engine mounts along with the others to eliminate all driveline hop.

No wheel hop here with Whiteline diff insert and Racecomp coilovers.

Having tires with a little sidewall height will help as well. 235/35 tires are more likely to skip than a 235/50.
There's a tradeoff there, but every modification is a compromise on some level.

Which brand engine mounts if I may? STI, Cusco and are there others?

By your reference diff/subframe/mounts. What were the others? Are you into the control arm bushings as well. Don't mind doing the work, just trying to get the biggest offenders first and see Whiteline has a pretty extensive bush kit for front and rear of these cars.

Also, where did you end up on NVH, mild or wild in the end?

I can definitely feel more deflection of everything with the added SC power. I'm starting with Whiteline KDT922, KDT923 and KDT925 bushings in the rear since my wheel hop on launch is stemming from that area.

With the transmission mounts you will see almost no change. I did Whiteline subframe and diff inserts. This will bring dif noise into the cabin on deceleration. I did torque solutions mounts to keep the engine from flexing due to the turbo. It does increase NVH but does not bother me. I drive this car for what it is and have done 600+ mile road trips. I just turn the stereo up louder.

My deflection is excessive, I'll take the tradeoff. Thanks for the input that it's not too obnoxious.

I added a 3 Whiteline bags'o bushings to the rear and Torque Solutions motor mounts.

NVH is the motor mounts for sure. Hella noisy in first and reverse moving slow, fine after that. But you definitely hear and feel it more while driving. Keep the radio off and cabin engine noise is up. These cars aren't quiet anyway. With these motor mounts and the previously installed Whiteline trans connection insert, much better for driving feel hard throttle to hard braking and vice versa.

The rear end Whiteline kits are impressive. The complete bushing change on the diff itself didn't add near the NVH expected. Hysteresis feel when laying into the power and suddenly out is much better. not entirely happy though. The rear kit that bolts to the diff, still chatters occasionally on some launches. At least it's the easiest one to change should I decide to go to the dedicated kit instead of the add on kit.

Overall, car feels much better when getting sprited, it did cost me some cabin noise, and I think that's nearly all from the engine mounts.

Now living with this setup months later, still happy with it. Only get the chatter on the slow stuttering start stop moves. Still happy with the performance, but am continuing to think STI N motor mounts may be in my long term future. I've talked myself in and out of it couple times now.

I know this was posted months ago but i noticed quite a bit of change with the transmission mounts and rear shifter mount. I find it easier to downshift and also i get less fore and aft jerky movement when pissing around in stop and go traffic.

You really should not be taking a piss inside the car - stop and go traffic or not

i fitted the STI mounts and no noticeable difference in NVH but improvement n power to the ground was very noticeable. Also running the Whiteline trans bushes and subframe/diff inserts minus the the diff front ones. Just found it was just to mechanical and race car noisy to enjoy. Nowhere near the NVH with just the rears in and delivers plenty of stiffness to drive train.

Consider the MCA traction mod. Pretty sure there’s a US equivalent now. Well worth the effort and cost.

I’m also running the Verus CF driveshaft but that’s next level in cost and no longer available, I think.

I'm close to your setup minus the driveshaft, plus the front diff bushings and the MCA brackets. The Torque Solution mounts are my issue. I'll finally going to pull the trigger and get some N-group ordered soon. I've been on and off the fence several times, but the NVH from the Torque Solutions is just too much for me for a commuter. Really like them otherwise, the in/out of throttle feel is astounding now. But watching the rearview mirror rattle at a stoplight and finding several resonances in the cab at parking lot speed, it's just too much. Great product, but more racecar than daily for me is all.


Pivoting to the rear end, I still have occasional wheel hop issues when I have the feather the clutch a lot, any comments on STI bushings in the rear end. It's all in the diff bushings that mount to the end of the diff. I've seen STI units for these, curious if anyone has tried them. I have the Whiteline kit in now, helps a lot, but still doesn't address that low speed suspension windup. I blame this on the 350ftlb stage two clutch, just need to find the right rubber to keep it in check, hopefully not with more glaring NVH like the Torque Solution motor mounts.

I'm trying to find the STI mounts. The numbers I had were wrong at the dealership. I had ST41022AS000 and same ending 010 for left/right. Subaru didn't even have the number.

I'm not sure which STI model matches our mounts, but I guessed at 15 WRX STI, but didn't make an order. Subaru parts counter gave me a number of 41031VA000 and same ending in 010. He couldn't find N group or anything when I asked. I was kind of puzzled, am I looking for a Aussie/UK part instead that's not on US books? I'm confused.

Moreover, what STI year range match the BRZ/FRS platform?

ST41022AS000 and ST41022AS010

When i bought these, cheapest place was rallysportdirect. Maybe a Japan part number? You don't want to pay their markup anyways.

I had those part numbers, thanks for confirming. Went to Rallysportdirect, punched in the number and got it. Still puzzles me the dealer couldn't find it, but oh well. Thanks for sending me the right direction. Just ordered them.

We're having a cold snap in April here in the PNW, those TorqueSolution mounts really lay on the NVH when cold. Look forward to alleviating that, as much as I like them, I have to peel back a level for my car.

Just had the Group N mounts put in today. No more parking lot speed NVH. The Group N put the power down well, but nothing like TorqueSolution mounts that I just took out. Definitely lost some of the racecar feel and sound because the TS models were so direct and stiff with 75 durometer bushings. Lift or press throttle with the TS mounts and you had instant decel/accel feel. The Group N makes the in/out throttle response softer and not as direct or instantaneous. Group N feels about 1/2 as firm and the in/out throttle feel is no where near as good as the TS mounts.

Again, this car is a SC'd commuter and wanted to get better bushings, but went too far with the TorqueSolution mounts. I like how they drive, but the NVH was just a bit much for my daily. I hate rattles, these created a lot of them at takeoff and driving at parking lot speeds.

Next, I'm going after the bushings that mount to the rear of the diff. Bushings are still too soft back there even with the Whiteline insert kit. Saw STI parts for that. Can anyone comment on that end?

I have the MCA traction kit, just haven't got it in yet. No time today.

Recent updates

STI Group N engine mounts traded out for the Torque Solutions. NVH is gone. In/out throttle isn't as crisp as the torque solutions.

I was still having suspension hurky jurkies occasionally in first when trying to feather the clutch around the parking lot or freeway start stop traffic. The Whiteline insert kit helped, but I still had a problem Still not happy, I ordered a Cusco rear diff brace.

Just installed it this morning. Took about an hour from start to finish in the driveway. You cannot use the Whiteline kit and the Cusco brace together. Pull your LCAs down for access, worth the room to reach inside the rear housing to mount the bolts.

I'm impressed, it appears it fixed that occasional diff flop, clutch chatter, wheel hop suspension bind I was getting in the back. And there is no discernible difference in NVH, really impressed with that. I have to admit I was a bit nervous thinking I was going to reintroduce the Torque Solution level NVH. You can still feel the deflection of the whole system, it's just stiffened quite a bit stiffer and no longer floppy soft feeling in first gear.


In the end, my boost and bushing journey has landed me here.

JDM STI engine mounts
Whiteline trans bushing insert
Whiteline rear diff outrigger mount bushings
Whiteline rear subframe insert bushings
Cusco Rear diff brace

I do attribute my low speed problems to my South Bend 350 ftlb clutch. It bites harder than the OEM rating and deflects the bushings more making this issue more prevalent. I put the clutch in before the boost and don't regret it. But it took these parts to make the new clutch and boosted power feel similar to the OEM car manners again.

The Torque Solution motor mounts are a bit too stiff. I couldn't find any softer urethane bushings for them. 75 durometer only, a 50 or 60 might be a good compromise, but I haven't found them yet. I also appreciate them for what they are, true race car gear. Their driving feel was really superb, the NVH was just too much for my commuter. As for the Whiteline diff inserts I removed, these were a big help, but still too soft with the stiff clutch. The Cusco brace really shines here now. I was nervous I'd bring more racecar NVH into play, but that was a pleasant nothing burger. NVH is same as it was before. The stiffer bushes in the rear have me hearing the diff more, but it's not obnoxious like the TS motor mounts could get.

Still have the herky jerky behavior with this setup when you have to feather the clutch too much. Still not thrilled, too soft in 1st.

Looking for feedback on Whiteline, Megan Racing or similar full bushing changes. I see solid aluminum inserts like the whiteline urethane insert kit, but just for other cars.

Megan Racing
https://www.rallysportdirect.com/par...-diff-bushings

Whiteline
https://www.rallysportdirect.com/par...-diff-bushings

I've seen STI bushing for these mentioned, but cannot find them on any JDM resource I have reviewed.

I never solved the herky jerky problem at low speed. These cars have it stock, and adding boost and an aftermarket tune i guess doesn't help. Doubt it's your clutch tbh.

At cold temperatures it could be pretty bad if i wasn't careful.

I didn't have these issues before the higher rated clutch, which came many miles before the HKS supercharger. The harmonics are the issue with the bushings.

Leaning toward Megan being they state their urethane durometer and I have the subframe bushing connection to buffer the extra stiffness they'll bring. Might be Torque Solution race gear all over again, not sure. Still appreciate that ripping racy feel the high durometer brings, but I'm going for touring car, not a track car.

I didn't have these issues before the stiffer clutch. The first gear low ratio really brings it out. May not pose the same issue as the motor mounts going stiffer due to the subframe connection providing some NVH barrier as well.

The jerky first gear is a problem when grippier tires are fitted. I just went from summer to winter tires and the jerkiness went away. But I am putting in diff bushing inserts on the weekend so that should fix the problem for good

Are you sure it wasn't cured by more compliant side walls? Remember these cars were designed for some pretty compliant tires.

Definitely possible, not an expert. All I know is I didn't have this problem when I had the Whiteline diff bushing inserts installed. Then I took them off cause they were making my 1-piece driveshaft vibrate at higher speeds and it became jerky. Took off the driveshaft and that made it even worse. Now the inserts are going back in