Background
I decided to upgrade the rear end of my '00 Dinan 5-speed 328Ci from the stock 2.93 to an open 3.15 differential. This Dinan differential upgrade completes the list of available Dinan "performance" upgrades available for my car, save suspension upgrades. My car now has the following Dinan upgrades: Dinan Stage II software, Dinan CAI, Dinan exhaust, Dinan front strut, and now the 3.15 open differential. For more of my car's technical specs and pictures, please check out my site.

Well, today was the day and I was pretty nervous, probably more so than excited. Why? This differential, although not a limited slip differential, still was not cheap and with install, was going to push the grand total towards a significant chunk of money. So how much? Well on to the numbers then.

Financial Numbers
Despite the discrepancies involved in Dinan's listed price and my Dinan dealer's "book" price, I was fortunate that Dinan stood behind their listed price on their site of $1295, which was $100 cheaper than the book price. I know Justin, aka DinanThree, had to pay $1397 which was the price listed in my Dinan dealer's book. Like most Dinan dealerships, my dealer went with Dinan's labor guide that equated to 45 Dinan units or a total of 4.5 hours worth of labor. Therefore, my damage was $1811 total (unit, labor, sales tax, shipping? and a 10% CCA discount).

Negative Numbers
As realized by many, including myself, going to shorter gearing results in less fuel economy as a result of greater engine speeds. The question is though, how much? In all honesty, I cannot honestly answer since I have only driven 30 miles or so with the new rear end. However, a good indication though is the increase in engine speeds before and after a new rear end. Prior to the 3.15 differential, I believe I was around 2850-2900 RPMs at 70 MPH while in 5th gear. With the 3.15 differential, I see somewhere around a 150-200 RPM increase in engine speeds. As in the above example, at 70 MPH in 5th gear, I am now sitting right at 3000 RPMs with the 3.15 rear end.

Another factor that I wanted to retain in going to the new rear end is the ability to reach 60 MPH in 2nd gear. Well, as speculated, it will take ALL of 2nd gear to achieve this, given an increase in rev-limits brought on by an engine software upgrade (i.e. Dinan). However, do note that I am currently running 235/35-19 tires, up from the stock rear tire size of 245/40-17, thus equating to a rough increase of 0.8" in overall diameter. Although I did not officially test this condition, I think it is safe to say that it will at least take 6500 RPMs and possibly up to 6700 RPMs to reach 60 MPH in 2nd gear. Therefore, I do not know if one will be able to do the same with a 3.23 as some have calculated and/or believed.

Finally, another consequence of going to shorter gearing is the reduction of top speed, and in this case, a reduction in effective top seed. Please note the emphasis on the word effective. Although the speedometer is more than likely a little off at high speeds, I have been able to take the car to 150 MPH with the just the Dinan Stage II software and Dinan CAI; although I am unsure of what RPM I was at during this run. In addition, I never was able to do another top speed run with the addition of the Dinan exhaust. Unfortunately, I will not officially be testing my car's new top speed anytime soon, but I will be extremely surprised if my top speed will decrease more than 15 mph, bringing me to a top speed of "only" 140 MPH. As a result, I do not feel in any way that my "effective" top speed has been negatively affected.

Positive Numbers
Despite Dinan's wording on their site, this 3.15 rear end should not extract any more power (HP and Torque) from my engine through "torque multiplication." HOWEVER, this "torque multiplication" of 7.5% should result in more power being delivered to the ground. From a different point of view, with this rear end, I will essentially have a more efficient drive train, losing less power through the drive train and delivering more to the ground or to the wheels. So is this 7.5% addition delivered to the ground felt? I say yes. Significant? Depends on what you consider "significant." But I would say 12-15 lb/ft. of additional torque being delivered to the ground would not be surprising.

What I have really noticed the most so far is the car's ability to "pull" in the higher RPM range. I would say from 4000 RPMs and beyond, the car really pulls all the way through the rest of the range. Granted that the Double-Vanos in our engines results in a pretty strong engine in stock form in the higher end, especially relative to its predecessor in the e36 where there was a significant drop off in the 5k RPM range, the engine now "pulls" even more in this upper end. In addition, the engine now climbs even faster through the RPM range.

So what should be the results if I were to dyno my car now? As mentioned earlier, this rear end will not extract any more power from the engine. On the other hand, more power should be delivered to the wheels, more than likely 7.5% worth. From a graphical standpoint, imagine the whole power curve shifting up in the positive y-axis while retaining its same shape.

Although I do not have any G-Tech numbers or the like to report at this time, I feel that my car is capable of ripping off 5.9s on average in 0-60 MPH runs, and 5.8s with a more than good launch. At the least though, I feel that this new rear end is worth about 2/10s shaved off of the traditional benchmarks (0-60 MPH and 1/4 mile).

Quick Summary
In short, I really feel like this is a great addition to my car's performance abilities, even as an open differential. I really feel that by upgrading the sway bars and the rear end, the entire driving experience has been significantly enhanced. Although, limited slip would have been nice, that feature would have significantly increased the price on the unit. Furthermore, I do not think our cars put enough power to the ground to really make limited slip a requirement in a differential. But again, it is a nice feature.

In continuing, I feel that many "average" performance enthusiasts fail to realize the importance of a car's rear end. Perhaps the non-association of power-extracting abilities is the cause. Nonetheless, my Dinan dealer is very involved in the racing scene (i.e. SpeedVision series) and he told me that he not only felt the difference in my car, but differentials are such a big factor that they change the rear ends in their race cars all the time.

So was it really worth it? Only time will truly tell, but I would definitely do it again. It is really nice that I have greater ease with passing cars on the freeways since the engine will be closer to its optimal performance range as a result of greater engine speeds. Plus, the requirement to downshift for passes is not as big now.

I hope I have provided you all with some valuable feedback and information regarding this upgrade, especially for you 328/330 5-speed owners (I believe 323 5-speeds and probably 325s too already come stock with a 3.15 rear end).

Feel free to offer up your comments on the E46Fanatics.com general forum, both Justin and I will try to answer any of your questions.