[Hoody007]

Hi All,

As I briefly mentioned in the “what did you do to your car today?” thread, I recently had two front struts replaced on my E90 ZSP. At the time, I thought they were installing B8s, but my alignment shop has since told me they are actually B6s. Additionally, I have a rear set of B4s installed by the PO, which I was planning on also switching to B8s. The B4s are quite new, but I’m having some ride issues in the rear - it almost seems like the rear is oscillating more than the front - it is because the B4s are softer, or is it possible that non-sport bump stops are to blame? I’ve verified that the rear shocks themselves are designed for the sport suspension but I don’t know if the correct bump stops were used.


I’ve heard some conflicting things about B6 vs. B8 fitment, so I wanted to get your opinions on what do do next. I’m really not wild about taking my car back to the shop I had the front struts installed at, partially because if they do agree to swap out the B6 for B8s, I really can’t trust their work (they left my DS control arm loose). Ride height is higher in the front than it was with the blown out Sachs, but seems to be inline with what the car originally sat at when it was new. The front ride quality is really quite wonderful - no bobbing I’ve heard associated with B6 + M Sport springs.

So, what should I do? Have the B6 swapped for B8s? Replace the rear B4s with B6 instead of B8s? Run B6 front and B8 rear? Or keep the B4s in the rear and swap in a different bump stop?

For the record, I have a 335i ZSP with the M Sport suspension from the factory. Thanks in advance!

I’ve attached an image of how the car sits in the front after the B6 install.

[Buug959]

As I understand it the B6 and the B8 have the same valving, hence the same damping ability. The B6 is a firmer shock than stock and designed to replace the original non-sport shocks whereas the B8 is shorter and designed for lowered springs such as the sport suspension springs. B4 is a direct replacement and equivalent to the non-sport original shocks.
I am sure some others here will have more details than I have and will correct any errors I may have made.

[Efthreeoh]

Originally Posted by LemansE90335xi

As I understand it the B6 and the B8 have the same valving, hence the same damping ability. The B6 is a firmer shock than stock and designed to replace the original non-sport shocks whereas the B8 is shorter and designed for lowered springs such as the sport suspension springs. B4 is a direct replacement and equivalent to the non-sport original shocks.
I am sure some others here will have more details than I have and will correct any errors I may have made.

IIRC when I was going through my Bilstein selection process was the B6 is intended to replace the BMW Sport suspension dampeners and is valved stiffer than the B4 yet softer than the B8 and the B6 is available for the standard suspension spring height and for a lowered sport spring height. The B4 is standard suspension spring height and the B8 is sport spring only.

[Buug959]

Originally Posted by Efthreeoh

IIRC when I was going through my Bilstein selection process was the B6 is intended to replace the BMW Sport suspension dampeners and is valved stiffer than the B4 yet softer than the B8 and the B6 is available for the standard suspension spring height and for a lowered sport spring height. The B4 is standard suspension spring height and the B8 is sport spring only.

So I was fairly close but not quite. Thanks for jumping in Efthreeoh.

[Hoody007]

Thanks for the replies! My confusion was due to the part number of the shock, 35-120384, showing as fitting cars with standard suspension, not sport. Guess I’m fine then?

I thought the difference between B6 vs. B8 was the shock length - the entire thread here confuses me a bit, especially the last post where the B8 is shorter than the OEM M sport. Thanks for clearing that up!

For the rears, I should replace with B6 then? I’m assuming there’s no way to get the rear B4s (which are quite new) to complement the B6 in the front?

Thanks!

[networker67]

I am running B8's with lowering springs front and back on 325xi (w/M Sport). I live in CHICAGO where every block seems to have as many potholes as we have shootings in this city. And I must say 6's were never an option for me. And the 8's went in fine all around. And all my factory 4's were on the car FAR TOO LONG.

I understand your 4's on the rear haven't been there long. But running 8's front and 4's rear just reads like an off balanced ride. From a Suspension perspective. Now I am no Mechanic, but the differences in 8's and 4's as I read here just doesn't make that sound feasible although it is workable.

May I suggest TRYING to sell the relatively new 4's? To help off set the cost of the 6's or 8's in the rear.

[gjm120]

Originally Posted by Hoody007

Thanks for the replies! My confusion was due to the part number of the shock, 35-120384, showing as fitting cars with standard suspension, not sport. Guess I’m fine then?

I thought the difference between B6 vs. B8 was the shock length - the entire thread here confuses me a bit, especially the last post where the B8 is shorter than the OEM M sport. Thanks for clearing that up!

For the rears, I should replace with B6 then? I’m assuming there’s no way to get the rear B4s (which are quite new) to complement the B6 in the front?

Thanks!

Looked into this a lot when re-doing my suspension a couple of years ago, and yes confusion reigns. Helped by the fact that Bilstein has given different advice at different times.

If I had sport springs would go with B6 all other things equal. I've got BMWP springs which are a slight drop from Sport and probably would have gone B8, but I could have flipped a coin B6 v B8. I think the internal bumpstops are part of the difference between B6 and B8. Maybe shock length is as well and maybe a bit of damping difference.

I would do B6's in the back to match the fronts and all should be good.

[Hoody007]

Originally Posted by gjm120

Looked into this a lot when re-doing my suspension a couple of years ago, and yes confusion reigns. Helped by the fact that Bilstein has given different advice at different times.

If I had sport springs would go with B6 all other things equal. I've got BMWP springs which are a slight drop from Sport and probably would have gone B8, but I could have flipped a coin B6 v B8. I think the internal bumpstops are part of the difference between B6 and B8. Maybe shock length is as well and maybe a bit of damping difference.

I would do B6's in the back to match the fronts and all should be good.

Thanks! I’ll order the B6s + M Sport bump stops and see how that goes. And you’re totally right about the conflicting advice from Bilstein, so thanks for clearing that up! I had told the shop which installed the fronts to install B4s, they “did me a favor” and installed B6s instead...

This will be my first suspension install, but it looks pretty straight forward. I’ve never lifted the rear of the car up before - I assume I jack it from the rear diff? I can’t put the car on ramps and install the shocks that way, right?

Thanks for all of your help!!!

[Efthreeoh]

Originally Posted by Hoody007

Thanks! I’ll order the B6s + M Sport bump stops and see how that goes. And you’re totally right about the conflicting advice from Bilstein, so thanks for clearing that up! I had told the shop which installed the fronts to install B4s, they “did me a favor” and installed B6s instead...

This will be my first suspension install, but it looks pretty straight forward. I’ve never lifted the rear of the car up before - I assume I jack it from the rear diff? I can’t put the car on ramps and install the shocks that way, right?

Thanks for all of your help!!!

The rear center jack point is the differential housing (not the aluminum cover!). Be safe and put the up on 4 jackstands. Pull out the trunk lining and remove the top mounts BEFORE you raise the car.

[gjm120]

Originally Posted by Hoody007

Thanks! I’ll order the B6s + M Sport bump stops and see how that goes. And you’re totally right about the conflicting advice from Bilstein, so thanks for clearing that up! I had told the shop which installed the fronts to install B4s, they “did me a favor” and installed B6s instead...

This will be my first suspension install, but it looks pretty straight forward. I’ve never lifted the rear of the car up before - I assume I jack it from the rear diff? I can’t put the car on ramps and install the shocks that way, right?

Thanks for all of your help!!!

Yes you need to get the car on jackstands and remove the wheels. Lot's of diy's on here on the best way to R&R the shocks - it's not too difficult. Make sure to get the differential itself and not the cover with your floor jack.

While you're at it might want to consider changing out the shock mountings both bottom and upper. For the upper mounts many of us have gone with Monroe - cheap minor upgrade over the stock ones. I think the part no. is 907984.

[Hoody007]

Thanks everyone for their help!

I contacted the shop who installed my B6s in the front, and they agreed to install the B6 in the rear w. labor free to make up for the install errors in the front. Parts should be in by the weekend, and I’ll update with the results. gjm120 I told them about the Monroe shock mounts, and they’re installing them too - thanks for the tip!

[Hoody007]

Final update below - thanks everyone for their input!

The shop installed the B6 in the rear - though not before trying to tell me they were the incorrect parts - apparently they thought they were for the front… Absolute morons!

Anyway, after some back and forth, they installed the two rear B6 for free - no parts or labor on my end. The B6 are definitely firm, and will probably need a bit to break in, but they are phenomenal - handling is excellent, with little body roll. They’re much more comfortable over bumps and potholes; while the car doesn’t glide over them like a Lexus, the dampeners soak up the harshness and basically “notify” you that you hit a bump.
Car feels glued to the road. The bounciness in the rear is gone as well, as all four dampeners are evenly matched. The B6 are an excellent match for the M3 control arms which I already have on the car.

Highly recommend!