Running a higher diff ratio puts more pressure on the final drive. Longer race durations simply multiply that even further! Standard drive ratio for the 330Ci manual is 2.93, and we’ll be running a 3.73 after the rebuild (April 2021). This all means that the input shaft is spinning faster for the same forward speed. There are more crown teeth engaging, therefore more contact, more friction, and more heat generated. We do this to improve acceleration at the expense of outright top speed.
This heat energy is dissipated by the oil, and the oil is cooled by the case.
For e46 autos with the “performance” package, BMW added cooling fins to the case to assist cooling as they used a shorter 3.43 final drive. Who knows why? Maybe it gave them a similar acceleration to the manuals and compensated for the losses in the auto box? I digress. The OEM part number for the upgraded, finned, cover is 33117512980 and also a longer M10x60 bolt (below the diff mount in picture) is needed. Part number 07119905148. Cost for both is less than £100 and is a great place to start to keep the diff oil cool.
When driven hard the oil temperature rises and the oil viscosity reduces, making it more ‘runny’. At extreme temperatures (outside the oil design limits) the molecular bonds start to break down under these excessive temperatures and the lubrication properties diminish, causing excessive wear, and ultimate failure. We’re developing this car to eventually compete in endurance races of up to 2 hours and diff cooling for reliability is a must.
The cover above also seems to have a breather at the top. This is welcome as it could provide a means to return oil through a cooler as the ultimate upgrade.
