The power limit of BMW 168mm LSD - Should I change to 188mm?
November 22, 2022
We get such questions all the time: what power can BMW small case limited slip diff (LSD) can hold? Should I get 188mm medium case?
The answer is, as always in mechanic world, it depends.
If you run the factory setup, there are some week points which might let you down.
The fist one is the cap. Factory LSD unit (carrier and the cap) is made from cast iron. As you know, casting is the procedure used by large factories to optimize production expenses. Therefore quality of a final product is limited. For that reason, BMW used small case LSD (168mm) mostly in 4 cylinder models. They rated factory (ZF) LSD unit to 150 HP. Which leaves E36 318is as the most powerful model with 168mm Limited slip diff. All S14 engines in E30 used 188mm differential size.
But if you use Billet steel cap, you will double the power limit in this point. We don't spare expenses in the production procedure in order to get the best quality products. We begin the procedure from forged steel block which is than machined on CNC lathe to obtain the highest possible precision. At the end, pieces are heat treated and nitrated. With this addition to your differential, you won't need to worry about braking it ever again. A must have for 6 cylinder swaps, or turbo applications.
Next thing is spider gears (planetary gears, side gears or satellites). Due to limited space in small case LSD carrier, they are prone to breaking. Especially if your clutches are warned out. Worn clutches allow differential to slip more than it should, thus creating excessive heat which is not good for gears. First central pins break, than small gears, and eventually large spider gears. What can you do to prevent that? Inspect your clutches and change them in time. This will keep your gears safer. But if you crossed the power limit of factory gears, we have Forged spider gears available for all types of BMW limited slip diffs.
At the end, you are left with clutch plates. If you have more power than factory setup can handle, than you might need to consider switching to 4 clutch pack instead of 2. Factory LSD uses 2 clutch discs and 2 outer (dog ear) plates. We have Stage 2 and Stage 3 upgrade packs which includes 4 clutch pack for 168mm small case LSD. All RacingDiffs clutch packs are Plug & Play so no modifications are required.
If you need any advice regarding your Limited slip differential setup, please feel free to write us an email. Read more blogs and follow us on social media.