[phsullivan]

Hi, I have a 2006 E90 330i with about 130k miles. Four years ago at 106k miles I replaced the sport spec shocks/struts and related rubber parts along with M-spec front control arms and tie rods. I loved the results of this upgrade. The engine has had all its major BMW leaks for the era dealt with (oil pan, valve cover, oil filter housing) and associated sensors parts replaced, and the engine is running very strong.

I had intended to get to the next phase of rejuvenating the suspension sooner but covid and life got in the way. My question is what option to do next. The rear of the car feels a bit more unsettled over bumps than it used to, so it's time to start tackling the 17-year rubber parts in the rear.

Option 1: Replace subframe and diff bushing with M-spec parts. Parts cost $690 from FCP. Plus some cost to rent or put together a kit to R/R the bushings.

Option 2: Replace all rear control arm/trailing arm/guide rod bushings with M-spec arms where possible, mono boll trailing arm bushings, and regular spec every else. Cost $993 including the forward lower trailing arm, about $890 reusing the existing arms.

At the moment I only expect to have time and money to do one of these jobs this winter/spring. Maybe I'll get to the other one later next year, but otherwise the next spring.

This is my daily driver (30-40 mph smooth back roads, and about 8 miles each way), 2 or 3 auto-cross per year and hopefully () one track day a year.

Which option do people think I should do next?

[streetpro]

Replace subframe and diff bushing + Diff-lock or brace... I have done all parts in both options bit by bit, subframe and diff-lock was the real improvement...

[335i54n]

solid subframe mounts are probably one of the best suspension mods imo. for value i would do subframe inserts, diff brace and m3 front-most rear arms.

[ktish]

I would go right to solid aluminum subframe bushings. There's no added NVH and are probably equally difficult to install when compared to the M3 rear subframe bushings.

I have alu subframe bushings in my 06 330i also. For the diff bushings I also upgraded and used the whiteline kit. From my research, the whiteline bushings are the softest poly bushings available on the market. There's some whine in the cabin during high load situations but nothing I personally would find annoying.

If you were looking to further improve your autocross time, the next obvious upgrade would be an LSD.

[phsullivan]

Thank you everyone for the recommendations. I had not been considering solid mounts but after the posts above I looked into some and am onboard with the idea. From my short search it looks like aluminum ones are not DIY friendly as the need a press to install them.

I am starting to look into delrin mounts and came across these two part UHWP mounts from Condor Speed Shop:

https://www.condorspeedshop.com/collections/e90/products/e90-subframe-bushing-kit-e8x-e9x

Any thoughts on these bushings or other recommendations for DIY friendly(er) bushings.

[335i54n]

I have the condor mounts but kinda always have a lingering feeling should have went with solid metal, though condor given me no reason so far to doubt them and no sign of any movement/deflection. 1st i would order the 2 short rear brake lines (better yet full ss lines upgrade) since you need to remove those to drop the subframe enough, install is ez, removal not so much in the car unless you have the tool.

[AWD Addict]

I've been pleased with changing all my rear end pieces to m3.

If you're in a temperate climate, take a hard look at the subframe. It's the only part of my undercarriage that rusted. Bought used ones from below the Mason Dixon, and had them cleaned up and rust proofed.

I liked the whiteline inserts, but they got too soft pretty quickly. If I'd had just done the m3 bushings, I have only needed to do that work once.

[whyzee125]

I would do subframe inserts + the control arm bushings/monoballs. Leave the diff bushings alone.

A lot of people say the M3 subframe bushings are the worst value bc you can get similar results for less money and time with the inserts. That will eliminate a lot of slop and allow you to kind of do both with very little compromise