Very informative write up. Thank you!
I currently own a 2004 BMW Z4 3.0 Manual transmission with a bad
differential. I had a mechanic drain a little bit of the gear fluid and there was metal bits coming out of it meaning my
differential carrier bearings is bad and my car is currently making very loud rear tire noise it drives me crazy. I need to swap it to a good
differential not necessarily a higher gear ratio, but just a regular 3.07 gear ratio should be fine and if I am not mistaken I think that is the standard ration on my car right now.
Now I have a very important question before I go out and buy the wrong
differential. I have been told that a 2.5 BMW Z4 manual or automatic transmission will fit my 3.0 BMW Z4 manual transmission and will bolt in just fine. Is that right?? I can see maybe a
differential from a manual 2.5 Z4, but an automatic transmission too?
One more question, If I choose a higher gear ratio such as the 3.46, I have been told that my speedometer will be inaccurate. example, the speedometer says I'm going 40mph, but my actual speed is 35mph. Is this true?
I would appreciate some answers and thank you again for this great post.
With regards,
Danny
bimster wrote:Changing the differential gear ratio can be a great performance modification. What follows is a summary for anyone who may be interested.
It’s normally done to increase acceleration. A differential swap takes advantage of the gear ratio in the rear differential to change the wheel torque in a given transmission gear. A higher ratio differential increases the wheel torque. A lower ratio differential reduces the wheel torque.
For example: replacing a 3.07 differential ratio with a 3.46 differential ratio increases the wheel torque by 3.46/3.07 or about 12.7%. That's about a 25 ft-lb increase in wheel troque for a typical 3.0 liter BMW engine...without any engine modifications. Believe me you'll definitely feel that much torque! Non-M BMWs come with a standard differential gear ratio that permits reasonable operation at very high speeds. Those of us who never drive at very high speeds (e.g., on the Autobahn) don’t care much about that end of the performance envelope. What we do care about is the performance available to accelerate through a curve, when merging into faster traffic, when climbing a hill, and so forth.
The differential ratio change is an ideal modification for improving performance in these daily driving situations (or at the track) because it increases wheel torque by the same percentage throughout the entire engine speed range. Mechanically, it provides the same performance improvement as increasing engine displacement by the same percentage, e.g. with the 12.7% torque increase from the example above, a 3.0 liter engine with a 3.46 differential accelerates like a 3.38 liter engine with a 3.07 differential!
Another benefit of larger ratio changes (I drive with a 3.64 ratio that added 18.5% wheel torque) is that they make it quite easy to start off in second gear without slipping the clutch. I can start in 2nd and still comfortably out-accelerate most cars on the road from a stop. I now use 1st gear only for those much rarer situations when I need to get going as fast as possible such as in AutoX events or merging into a tight spot in cross traffic, or when starting off on a steep uphill grade. So, even though the speed ranges of the transmission gears are now shorter, I find that I do less shifting, which certainly helps in commuting traffic.
And, I still get approximately the same fuel economy as before even though my rpms run 500 higher at 70mph and the greater sportiness of the car encourages more spirited driving. When cruising at lower speeds I’m still selecting a gear such that the engine runs between 1500 and 2000 rpm.
In the USA, where 90% of the BMWs (even Z4s) have automatic transmissions, finding a suitable salvaged higher ratio differential like a 3.46, 3.64 or 3.73 is quite easy. Prices generally range from $300 - $500 plus $200 labor for the swap, so swapping a whole salvaged differential is the most common approach. It may be harder to find a higher ratio differential in other countries, such as the U.K., where automatic transmission models are rarer. So, in that case, it may be necessary to order one from the factory or to have a properly trained technician change the gears themselves.
