[Zoomz]

I have a 2018 440i Gran Coupe X-drive in great shape with 40K miles I've owned about a year. It has all the options/packages (M-Sport, Track Handling, etc.). I enjoy the practicality and power and consider it an ideal size for a family sports sedan but have been disappointed in the so-so handling and soft suspension (tho it was improved by an upgrade to the control arms and Dinan shockware). I mainly drive it in winter and when carrying multiple passengers, as my Cayman S is the go-to whip in good weather.

Since there are so few other cars out there with this combination of features--liftback, AWD, 300+ HP, and midsize 4-door body--I've been considering bringing out the car's true potential by installing coilovers (Bilstein B16 damptronic), a Valvetronic exhaust, a BM3 tune, and possibly upgrading the sway bars. However, I also know I won't recoup the investment that would require ($6-9K), tho it's still cheaper than, say, upgrading to an AWD M5.

Any thoughts welcome on whether you think it's worth making these upgrades to this platform and nearly 6-year-old car, or you'd suggest a different route or car. Also interested in hearing about your liveability/comfort experience if you've put yours on coilovers.

[Tommy D]

My thoughts, though I never installed coil overs on my cars , just road in friends cars.
If you love the car and plan on keeping it for many years, investing in the upgrades shouldn't be a worry. If you plan on getting a newer vehicle soon, then don't upgrade.
I assume you have the adaptive suspension, I'd keep the original parts if you do the upgrade and then reinstall if you do sell in the future.

The 440I gran coupe is a big car with a long wheelbase and is more a highway cruiser than corner carver but it has a nice balance, that's why I keep my E46 around to have a vehicle I can throw around.

Ill let others chime in on the install and ride quality of the coil overs. The looks of lowering the car are one of the main reasons to switch and cost, but ride quality will suffer. Also gone are the days of switching between comfort mode and Sports mode.
Have you driven a M5 yet to get an idea on ride quality? Try, dare I say test drive an Audi RS5?

[johnung]

Originally Posted by Zoomz

I have a 2018 440i Gran Coupe X-drive in great shape with 40K miles I've owned about a year. It has all the options/packages (M-Sport, Track Handling, etc.). I enjoy the practicality and power and consider it an ideal size for a family sports sedan but have been disappointed in the so-so handling and soft suspension (tho it was improved by an upgrade to the control arms and Dinan shockware). I mainly drive it in winter and when carrying multiple passengers, as my Cayman S is the go-to whip in good weather.

Since there are so few other cars out there with this combination of features—liftback, AWD, 300+ HP, and midsize 4-door body—I've been considering bringing out the car's true potential by installing coilovers (Bilstein B16 damptronic), a Valvetronic exhaust, a BM3 tune, and possibly upgrading the sway bars. However, I also know I won't recoup the investment that would require ($6-9K), tho it's still cheaper than, say, upgrading to an AWD M5.

Any thoughts welcome on whether you think it's worth making these upgrades to this platform and nearly 6-year-old car, or you'd suggest a different route or car. Also interested in hearing about your liveability/comfort experience if you've put yours on coilovers.

Sounds like you really love the car and mileage is low so why not enhance it. What control arm upgrade did you do?

I would recommend against coilovers and the B16-D in particular. Bilstein did a poor job of matching springs and dampers. The front spring rates are way high. A buddy installed them on his car and he hated them. So harsh it would rattle your teeth.

I suggest keeping your stock adaptive dampers, but swap out the stock springs for an Eibach spring kit. See photo for part number. They will provide more control and a mild lowering of 1” in front and 0.8” in rear. So it will eliminate some of the BMW reverse rake (larger front tire/fender gap).

Trim off about 1/2” from the tops of the stock bump stops with a serrated knife to allow for the drop and prevent premature contact with the bump stops on rough roads.

At the same time swap out your old top hats and rubber bits like spring perches for fresh OEM pieces (FCP Euro sells some OEM kits and single parts). The original parts may appear okay but after 6 years they aren’t. A refresh helps it ride like new.

In the future if any of the stock Adaptive dampers develops a fluid leak then replace just that pair with Bilstein B4-DampTronics.

A sway bar upgrade can make a huge difference in controlling body roll. Makes the car even more fun to drive. It’s not done as often with F3x simply because the labor can be high because there’s a lot of stuff to remove to get access to them. But it’s well worth it. Rear sway is easier to install than front. It’s often best to upgrade the rear first anyway since the car naturally understeers.

Your car has two different front and rear sway bar options from the factory. See photos. Put your full VIN into the mdecoder website and scan through the report for your car looking for the S2VFA factory option code. Whether you have that or not will determine which factory sway bars you have, and which sway bar upgrades might be your best option. For example, if you have the 12mm stock bar then there are 15mm, 16mm and 17mm options. But if you already have the 15mm factory bar then there are 17mm, 19mm and 20mm options.

Note: I only recommend solid steel sway bars. Hollow bars are cheaper to manufacture, but they have been known to suddenly snap. A safety concern in my book.

I would make the other changes to springs and tires first. Then upgrade the rear sway bar first. Only after that decide whether or not to upgrade the front sway bar.

What is your current tire model and size(s)? What is your winter weather and local road conditions? If you get snow is it plowed quickly or do you have unplowed mountain roads to deal with. Tire model and size can make a huge difference.

For summer I have square Michelin Pilot Sport 4S 255/40-18 over APEX VS-5RS.

For winter, Michelin X-Ice SNOW in 245/40-18 over my old stock BMW 400M wheels. (The stock 225/45-18 size would have been better if we got a lot of unplowed snow locally because wider tires tend to hydroplane in heavy snow.) Winter tire compounds make a huge difference in road grip in cold weather and these Michelins are fantastic on Black Ice.

If I was going with an all Season tire it would probably be the Continental DWS06+ in square 245/40-18 or 255/40-18 depending on the wheel size.

I highly recommend a Bootmod3 tune. No need to start throwing hardware like downpipe and fuel pumps at it. Just install a Bootmod3 Stage1 tune. Drive it and enjoy it.

I’m not too familiar with the Valvetronic exhaust. My favorite sounding exhaust is the AWE axle-back exhaust. Deep full sound like a fine European sports sedan should have.

Hope this helps!

[B58 parts]

The b16s are really stiff. They're more on the race side of things. I took them off for a few different reasons and switched to the kw v3s, which overall is a way better set but still pretty stiff for daily driving. If you have passengers more often you're going to want to get something softer. The KW Street comforts come to mind but I don't know much about them, there is some issue with the springs being too soft on those. I would probably get koni yellows and eibach springs of the correct rate which would take me time to figure out.

These cars absolutely do need sway bars but it's a hard upgrade to do as you have to lower the front or rear subframe.

Tires you really can't go wrong with the Michelin pilot sport 4S. The bootmod3 stage 1 tune with an intake was actually great I would love to just go back to those days. A high flow cat downpipe will give you a little bit more sound before going to a full exhaust which might be too loud.

[tturedraider]

Based on how you said you like the car I think it’s worth doing.

[vinode]

Originally Posted by johnung

Sounds like you really love the car and mileage is low so why not enhance it.

johnung, really liked your suggestions as I am thinking of doing most of this to my 99k kms 430i. Before all that, I should probably replace some worn rubber stuff as the car no longer feels tight at this age / mileage. What would you suggest as a must at the 100k kms mark? Engine and transmission mounts? Bushes and links etc? Love the car and not too keen on the newer models atm.

[B58 _gran_coupe_]

I just refreshed the suspension on my 2018 f36 440i msport (non adaptive) at 51k mules

It's my only car and my daily so nothing too low or stiff.

I ended up using billstein b4 with h&r sport springs.

I also changed the top mounts, dust covers, spring seats and bump stops (shorter m3 bump stops for extra travel)

I have been absolutely thrilled with the difference in ride quality and responsiveness. VERY smooth yet a good amount of increased feedback and sharpness. It has not reduced ground clearance enough for it to be troublesome either.

[johnung]

Originally Posted by vinode

johnung, really liked your suggestions as I am thinking of doing most of this to my 99k kms 430i. Before all that, I should probably replace some worn rubber stuff as the car no longer feels tight at this age / mileage. What would you suggest as a must at the 100k kms mark? Engine and transmission mounts? Bushes and links etc? Love the car and not too keen on the newer models atm.

Really at 60k miles/100k Km you don’t need to replace much, certainly not engine/trans mounts. Whenever I already have to pay labor to do something else, I look at what else to tack on because it’s unlikely that area will be touched again, maybe for years.

So when plugs are changed I consider whether new Eldor coils are needed, etc.
Plugs and Eldor coils can be done at 60k miles. Just stock plugs and stock gap.

When shocks/springs are touched I consider also doing top hats, spring perches and all of the other bits.

When brake fluid needed to be changed (every two years) or I was upgrading calipers, then I upgraded rubber brake hoses to stainless steel brake lines.

When stock brake pads and rotors needed to be replaced, I upgraded to Hawk 5.0 pads and StopTech full crossdrilled rotors

When 225/45-18 runflat tires needed to be replaced, I switched to 245/40-18 Michelin Pilot Sport 4S tires.

When coolant needed to be replaced I also upgraded the Mickey Mouse shaped coolant flange that likes to break from plastic to aluminum. I also replaced the top coolant return hose with the fitting that likes to break and spray coolant everywhere. That’s something to do by 60k miles!

60k miles is the perfect time to do the automatic transmission service using the kit sold by ZF who manufactured the tranny. Kit sold by FCP Euro. When doing fluids I do them all because it’s cheaper if shop only charges you by overall time, NOT individual book rate for each. So rear differential fluid, and if XDrive then also Transfer Case and Front Differential fluids.

Hope this helps!

[B58 parts]

In line with what he saying above, when you lower the car, the sway bar end links are so long it puts preload on the sway bar. So while you're in there you should do adjustable sway bar end links. You put the car on the ground with the end links loose, adjust them till there's no tension on the bar and then torque everything to spec. It's a difficult reach under the tire but it's doable without an alignment rack. If you adjust the end link really long like stock it's quite a bit of preload.

It's also a good time to consider camber plates if you want to run more aggressive wheels. Either way I would definitely just get new top hats instead of the whole process of the spring compressor and pulling the stock top hats apart. I did all that work and the top hats started making noise within a month. Just get new ones. Also new rear control arm bolts if you're in the north.

[stoked335d]

I am considering a 5-6 year old F36 as a daily and this thread is very helpful. Was thinking of 60k or less miles but after reading all this may need to get one with less miles.

[cfm56d7b]

Originally Posted by stoked335d

I am considering a 5-6 year old F36 as a daily and this thread is very helpful. Was thinking of 60k or less miles but after reading all this may need to get one with less miles.

You will not be disappointed with a well maintained F36. My 2017 F36 440i GC was and still is the best BMW I owned.

[johnung]

Originally Posted by YuminNuman

In line with what he saying above, when you lower the car, the sway bar end links are so long it puts preload on the sway bar. So while you're in there you should do adjustable sway bar end links. You put the car on the ground with the end links loose, adjust them till there's no tension on the bar and then torque everything to spec. It's a difficult reach under the tire but it's doable without an alignment rack. If you adjust the end link really long like stock it's quite a bit of preload.

It's also a good time to consider camber plates if you want to run more aggressive wheels. Either way I would definitely just get new top hats instead of the whole process of the spring compressor and pulling the stock top hats apart. I did all that work and the top hats started making noise within a month. Just get new ones. Also new rear control arm bolts if you're in the north.

Good suggestions. FaRKle! did a nice installation video below

https://youtu.be/Q_sBjb82glc?si=79-r00WJ8aHhYbZ1

[stoked335d]

Originally Posted by cfm56d7b

You will not be disappointed with a well maintained F36. My 2017 F36 440i GC was and still is the best BMW I owned.

There are so many BMW models and sun-models hard to keep up with all the options offered.

Do you know if 440i or ix came with a cold weather package or at least heated steering wheel option? Which option comes with sport diff in F36?

Also considering a used B9 S5 sportback with at least a cold and sport pkg which seems to be in the similar segment. Had a B8.5 S4 before so very familiar with maintenance schedule and cost.

[cfm56d7b]

Originally Posted by stoked335d

There are so many BMW models and sun-models hard to keep up with all the options offered.

Do you know if 440i or ix came with a cold weather package or at least heated steering wheel option? Which option comes with sport diff in F36?

Also considering a used B9 S5 sportback with at least a cold and sport pkg which seems to be in the similar segment. Had a B8.5 S4 before so very familiar with maintenance schedule and cost.

My F36 has Cold Weather Package, which includes front heated seats and a heated steering wheel.

I believe BMW LSD or Limited Slip Differential (sport differential in your prior message?) has always been an option. It's not inexpensive at $2.5K.

https://www.turnermotorsport.com/BMW...-differentials

[stoked335d]

Originally Posted by cfm56d7b

My F36 has Cold Weather Package, which includes front heated seats and a heated steering wheel.

I believe BMW LSD or Limited Slip Differential (sport differential in your prior message?) has always been an option. It's not inexpensive at $2.5K.

https://www.turnermotorsport.com/BMW...-differentials

Yes. Was referring to LSD. So, the F36 was not offered with an LSD option from the factory? If it was offered, was it a stand alone option or part of some pkg?

[tturedraider]

Originally Posted by stoked335d

Yes. Was referring to LSD. So, the F36 was not offered with an LSD option from the factory? If it was offered, was it a stand alone option or part of some pkg?

It was either a port installed or dealer installed option and you will be hard pressed to find one that has it. It was not cheap. Around $2,500 - $3,500.

[Livetodrive]

“When coolant needed to be replaced I also upgraded the Mickey Mouse shaped coolant flange that likes to break from plastic to aluminum. I also replaced the top coolant return hose with the fitting that likes to break and spray coolant everywhere. That’s something to do by 60k miles!

60k miles is the perfect time to do the automatic transmission service using the kit sold by ZF who manufactured the tranny. Kit sold by FCP Euro. When doing fluids I do them all because it’s cheaper if shop only charges you by overall time, NOT individual book rate for each. So rear differential fluid, and if XDrive then also Transfer Case and Front Differential fluids.”

Johnung,
Great service summary. My 2016 340 iX has 60k miles. We drive it gently. My dealer does not perform the transmission fluid change, saying it’s not recommended. Where in the Philadelphia area could I get the tranny, diffs. and transfer case fluids done, also with a coolant, hoses, fittings and maybe a thermostat replacement? How much might this cost?

[StradaRedlands]

Originally Posted by Livetodrive

“When coolant needed to be replaced I also upgraded the Mickey Mouse shaped coolant flange that likes to break from plastic to aluminum. I also replaced the top coolant return hose with the fitting that likes to break and spray coolant everywhere. That’s something to do by 60k miles!

60k miles is the perfect time to do the automatic transmission service using the kit sold by ZF who manufactured the tranny. Kit sold by FCP Euro. When doing fluids I do them all because it’s cheaper if shop only charges you by overall time, NOT individual book rate for each. So rear differential fluid, and if XDrive then also Transfer Case and Front Differential fluids.”

Johnung,
Great service summary. My 2016 340 iX has 60k miles. We drive it gently. My dealer does not perform the transmission fluid change, saying it’s not recommended. Where in the Philadelphia area could I get the tranny, diffs. and transfer case fluids done, also with a coolant, hoses, fittings and maybe a thermostat replacement? How much might this cost?

Dealers don't want/like to do those things as preventive service. It's just not in their nature. Plus they would charge you a lot more to do it than you can find at a good independent. No recommendations for Philly, but there has to be several out there!

I'd get the trans done at now at 60k as that is what ZF recommends. The other fluids I personally would hold off to 100k and then do them all including the transmission again. No harm getting them done sooner though. I think 100k is the recommended service for diffs and transfer cases. Do the coolant and brake fluid if you haven't already. Hoses and thermostat may be a bit aggressive on service interval though. If they're not showing wear/hardening, maybe hold off. But if you want to, sure, get them done. Oh, don't forget accessory belt and tensioner. That's a good one to replace at 50k intervals.

[johnung]

Originally Posted by Livetodrive

“When coolant needed to be replaced I also upgraded the Mickey Mouse shaped coolant flange that likes to break from plastic to aluminum. I also replaced the top coolant return hose with the fitting that likes to break and spray coolant everywhere. That’s something to do by 60k miles!

60k miles is the perfect time to do the automatic transmission service using the kit sold by ZF who manufactured the tranny. Kit sold by FCP Euro. When doing fluids I do them all because it’s cheaper if shop only charges you by overall time, NOT individual book rate for each. So rear differential fluid, and if XDrive then also Transfer Case and Front Differential fluids.”

Johnung,
Great service summary. My 2016 340 iX has 60k miles. We drive it gently. My dealer does not perform the transmission fluid change, saying it’s not recommended. Where in the Philadelphia area could I get the tranny, diffs. and transfer case fluids done, also with a coolant, hoses, fittings and maybe a thermostat replacement? How much might this cost?

Actually I have a great recommendation near Philly. They did the fluids that I recommended above and supplied the aluminum Mickey Mouse flange. They installed my M Performance LSD. For the transmission service they wanted my car dropped off the day before. Usually I schedule way in advance because they are so popular. I don’t have my own home shop anymore. I’m glad I found these guys years ago. They specialize in BMW, Mercedes & Mini
MD Motorwerke. Factory trained. Extremely experienced. Impeccably honest. Their customers are incredibly loyal. Talk to the owner Matt. (856) 435-0525

[Livetodrive]

Johnung, thanks for your recommendation. I’ll give them a call

[Zoomz]

Originally Posted by johnung

Sounds like you really love the car and mileage is low so why not enhance it. What control arm upgrade did you do?

I would recommend against coilovers and the B16-D in particular. Bilstein did a poor job of matching springs and dampers. The front spring rates are way high. A buddy installed them on his car and he hated them. So harsh it would rattle your teeth.

I suggest keeping your stock adaptive dampers, but swap out the stock springs for an Eibach spring kit. See photo for part number. They will provide more control and a mild lowering of 1” in front and 0.8” in rear. So it will eliminate some of the BMW reverse rake (larger front tire/fender gap).

Trim off about 1/2” from the tops of the stock bump stops with a serrated knife to allow for the drop and prevent premature contact with the bump stops on rough roads.

At the same time swap out your old top hats and rubber bits like spring perches for fresh OEM pieces (FCP Euro sells some OEM kits and single parts). The original parts may appear okay but after 6 years they aren’t. A refresh helps it ride like new.

In the future if any of the stock Adaptive dampers develops a fluid leak then replace just that pair with Bilstein B4-DampTronics.

A sway bar upgrade can make a huge difference in controlling body roll. Makes the car even more fun to drive. It’s not done as often with F3x simply because the labor can be high because there’s a lot of stuff to remove to get access to them. But it’s well worth it. Rear sway is easier to install than front. It’s often best to upgrade the rear first anyway since the car naturally understeers.

Your car has two different front and rear sway bar options from the factory. See photos. Put your full VIN into the mdecoder website and scan through the report for your car looking for the S2VFA factory option code. Whether you have that or not will determine which factory sway bars you have, and which sway bar upgrades might be your best option. For example, if you have the 12mm stock bar then there are 15mm, 16mm and 17mm options. But if you already have the 15mm factory bar then there are 17mm, 19mm and 20mm options.

Note: I only recommend solid steel sway bars. Hollow bars are cheaper to manufacture, but they have been known to suddenly snap. A safety concern in my book.

I would make the other changes to springs and tires first. Then upgrade the rear sway bar first. Only after that decide whether or not to upgrade the front sway bar.

What is your current tire model and size(s)? What is your winter weather and local road conditions? If you get snow is it plowed quickly or do you have unplowed mountain roads to deal with. Tire model and size can make a huge difference.

Thanks so much for taking the time to provide such a detailed reply, johnung! Very helpful. Only now seeing this and all the other discussion on the thread.

I've gone ahead and ordered the Eibach springs and will try that and the sway bars--what would be the risk of going ahead and doing the front one at the same time, btw?

I was thinking coilovers because I've seen many people say they were never satisfied with lowering spring ride quality and ended up getting coilovers anyway. But, I just put TechArts (made by Eibach, I believe) on my Cayman and am very impressed with them, so willing to give that a run.

Any reason I couldn't just use the Eibach springs in the B16's in place of the Bilstein ones? I have picked up a set of B16's, but haven't installed and have been deciding whether to just return them for a refund. I've heard they're harsh, but others say they're fine and compare favorably to F80 suspension, which I found very liveable on a test drive recently.

To answer your questions, I switched to VAC control arms and they absolutely improved handling and got rid of the front wheels' disconcerting wandering habit. Wheels are 19" and I'm running Pilot Sport All Season 4s. And, agreed on solid sway bars only--the ones I have, but haven't installed yet, are H&R.

[johnung]

Originally Posted by Zoomz

Thanks so much for taking the time to provide such a detailed reply, johnung! Very helpful. Only now seeing this and all the other discussion on the thread.

I've gone ahead and ordered the Eibach springs and will try that and the sway bars—what would be the risk of going ahead and doing the front one at the same time, btw?

I was thinking coilovers because I've seen many people say they were never satisfied with lowering spring ride quality and ended up getting coilovers anyway. But, I just put TechArts (made by Eibach, I believe) on my Cayman and am very impressed with them, so willing to give that a run.

Any reason I couldn't just use the Eibach springs in the B16's in place of the Bilstein ones? I have picked up a set of B16's, but haven't installed and have been deciding whether to just return them for a refund. I've heard they're harsh, but others say they're fine and compare favorably to F80 suspension, which I found very liveable on a test drive recently.

To answer your questions, I switched to VAC control arms and they absolutely improved handling and got rid of the front wheels' disconcerting wandering habit. Wheels are 19" and I'm running Pilot Sport All Season 4s. And, agreed on solid sway bars only—the ones I have, but haven't installed yet, are H&R.

Sounds like you are accumulating parts. A habit of mine too. Great choice with the VAC monoball front control arms. They are on my Top5 F3x Mods List.

What is your VIN? Have you looked up which factory sway bars you have now?

Stock & aftermarket springs are not interchangeable with coilover springs because they have different diameters. It’s a physical fitment issue. So the Eibachs shouldn’t fit a Bilstein coilover or other coilover.

There are companies that make coilover sized springs in various spring rates. But you really have to be knowledgeable about the coilover dampers that they would be paired with. Suspensions really are a system of many parts that need to work together. Very easy to make the wrong choice. If you really want to keep the B16-D’s while swapping their springs, I’d search on posts from FaRKle! He knows more on this subject than anyone.

ADAPTIVE COILOVER OPTIONS
I may not have mentioned that besides the B16-D’s there is another coilover option that allows you to maintain your investment in the Adaptive suspension. KW DDC coilovers, part# 39020034. Link below.

https://www.kiesmotorsports.com/prod...0&_ss=r?aff=22

KW is known for high quality materials similar to Bilstein, and for doing an excellent job of matching spring & damper rates. A buddy of mine who has driven many exotics and is used to their super stiff suspensions, said that the most comfortable exotic that he ever drove had had its stock suspension replaced by KW. He actually replaced the B16-D’s on one of his cars with KW’s.

CHASSIS DROPS
It is important to measure your BASELINE CHASSIS HEIGHT at all four wheels before starting a suspension project. On a level surface simply use a tape measure to obtain the length from the center of the BMW wheel cap logo straight up to the edge of the fender lip. Don’t be surprised if left/right differ by as much as 1/2”.

For XDrive it’s a good rule of thumb not to drop the front more than 1.5” from stock. More can get outside of the XDrive suspension geometry and break front drive shafts.

I don’t know the stock chassis heights on your 440ix GC, how much stock reverse rake there is, or your chassis height preferences. It’s also interesting how the eye looking at different shaped front and rear fender lines can perceive different gaps than actual measurements. And spacers changing the poke of a tire can affect that visual too. In other words some people lose their minds staring at tire/fender gaps when they start messing with chassis height.

It’s important to note that there are different chassis drops for the three options. (25.4mm = 1 inch)

1) Eibach Spring Kit
Front 25mm/Rear 20mm. This is a moderate spring drop that won’t have the car scraping. It is pleasing to the eye because it removes some of the excess front gap or reverse rake.

2) Bilstein B16-Damptronic Coilovers
Front & Rear minimum 30mm, maximum 50mm

3) KW DDC Coilovers
Front minimum 1.0” to max 2.1”
Rear minimum 1.4” to max 2.4”

Remember the XDrive rule of thumb not to drop the Front more than 1.5” (38mm)

SWAY BARS
On many cars sway bars easily bolt up. So labor is cheap and it’s easy to try out different thicknesses. But the F3x sway bars are buried so a lot of things need to be removed to access them. Not difficult, just time consuming, so labor is expensive. The front is more difficult than the rear. Rear probably takes at least three hours.

The F3x suspension is designed to understeer. Adding a thicker rear sway bar to a car will reduce understeer. That combined with it being easier/cheaper to upgrade the rear bar is why the rear bar is typically done first. And often the front bar is subsequently left stock.

Many of the aftermarket sway bar kits use a front 28mm and a rear 16mm bar. Some are solid steel and some are hollow. Hollow are cheaper to make so more profitable. I avoid hollow because they can suddenly snap. (Decades ago I lost a friend when he was just driving along at 50mph and a front suspension piece snapped, sending his car into a barrier. I never chance safety.)

H&R makes a thick solid steel sway bar kit at F28mm/R20mm. The rear 20mm can be a little strong for many F3x suspension setups because in a high speed curve there can be a slight lift reducing rear tire patch. I have the H&R sways on my car. I’ve felt it a little when I had square 225/45-18 tires. But now with square 255/40-18 tires (and lowered) it feels much more planted in high speed curves. Again, suspension systems are interrelated parts.

Sway bars main purpose is to control body roll during turns. However springs also contribute some of their function to controlling body roll. So the correct order is to choose dampers and springs that compliment each other. Once the spring choice is known, then choose sway bars.

Some examples:

Example #1
For a stock F3x with say a stock 12mm rear bar, good options might be upgrading the rear sway bar to 15mm (BMW), 16mm (Eibach) or no more than 17mm (KC Design). Leaving the stock front sway.

Example #2
For an F3x suspension that has been moderately lowered with say Eibach or even harsher H&R or Dinan springs, and the dampers upgraded to something more sporty like Koni Special Actives or Koni Sport Yellows, then a good sway bar match might be a Front H&R 28mm and a Rear KC Design 17mm.

Example #3
For an F3x suspension with B16 coilovers, the front spring rates are so high that going to a larger front sway wouldn’t be recommended. The solution to try to balance things out with those springs might be to use the stock front sway bar with the large H&R 20mm rear sway bar. I recall FaRKle! suggesting this solution for a car with B16’s installed.

What size are your 19” MPS4S tires? Square setup? Is your stock rear sway 12mm or 15mm? I’m thinking that if you are already at 15mm, then you could install the rear H&R 20mm and it would work for whichever of the three spring/coilover options that you ultimately choose.

As you suggested you could install the Eibach spring kit, along with new top hats, new rubber spring perches and any other rubber bits. FCP Euro sells an OEM top hat kit with some of that stuff that’s cheaper than Ala carte. Buy new OEM bump stops and trim maybe 5/8” off the top of the fronts and 1/2” off the top of the rears with a serrated knife to avoid prematurely hitting the bump stops after the spring drop.

It’s great that you are willing to try things in steps. That’s what I tend to do when possible. You may find that you really like the Eibachs with the refresh along with the rear sway upgrade. If you want to stick with it and you think there’s a bit much front body roll then install the front H&R 28mm bar. Or if you ultimately want to change to one of the coilover options, then just keep the rear 20mm sway.

Curious to hear what you decide. Hope this helps!