[Kraklz]

Just wanted to share details how to retrofit an M Performance LSD on your F chassis 20i/28i/30i. The M Performance LSDs are made for the 6-cylinder 35i/40i models. There aren’t many options on the market to for an LSD. Quaife and Wavetrac do have some options if you reach out to them but they cost over $3000 USD for a complete unit. Yes I know you may find some that are cheaper but those are usually just the core and don’t include machining the diff or a core charge. I also don’t recommend machining your stock diff for high power applications because it is a smaller unit that will hold less power than a 6-cylinder diff. I am not saying it will break at x amount of horsepower but considering cost it is probably a better idea to go with a bigger diff if you are regularly drifting or drag racing.

I have a 330i and decided to buy the M performance LSD for the automatic 340i. They share the same 2.81 gear ratio. If you own an automatic 328i then it will have a different gear ratio of 3.15 in which you will need to source the M Performance LSD for the automatic 335i (Good luck finding one they are almost impossible to find since they are not in production anymore, the closest ratio you can get is 3.08 #33108659987 which is not a huge deal). As for manual 20i/28i/30i they use a 3.91 gear ratio which is NOT used in the manual 35i/40i cars 3.23 ratio. That is a drastic difference in which case your ratios will be too long so I recommend you go with the aftermarket option or swap the transmission too.

The M Performance LSD is bigger than the factory 4-cylinder diff. You will not be able to use the stock axles. 6-cylinder axles are 35mm in diameter whereas the 4-cylinder axles are 31mm. You NEED to source 6-cylinder RWD axles for this retrofit even if you are attempting this on your AWD 4-cylinder. The good thing about the axles is that they are the same in every 6-cylinder RWD car so if you are trying to do this on your 328i you can buy 440i axles and it will work. You will need to source left and right axles because they are different. You cannot buy 2 driver side axles.



As specifically for the diff it will mount up to your subframe with no issues. The exterior dimensions are slightly bigger because of the increased size of the ring gear. BMW for some reason decided to use different coupling methods on the diff ends for the 4-cylinder and 6-cylinder cars. The 4-cyl uses a 3 bolt flange whereas the 6-cylinders use an inverted cv joint that is held by a 50mm nut. You CANNOT transfer over the 3 bolt flange over from your stock diff over to the M Performance diff because that will make your driveshaft too long. You also cannot use an oem 6-cyl driveshaft because the 3-bolt flange on the transmission side are different sized from the 4-cyl to 6-cyl. 4-cyl is 91mm/12 and the 6-cyl is 105mm/12 (Basically the 3 bolt flange is bigger/more spread apart). This leaves you with the only option of a custom driveshaft. The easiest is sourcing a 340i driveshaft and swapping the transmission side flange for a 4-cyl flange. This requires cutting and welding which any competent driveshaft shop can do. There is a way you can use an oem 6-cyl driveshaft and that is by swapping the flange on the transmission itself but the part cost around $300 and it requires specialty tools so I recommend spending money on a custom driveshaft that’s stronger.



 
I had a shop custom build me a driveshaft using 340i measurements with a 330i transmission side flange. This driveshaft is stronger than OEM because I had already broken 3 OEM driveshafts drag racing my car.




To summarize you need a
1. M-Performance LSD with the correct gear ratio
2. 6-cylinder RWD axles
3. Custom driveshaft (6-cylinder driveshaft with the 4-cylinder transmission side flange)

I know this may seem an intimidating retrofit for some but is very worth it. The difference is very noticeable. I know that the cost of the axles/driveshaft/m-perf diff might cost about the same or more as an aftermarket diff but you need to consider that you will have a much stronger driveline. Since I am drag racing my car it was the better decision for me.

I have put together a kit that will make the process extremely easy. It comes with the brand-new OE axles and the custom built driveshaft that is stronger than stock (800whp+ with no issues). You have the option of sourcing your own diff or including it with the kit. You can get one here https://iposmotorsport.com/products/f-chassis-20i-28i-30i-m-performance-limited-slip-differential-retrofit-kit

[MaverickCruise]

nice work.
RWD + LSD = more fun