Upgraded differential - other related mods needed?

[dcaron9999]

I have a N55 2011 135i with a Cobb Stage 1 Agressive map. This is a dual duty car, which I started taking to the track for the first time in August 2013. I developed the bug for lapping.

Among other mods (see my signature) I upgraded to M3 rear subframe bushings, which got rid of the rear wiggle.

Just about to get my stock open 3.08 large body differential swapped out for a Quaife 3.46 large body.

While Im in there (or my mechanic is), maybe I should replace the three stock differential bushings with firmer ones. The front E9x/M3 ones are a direct replacement, but not the large rear.

With the added load on the rear end, due to more aggressive 3.46 final drive ratio, Cobb map, sticky tires, will the stock diff bushings be OK? Ideally, I dont want to introduce too much noise into the cabin, so solid aluminum ones or the DEFIV lock down kit are out of the equation.

Probably will get the diff cover drilled and drain bolt fitted by a machine shop. Gotta love our designers at BMW for omitting this on the 1-series...

For the rear end, here are the planned upgrades, budget permitting:
In order of preference:
#1 - Quaife ATB 3.46 LSD, two front M3 diff bushings (maybe), finned dif cover (maybe but the suckers are expensive!)
#2 - TRW Rear upper control arm kit (Rear Upper Guide Rods, and Rear Upper Links)
#3 - Aftermarket adjustable toe arm
#4 - Thicker Rear Sway bar

[Kgolf31]

Why a thicker rear sway bar?

If you're tracking the car, better pony up for track pads

[dcaron9999]

Originally Posted by Kgolf31

Why a thicker rear sway bar?

If you're tracking the car, better pony up for track pads

Track pads and fresh rotors are lined up too, just not directly related to this thread.

The thicker rear sway bar due to mixed reviews that it is a nice complement to the LSD. Notice that it's further down the list, because Im not completely sold on this.

[Kgolf31]

Just making sure. People like to boast about power but have no clue what it takes to stop.

As far as a rear sway, it will eliminate the weight transfer you need inorder to successfully track out and put power down. it will also increase understeer

[dcaron9999]

Originally Posted by Kgolf31

Just making sure. People like to boast about power but have no clue what it takes to stop.

As far as a rear sway, it will eliminate the weight transfer you need inorder to successfully track out and put power down. it will also increase understeer

Good points. Nice to put more power down, and track out faster, but also quite important to be able to shed increased speeds consistently. I may skip the rear bar, because I dont need to introduce understeer. I recently tamed the understeer with camber plates, and M3 control arms.

Before front or rear sway bars, I will probably invest in Koni sports and Swift springs, but not until next year. Already have $5000CAD planned in Quaife LSD Diff and brake pads+rotor refresh, and season hasn't started yet. ;-)

[RimasRS]

Originally Posted by Kgolf31

Just making sure. People like to boast about power but have no clue what it takes to stop.

As far as a rear sway, it will eliminate the weight transfer you need inorder to successfully track out and put power down. it will also increase understeer

I think thicker rear sway bar will increase oversteer that is why we have to be careful because of the short wheelbase.
I think nice would be to use from Xi or cabrio it is 2mm thicker.

[Kgolf31]

Originally Posted by RimasRS

I think thicker rear sway bar will increase oversteer that is why we have to be careful because of the short wheelbase.
I think nice would be to use from Xi or cabrio it is 2mm thicker.

No, that is completely the opposite. I think you have it backwards

BMWs are weird, you typically don't add a front bar on a RWD car and see a decrease in understeer and an increase on oversteer, but that is what happens.

Adding a bigger rear bar will transfer weight to the front and increase understeer.


This is why you typically see alot of BMW racers running a stock bar, or no rear bar at all.

IMO an aftermarket rear bar does nothing good.

[RimasRS]

Originally Posted by Kgolf31

No, that is completely the opposite. I think you have it backwards

BMWs are weird, you typically don't add a front bar on a RWD car and see a decrease in understeer and an increase on oversteer, but that is what happens.

Adding a bigger rear bar will transfer weight to the front and increase understeer.


This is why you typically see alot of BMW racers running a stock bar, or no rear bar at all.

IMO an aftermarket rear bar does nothing good.

read this thread it is really informative... Where he installed M3 rear sway the oversteer was crazy. http://www.1addicts.com/forums/showthread.php?t=510253

[Kgolf31]

Originally Posted by RimasRS

read this thread it is really informative... Where he installed M3 rear sway the oversteer was crazy. http://www.1addicts.com/forums/showthread.php?t=510253

Typically, adding a sway bar on the front or rear end will reduce that end's grip.

However, this isn't the case with BMW.

As well, to touch on that thread, there are a lot of factors. Alignment, spring rate...etc can all effect oversteer.

[dcaron9999]

Thanks for all the input so far guys.

No comments yet on stiffer Rear Differential Carrier Bushings? While Im in there replacing the Diff pumpkin, I could replace the stock ones, with the M3 ones here:
http://www.ecstuning.com/ES1844489/

There are 3 bushing in all (2 front, and one larger rear one). The parts above are only the two front ones.

I wonder if these are worthwhile, and introduce a lot of NVH ... My stock ones seem in good shape, and I did M3 rear subframe bushings already, which are probably more beneficial.

[Dackelone]

Originally Posted by dcaron9999

I wonder if these are worthwhile, and introduce a lot of NVH ... My stock ones seem in good shape, and I did M3 rear subframe bushings already, which are probably more beneficial.

I think the M diff bushings would be a good choice. The ///M bits really don't introduce that much more NVH - at least when it comes to rear suspension bushings and trans mounts. I suspect its the same for the diff bushings as well.

[dcaron9999]

Thanks Dackel!
I just found this related thread on the E90 forum, which includes experiences of owners that have done this mod: http://www.e90post.com/forums/showthread.php?t=395250