So I went ahead and took the plunge on this. Bought a 50i 3.15 diff off ebay, $150 shipped w/ 60k on it, very reasonable IMO.
E70 3.64 to 3.15 Differential Swap:
Initial Impressions
I have driven ~500 miles with the new 3.15 diff installed and I wanted to share some initial thoughts. Here is a collection of observations in no particular order.
Noteworthy mods on the X5: RWD (XHP), Malone 2.9, ATM IC, custom intake, Sutphin Trans Tune, and HPR fuel.
- In short, it is working out how I had hoped. The actual swap was straight forward and there have been no hang-ups with the process so far.
- My tuned X5 never struggles to accelerate and driving is effortless in all conditions like it was before. You can leave from a standstill in 2nd without much fuss.
- In general the engine and trans seem to be handling the different gearing fine, shifting behavior seems normal but there are some times when the trans is holding onto a gear (close to downshifting) and with the 3.15 those RPMs are even lower and the engine is falling out the power brand. The motor and trans find themselves in the "RPM troughs" but maybe Rod can adjust these gear limits so the trans is a bit quicker to downshift?
- With the taller gearing WOT from a dig isn't as "exciting" ..which is to be expected but the decrease in straight line performance is most exposed at these 0-60 0-80 WOT periods. Acceleration at high speeds such as highway pulls, passing, etc. seem very similar to before except there is less shifting.
- The main goal was to give the X5 longer legs so the diesel motor isn't humming at more than half its redline at freeway speeds and the 3.15 delivers, if you reference the 2nd photo you can see I am doing roughly 80mph at 2,000rpm, wonderful. This is down 550-600rpms compared to before
- At highway speeds the powerband is very useful since 65-80mph buts you between 1,600-2,000prpm. Only if you get down to 60mph and are going uphill would you really need to downshift.
- My favorite part of the whole swap is how much more relaxed the X5 is in the 70-80mph range, There is less NVH and the X5 cruises along with less drama. IMO this is a pleasant surprise I wasn't expecting.
- Addressing fuel economy: Doing 60-65mph will net you similar fuel consumption as with the stock 3.64 diff. What's different is in the 65-75mph range where the 3.15 returns similar numbers but the 3.64's efficiency drops off quickly >65mph. In this 65-80mph range is where I am seeing my predicted +3-5mpg bump.
*I haven't done any 80+mph highway testing yet.
- My X5's digital fuel economy readout is usually optimistic but not by much, usually less than 1mpg variation. I the X5 had some urban driving prior to this road trip so unfortunately I wasn't able to get accurate hand-calculated numbers but .....
- I drove from San Diego to Orange County, driving flow of traffic 71-72mph. On the drive up I recorded 32.2mpg and this included 10 miles of urban driving to a shop. Return trip yielded 35.1 mpg. So let's call it 33.5mpg over 180 miles out-n-back. Nice
- Overall I am very happy and consider this swap to be 85% positive and 15% negative, well worth it IMO. If you care a lot about drag racing this might not be the swap for you, for the rest of the time it's a welcomed change.
- In terms of fuel economy I think there is more to be gained as well. I have some audible contact between a brake pad and rotor, this certainly isn't helping anything and the X5 doesn't seem to coast as well. The plan is to bleed the brakes and lube the caliper pistons. With the rubbing resolved I wouldn't be surprised if there is another couple MPGs to be had.
